small sailboat diesel engines

Yachting Monthly

• Digital edition

Best marine diesel engines: a buyer’s guide

• Sam Fortescue
• September 24, 2021

Modern marine diesel engines may look very similar to the large, noisy, oily chunks of iron of yesteryear, but a lot has changed under the bonnet, says Sam Fortescue

An inboard engine is a critical part of any boat, whether you're a purist who only motors when safety demands, or if you'd rather get into harbour in time for last orders. The chances are you also rely on the engine for battery charging. Credit: WorldFoto/Alamy Stock Photo

Best marine diesel engines: the new tech making inboard engines cleaner, more efficient, lighter and more reliable

Once referred to as the ‘auxiliary’ engine , your boat’s inboard diesel is now so much more than just a helping hand.

Not only is it central to your boat’s ability to manoeuvre safely into tight marina berths, it also allows you to keep up average speeds and make the tidal gate before it closes, charges the batteries that run the ever-expanding suite of entertainment and navigational tools aboard, and keeps the beers cold and showers warm.

Despite visual similarities to the noisy models of yesteryear, modern marine diesel engines are cleaner and more efficient, lighter and more reliable. And they are increasingly interactive.

If you’re considering re-powering your boat, here are some of the key features to look out for.

Marine Diesel Engines: Emissions

Tighter regulation in Europe has been forcing engine manufacturers to reduce the harmful emissions generated by their equipment.

In the marine sector, that means complying with the 2013 Recreational Craft Directive, known as RCD 2, and often with the EPA Tier 3 requirements in the USA.

Though most manufacturers anticipate further tightening in the future, there is nothing yet on the cards for marine diesel engines in the leisure sailing sector.

The focus on reducing emissions has resulted in a better understanding of the combustion process inside the cylinder.

A key pollutant, nitrous oxide, is formed at higher combustion temperatures, so one advance has been to reduce those temperatures.

A mechanical system, like this Beta Marine engine, offers ease of maintenance. You don’t need a laptop to reset the electronics

Increasing the swept volume of the cylinder theoretically creates more power, because there is room to burn more fuel each cycle, but if you keep fuel consumption the same, the diesel generates less heat in combustion.

Another area of development has been around the fuel injection system.

A lot has been written about the desirability and reliability of so-called common rail versus mechanical injection for marine diesels.

As Andrew Growcoot, CEO of Beta Marine puts it: ‘The benefits of a mechanical system [are] simplicity and ease of maintenance. One doesn’t need a laptop to reset the electronics ; a mechanical system is safe and will not power down at the wrong time.’

Beta Marine is a British marine diesel engine supplier that uses a Kubota block, and has no intention of introducing the technology any time soon on sub-100hp engines.

The same is true of France’s Nanni and Spain’s Solé, who both supply good, reliable diesel marine engines to the sailing market.

Hedley Beavis of Solé distributor Engines Plus says research to find a common rail injection system has been delayed by COVID-19.

‘It is not an easy task finding a reliable common rail engine but also at a reasonable price for the marine market,’ he adds.

But while common rail injection makes your engine reliant on an electronic control unit for precise high-pressure fuel injection and makes it more susceptible to poor quality fuel, this widely used technology can also offer significant benefits in terms of power and fuel consumption.

Continues below…

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Plus, it’s quieter and produces less vibration.

That’s because the diesel is injected as a huge number of tiny droplets, greatly increasing the surface area for better combustion.

Common rail injection is usually found on more powerful engines – Volvo Penta uses the system in its D3 engine and above, which starts at 110hp.

But Yanmar took a bold step in 2018 when it launched a new 40hp unit with electronic fuel injection. The 3JH40 is still the smallest marine diesel engine with this technology.

‘Through common rail technology, the 3JH40 offers minimal fuel consumption and exceptionally low noise and emission levels, exceeding EPA Tier 3 and EU RCD Tier 2 emission regulations for virtually smoke- and odour-free operation,’ says marketing manager Sander Gesink.

‘People don’t want to have the smell of diesel onboard making them seasick.’

The demand for more plentiful electrical power on board boats has led engine manufacturers to increase the output of their alternator systems.

Where a typical alternator on a 30hp engine might have been around 50A two decades ago, they are now often in excess of 100A.

So, for example, even Volvo Penta’s entry-level D1-13 engine packs a meaty 115A alternator with its own built-in charging regulator, for optimal battery charging.

The 50hp D2-50 offers the same alternator, as well as the option of expanding output with a pulley take-off for a second alternator.

Even Volvo Penta’s entry-level D1-13 engine packs a meaty 115-amp alternator with a built-in charging regulator

Beta Marine’s 43hp engine comes with a 70A alternator as standard, but can accommodate upgrades to 120A or 175A alternators, as well as a second 175A alternator for really hefty charging.

It is worth noting, however, that many of these alternators remain fairly basic bits of kit with an inflexible charging regime.

That’s one reason that Yanmar has worked with Mastervolt on its 150hp-plus 4LV engines to develop a secondary charging option in addition to the on-board 130A unit.

Using the Alpha III charge regulator, that alternator will put exactly the right charge into the battery bank on a three-step regime that works regardless of the RPM by varying the field current going into the alternator.

Units on smaller, older marine diesel engines can easily be retrofitted with a regulator, such as Sterling Power’s Pro Reg.

Interactive marine diesel engines

It used to be that marine diesel engines would buzz along in the background unless they overheated, in which case you were treated to an ear-splitting alarm from the control panel.

They still do sport their own dedicated warning lights which ping on if oil pressure drops or the temperature rises above a safe 80-90ºC.

But with the growth of instrumentation on board, and the development of fast, bi-directional networks, engine manufacturers have had to up their game.

That means enabling the engine to put data into your NMEA2000 instrument network about its speed, temperature and oil pressure, as well as many other potential parameters on more complex systems.

‘Captains just want to see their engine performance and details on their MFDs,’ says Yanmar’s Gesink.

Steyr’s unique monoblock marine diesel engine starts at 75hp. The MO 4-CYL uses a two-stage unit injection system with the benefits of high-pressure fuel without the requirement for electronic control

As a common rail injection engine, Yanmar’s 3JH40 already has the electronics necessary to connect directly to the boat’s NMEA2000 instrument network, as well as the VC10 electronic throttles and other controls via a J1939 CAN bus.

These include the YD42 smart panel display, which will toggle through engine data such as load percentage and engine hours, as well as standard navigational data like depth and wind speed.

‘Direct connectivity to NMEA and J1939 CAN-bus networks has been purpose engineered into the 4LV range, allowing future-proofed integration to any multifunction bridge display. Same counts for the 3JH40 and the rest of our common rail engine range,’ adds Gesink.

While 110hp-plus Volvo Penta marine diesel engines – the D3 upwards – have been built since 2006 with a proprietary Electronic Vessel Control system, its smaller D1 and D2 marine diesel engines were given a hybrid solution.

This Mechanical Diesel Interface (MDI) is a black box fitted to the side of the exhaust manifold, where heat and vibration have made it somewhat temperamental.

Without the MDI, the engine simply won’t run, although it is quite straightforward to bypass.

Volvo sells an Easy Connect adapter that plugs into the J1939 socket on the MDI and feeds its data out onto a NMEA 2000 instrument network, allowing it to be visualised on the plotter or other displays.

Volvo’s Easy Connect app allows you to monitor engine-specific data over Bluetooth on smart devices

Other manufacturers also offer products that can do this.

Volvo offers dedicated instruments to show fuel level, temperature and voltage if you want it.

It also produces its own glass bridge touchscreen displays from 7in to 24in, although this will mainly appeal to the powerboat market.

More interesting for sailors is Volvo’s Easy Connect app available for smart devices, which allows you to monitor engine-specific data over Bluetooth thanks to the Easy Connect adapter.

Other engine manufacturers have adopted a more agricultural approach.

Beta Marine, for instance, has worked with market-leading sensor firm Actisense to build a module that plugs into the wiring loom that runs between the engine and the control panel, and feeds data into the NMEA 2000 network.

It only works with one of Beta’s more expensive C and CW Deluxe control panels, and requires a bit of splicing in of wires.

Solé also offers an NMEA 2000 converter to get analogue data from the engine into your digital instrument network.

You still need a dedicated control panel, but the SDC2000 kit even allows the engine’s alarms to be transferred to your MFD.

It is compatible with all Solé diesel engines since 2008.

France’s Nanni uses a similar NMEA adapter, but also offers a dedicated interactive display to enhance its control panel – available for all engines above 21hp.

There is a small 4in model, and larger 7in and 9in display built around Raymarine technology, with charting, radar and CHIRP sonar support.

The choice is yours

Every owner will make up their own mind on common rail injection.

It looks like the way of the future, judging by the automotive sector, but critics point out that poor quality fuel and the need for maintenance in out-of-the-way places weigh in favour of mechanical fuel systems.

It depends whether you’ve got far-flung cruising in mind, or whether you plan to stick to home waters.

Weigh up your priorities. The Yanmar’s fuel consumption is certainly better than the competition.

But the Beta offers one of the best torque curves, equating to more usable power at cruising revs.

Thinking about going electric? Click here

Either way, don’t be tempted to overpower the boat, as marine diesel engines are most efficient at around 80% of their revs.

If you want the engine to feed data onto the network, it’s best to know from the outset, so the engineer can hook the whole thing up.

That said, it’s hardly a complex job, although the necessary kit amounts to several hundred pounds in the aftermarket.

If you’re looking to re-power, the key criteria will always be the space available, and access to filters, impellers and freshwater systems for maintenance .

While there’s broad consensus about hydrogen being the fuel of the future, the path to fuel-cell propulsion is distinctly unclear.

In the meantime, alternative fuels may play an increasing part in the energy mix for combustion engines: methanol or ammonia can be stored as liquids and mixed with diesel in a combustion engine.

There are challenges to overcome, not least the nitrous oxides produced when ammonia burns, but there is research time going into just that.

Shipbuilder CMB is working with manufacturers of bigger engines to make the design tweaks to run on dual fuels; focused on the injection system.

A more immediate possibility is the use of hydrogenated vegetable oil ( HVO ) or gas-to-liquids (GLT).

HVO is essentially biodiesel and can be made from waste cooking oil, while GLT is made from natural gas and is said to burn more cleanly than diesel.

‘The advantage of mechanical injection systems is that the engine can take advantage of using low- emission alternative fuels, such as HVO and GLT, without alterations to the engine,’ adds Beta Marine CEO Andrew Growcoot.

Marine Diesel Engines: A buyer’s guide

Volvo penta d2-50.

Volvo Penta D2-50: Easy to get NMEA data from the engine

Volvo has an odd gap in its engine line-up between the 28hp D1 and the 50hp D2, which exist in both shaft and saildrive formats.

The D2 marine diesel engines run at 3,000rpm, which is pretty standard for this power rating.

In general, slower rotation makes for less noise and vibration. Both series use inline injection and feature the MDI electronic interface which has suffered from reliability problems in the past.

On the plus side, the interface makes it easy to get NMEA data out of the engine and onto your instrument network. It also opens up a good range of snazzy remote displays.

Volvo’s ergonomic electronic controls won’t work with the D2 engine, however. They are only compatible with the EVC marine diesel engines that run on common rail injection.

Volvo has excellent, if expensive, global support.

• Power: 51hp (37.5kW) crankshaft
• Cylinders: 4
• Displacement: 2,200cc
• Fuel consumption at max rpm: 11.5lt/hr
• Dry weight: 249kg
• D2-50: £9,360
• Dealers: www.volspec.co.uk ; www.goldenarrow.co.uk

Yanmar 3JH40

Yanmar 3JH40: An award-winning 40ho engine

With a power output rated at 40hp, this hugely successful, award-winning engine was launched in 2018.

Its key feature is the common rail injection system which Yanmar has introduced with claims that it reduces smoke and odour to nearly nil.

This is due to the more complete combustion of the diesel fuel when injected under high pressure.

It also operates at a relatively low 3,000rpm, which makes it quieter than faster-spinning engines, and reduces vibration.

Yanmar has developed a series of digital controls for its engines. The VC10 and VC20 electronic throttles make data such as engine loading available on the network.

There are also dedicated displays like the YD42, which has a full-colour screen. Otherwise, a £350 analogue-to-digital converter will get your engine data onto the NMEA 2000 network.

There’s a saildrive option, which costs some £2,500 more than the shaft alternative.

Yanmar engines are also very well supported with servicing.

• Power: 40hp (29.4kW)
• Cylinders: 3
• Displacement: 1,642cc
• Fuel consumption at max RPM: 8.8lt/hr
• Dry weight: 192kg
• www.barrus.co.uk

Beta 43: Excellent, mid-range torque for its rating

Beta Marine diesel engines are based on the solid Kubota block, and represent good value, no-frills performance.

Painted a distinctive red colour, they offer flexible installation thanks to a good range of options.

This makes them well suited to repowering in tight corners.

Though the design is constantly being improved, this is a traditional mechanical engine with the minimum of electronic gimmickry.

It offers excellent mid-range torque for its rating.

The standard control panels are steadfastly analogue, although a NMEA2000 converter is available to get limited engine data onto the network.

A new digital display panel is now available, with a standard loom input and a NMEA 2000 output.

Reliable and well supported with spares, and readily serviced, these are good engines that are sold worldwide.

• Power: 43hp
• Displacement: 1,999cc
• Fuel consumption at max RPM: 9.3lt/hr
• Dry weight: 238kg
• www.betamarine.co.uk

Nanni N4.43

Nanni N4.43: Low rpm makes for a quieter unit

Nanni, a French-Italian brand, is well represented in the UK by AR Peachment. These distinctive blue marine diesel engines have helped make Nanni the world’s third largest marine engine supplier.

The engines are fairly traditional, with indirect mechanical fuel injection, natural aspiration and water-cooled exhaust manifold. Relatively low RPM makes for a quieter unit.

The N4.43 is in fact a de-rated version of the larger 50hp unit.

Being a mechanical engine, the N4.43 relies on an NMEA adapter to get engine data onto the network.

Though controls remain analogue, the N4.43 is compatible with a series of Nanni digital displays which start at 4in.

Support at the leisure end is not as widespread as bigger brands, but as it’s based on a Kubota engine block, it should be easy to find a competent mechanic.

• Power: 43hp (29.4kW)
• Displacement: 2,197cc
• Fuel consumption at max RPM: 10.3lt/hr
• Dry weight: 228kg
• www.peachment.co.uk

Solé Mini-44

Solé Mini-44: Decent torque at mid range

Spain’s Solé produces a very wide range of diesel marine engines, and the Mini-44 is aimed at yachts in the 10-12m LOA range.

Sam Fortescue is a freelance marine journalist and former magazine editor who sails a Sadler 34, which has taken his family from the Caribbean to the Baltic

It looks cheaper than the competition, but Solé’s model is to quote a fixed price and not engage in the inevitable discounting of the other brands.

This is the smallest four-cylinder unit the company builds, giving quieter running but greater fuel consumption at top speed.

Though rated 42hp, it has a smaller capacity and has to work harder to achieve it.

For that reason it compares better against 38hp models.

A purely mechanical engine, it relies on cam-driven indirect fuel injection and produces decent if not outstanding torque at the mid range.

Available with an impressive range of gearboxes, including saildrive fittings, the Mini-44 is a very flexible engine with lots of options.

A very expensive converter (SDC2000 – £800) is available to transfer the engine’s analogue data into digital signals for your NMEA 2000 network.

But the control panel options, although very attractively styled, remain resolutely analogue.

Solé has decent global support, naturally focused on Europe and the Spanish-speaking world.

• Power: 42hp (30.9kW)
• Displacement: 1,758cc
• Fuel consumption at max RPM: 10.4lt/hr
• Dry weight: 175kg (w/o gearbox)
• www.enginesplus.co.uk

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Specifications

NOTE: 1 kW = 1.3596 mhp = 1.34102 bhp Fuel condition: density at 15°C = 0.842 g/cm³ Technical data is according to (ISO 8665) / (ISO 3046) Fuel temperature 40°C at the inlet of the fuel injection pump (ISO 8665) Length includes KM35A marine gear (Dimensions may vary with other gears/elbow)

More Information

Winner of the miami 2018 innovation award for the inboard engine category.

The 3-cylinder 3JH40, with an output of 40 mhp, will enable a whole new category of smaller leisure boat owners and commercial vessel operators to benefit for the first time from the efficiency and performance advantages associated with the most recent electronically-managed CR fuel-injection technology.

With an output of 40 MHP @ 3000 rpm (29.4 kW @ 3000 rpm), the 3JH40 is the smallest common rail inboard marine diesel in:

• Displacement, with only 3 cylinders measuring 1.642 L (100 cu in)
• Physical size, measuring 774 x 580 x 632mm
• Weight, measuring 192kg without gear

Through common rail technology, the 3JH40 offers minimal fuel consumption and exceptionally low noise and emission levels, exceeding EPA Tier 3 and EU RCD Tier 2 emission regulations for virtually smoke and odor-free operation.

The 4-stroke, water-cooled 3JH40 is an ideal solution for new builds and repowering applications, specifically monohull sailing yachts and catamarans up to 45ft, small motor boats or sloops, and light duty commercial craft.

*The next largest common rail inboard marine diesel engines are the YANMAR 4JHCR line: 45, 57, 80, 110 MHP.

Drawings Download Drawings

Performance Curves Download Specification Datasheet

Standard Engine Package

• Exhaust mixing elbow [L-type]
• Alternator 12V – 125A
• Flexible mounts
• Mechanical throttle sensor
• Fuel pre-filter with water separator
• Fresh water expansion tank
• Belt safety cover

Optional Accessories

• High-riser mixing elbow [instead of L-elbow]
• Secondary alternator 12V – 125A or 24V – 60A
• Extension harness [3m, 5m, 10m]
• Second station
• TFX Sidemount control head
• VC10 Vessel Control System
• NMEA2000 Engine monitoring system
• Semi 2-pole
• SOLAS including Certificate [factory fit only]
• Hot water kit with cocks
• Extension saildrive leg [75 mm]
• Propeller cone nut kit SD60
• On-board spare parts kit

Operation Manual

Product guide, specification datasheet, technical drawings - bobtail, technical drawings - gear, technical drawings - saildrive, compatible products, gears / drives, controls / panels.

Kanzaki Gears

B25 & C35 Panels

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The Best Diesel Engines?

• Thread starter Ducati
• Start date Sep 16, 2009
• Forums for All Owners
• Ask All Sailors

Hello Who make the best most reliable and least expensive to repair diesels today? My pick is Yanmar because I consider them to be the Honda of diesel world. Thanks  

The only experience I have had is that I looked at a number of boats. I could find ready sources of supply for parts for a 1980's era Atomic 4 or Yanmar, but boats with Volvo's seemed to be selling at a drastic discount due to abysmal parts availability from Volvo which apparently had stopped supporting them.  

I have no trouble getting parts for my Volvo. But I think that Kubota probably makes the best diesel used in small boats. Several companies convert them for marine service.  

Rich Stidger

I agree with Ross- I have no problem getting parts for my Volvo. Regarding "least expensive", I only have experience with Volvo and Westerbeke, and neither of them is bashful about charging hearty prices for their stuff, but availability is good.  

Kabota is probably the cheapest. Volvo is one of the most expensive. Yanmar makes good units, but they are not the cheapest. Bukh makes an excellent unit but are not very popular. There are probably another handful that are also excellent units.  

I googled "lifeboat engines" and came up with this. http://cgmix.uscg.mil/Equipment/EquipmentDocuments/Engine List_Jan2009.pdf It makes some interesting reading.  

The beta marine Kubota units are nice with perfect raw water pump access a built in oil change pump ect  

2 sailboats with Yanmar engines, 10 years, no problems. I vote for Yanmar. Bill  

Had a Westerbeke 30 HP on my Cal 34 III... Was a great engine. Ran like a champ 4 cyl, heavy. New boat has a Yanmar 3HM35F....I love it. Smaller, no glow plugs, no engine zincs, quiet. Parts for both were available If I had to repower based on experience I would go with Yanmar. But I hear Beta Marine (Kabutu) are great and very user friendly. Good Luck  

Stu Jackson

NYSail said: (Kabutu) are great and very user friendly. Click to expand

Re: Most Creative Maybe he means Neil Cavuto...the guy who does the financial / money segment on T.V.  

Two boats with older Yanmars. I've been happy so far. I guess since I am used to them and don't know better I prefer them.  

Bad Obsession

The best diesel engine on the market is the one that is in a boat and it turns on when you want it, runs well, and gets you far enough away from the dock that you can put the sails up! After that, the best engine ever made goes to Mother nature. The brand name is WIND!  

Jim Kolstoe

[Rare is the man who does not truly believe that the very best boat in the wrold is the one he currently owns./QUOTE] stu, could you please explain the apparently common malady known as 3ft-itis. Click to expand

Mike Holibar

Volvo 2000 series were actually marinised Perkins 100 series which were manufactured by Shibaura of Japan which is Yanmar. Perkins is now owned by Caterpillar. Perkins have always made good reliable engines from about 45hp up and because of their prevalence in agricultural and industrial equipment around the world, it is usually easy to source parts and find a mechanic that can work on them. I don't know if they have reached the States yet, but if you get the chance take a look at the FOCS engines (13-40hp) by Lombardini Marine. They have a loto of advantages over the others in this class. I have owned two, and we are just in the process of repower our club rescue launch with one. Yanmar is a big name here but the Lombardinis are getting close to knocking them off the market leaders perch. Cheers,  

Come on, Stu ! That is (maybe?) the plural of Kubota ! I have had one boat/engine for 18 years.. Yanmar 3 GMF.. Has been an excellent engine.. troubles so far have been only the exhaust elbow and the hesitant/reluctant starter. Ohhhh yeah and the crappy air cleaner that turned to polysomething-or-other dust and was ingested by the engine. Mechanically outstanding engine. The 24 year old lift pump is probably the next part to fail. The Yanmar has always run when I've asked it to!  

Continuing: The highest hours engine I have experience with is a 1982 Westerbeke 4 cylinder, around 35 HP.. don't remember the model offhand. It is a marinized Mitsubishi. Went about 5000 hours before rebuild. Would have gone longer except that it had no intake air strainer and had ingested a small piece of stainless, about the size of a pinhead. That imbedded into the top of one piston and started a crack. The crack did a classic fatigue propagation across the top of the piston and down one side. (we wondered why it would immediately turn engine oil sooty black after a change!) The piston eventually siezed one day on the way back to the slip, and was subsequently rebuilt. It is about 2500 hours into the rebuild now and is fine. We had to replace the electric lift pump after about 4500 hours. The only real weakness on that engine is the gear driven raw water pump. Its relief holes were not large enough and the bronze sleeve bearings were not up to the load. sleeves would fail, the seal would fail then seawater would get into the crankcase. That would make a mayonnaise looking emulsion in the crankcase which would completely fill the engine and valve cover if ya let it run long enough.. No oil pressure warnings !.No obvious damage to the main and rod bearings. It was quite a job to get it cleaned out and flushed out after an incident. . I modified the pump to ball bearings and more limber hole area.. haven't had the emulsion problem since. A new Westerbeke of that same model will have a better pump on it, with better bearings and seals and more limber hole area. Other than the water pump, headache, the Westerbeke/Mitsubishi continues to be a fine engine.  

fox islander

I have the Universal (kubota) MXP25...Ive had no problems with it...and marinedeiseldirect.com gets the parts to you fast...I have noticed however that the Yanmars seem a little smoother, less vibration, less tractor-like.  

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Every sailor knows that operating and maintaining a diesel engine in a saltwater environment can be a challenge. Add to that the fact that many of these engines are upwards of twenty years old and may have a sketchy maintenance record. Enjoying sailing can often translate to being a part-time diesel mechanic .

Well, that’s where SaltWaterDiesels.com kicks in to gear. Here you can learn how to maintain, troubleshoot and repair your sailboat’s small marine diesel engine. There’s a little bit of history about the development of the diesel engine and some general advice about maintaining your engine.

We passionately believe that if you know how it works, then you will know how best to fix it.

This means that if your engine gives you trouble when you’re at sea, you have at least a fighting chance of getting it working again yourself. Self-sufficiency at sea (in this case the ability to solve minor engine problems) is the hallmark of a good skipper.

Let’s begin. Choose your engine by manufacturer…

The semi-diesel, or hot-head, engine is not a true diesel, but retains the operating functions of the diesel with the exception of high compression. On these engines, the head or a bulb in the combustion chamber is heated to near red heat, usually with a kerosene torch. The engine usually has a manual pump that will force an amount of fuel that will be sprayed through a nozzle in the combustion chamber against the heated area. As the engine is rolled against compression, the manual pump is activated and the sprayed fuel ignites. This in turn creates pressure in the combustion chamber against the piston, thus starting the engine.

Once the engine starts, fuel addition, under pressure, continues automatically and is controlled by the governor. As in a true diesel, the engine speed is controlled by the timing and length of fuel addition, during the power stroke. Also, as in the true diesel engine, this has no throttle on the air intake. Most semi-diesel engines are 2-stroke but there are 4-stroke versions also.

In a semi-diesel, once the engine starts the heat source for the hot-head or bulb can be removed; the heat is generated by the combustion itself to continue the ignition process. Typically, semi-diesel engines have from 5:1 to 7:1 compression ratios. They can use a variety of liquid heavy fuels, from kerosene to heavy bunker oil, and almost anything in between. Their ignition does not come from the pressure generated by the engine as a true diesel does.

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[bbp-topic-form]

The VOLVO PENTA MD4 is a marinisation of the Mercedes-Benz OM636 engine.

This version is also fitted with a BOWMAN heat exchanger but uses a different sea-water pump.

Front Projection

Side Projection

Side Projection with Sail Drive

The Solubility Rules of Salts

1. The nitrates, chlorates, and acetates of all metals are soluble in water. Silver acetate is sparingly soluble. 2. All sodium, potassium, and ammonium salts are soluble in water. 3. The chlorides, bromides, and iodides of all metals except lead, silver, and mercury(I) are soluble in water. HgI2 is insoluble in water. PbCl2, PbBr2, and PbI2 are soluble in hot water. The water-insoluble chlorides, bromides, and iodides are also insoluble in dilute acids. 4. The sulfates of all metals except lead, mercury (I), barium, and calcium are soluble in water. Silver sulfate is slightly soluble. The water-insoluble sulfates are also insoluble in dilute acids. 5. The carbonates, phosphates, borates, sulfites, chromates, and arsenates of all metals except sodium, potassium, and ammonium are insoluble in water but soluble in dilute acids. MgCrO4 is soluble in water; MgSO3 is slightly soluble in water. 6. The sulfides of all metals except barium, calcium, magnesium, sodium, potassium, and ammonium are insoluble in water. BaS, CaS, and MgS are sparingly soluble. 7. The hydroxides of sodium, potassium, and ammonium are very soluble in water. The hydroxides of calcium and barium are moderately soluble. The oxides and hydroxides of all other metals are insoluble.

seriesOne Installation Guide

The following is a brief description of a typical seriesOne installation. Detailed instructions are included with each product and should be followed closely.

• Remove shaft from transmission coupling.
• Disassemble and remove all of the existing shaft sealing system.
• Remove the hose clamps and old hose from the shaft log (stern tube). Discard them. DO NOT RE-USE these components as they will not function correctly with the Seal.
• Draw the shaft back up against the coupling. This will expose the portion of the shaft that was located under the old hose and stuffing box.
• Insert the end of the seriesOne unit into the articulated hose and push in as far as it will go.
• Position the seriesOne Unit while installed in the articulated hose next to the stern tube to determine approximately where the lip seal will ride on the shaft.
• Examine this area carefully. Be SURE that it is free of pitting, nicks or surface imperfections which could cause leaking. Clean this area thoroughly. Polish the shaft using 280 to 320 grit wet/dry sandpaper or emery cloth working around the shaft. Fore and aft actions could put flats or grooves in the shaft. Do NOT over polish the shaft or this will prevent the lip seal from sealing out the water. The assembly may be shifted forward slightly by adjusting the position of the hose on the stern tube at the time of installation.
• Carefully press the red seal protector into the front of the seriesOne. Make certain it covers the “lip” portion of the seal.
• Back the shaft away from the coupling to provide enough room to install the assembly.
• Carefully slide the assembly (hose-end first) onto the shaft so that the shaft passes through the red seal protector.
• Slide the assembly down the shaft and onto the stern tube. Push on as far as it will go or as needed to position the lip seal on a clean spot on the shaft. Make sure the hose is over the stern tube enough to be able to put both hose clamps on.
• Reconnect the shaft to the coupling. Make certain the coupling is firmly secured to the transmission.
• To help align the unit, grasp the assembly and slide it forwards on the stern tube an inch or so and rotate it side to side twice. Slide the assembly back into position on the stern tube. Space the two hose clamps over the stern tube end evenly and “snug”. Hose clamp screws should be on opposite sides to distribute the pressure evenly. Space the two clamps on the seriesOne end of the hose evenly and “snug”. Confirm that both the seriesOne shaft seal and stern tube are fully inserted into the articulating hose. Tighten all four hose clamps.
• Pull the red seal protector from the Seal. Separate the tabs to split the cone and remove it from the shaft.
• Connect the seriesOne to a pressurized water supply source (point in the engines raw water cooling system) by attaching the water injection hose to the stainless steel fitting on the housing. CHECK WATER SUPPLY TO SERIESONE BEFORE OPERATING VESSEL  (1 gallon per minute at idle)
• *** NEVER USE GREASE OF ANY KIND ***

Cutlass Bearing or Cutless Bearing

In short, Cutlass is a type of curved sword and Cutless is a type of rubber bearing although the two are often mixed up.

This type of bearing originated in USA and was manufactured in the 1960’s by a company called Johnson Cutless  which is still a registered trade  name.The  bearings seem to be marketed by  DURAMAX  who sell them as  “Johnson Duramax Cutless Bearings” . In the USA they are still referred to by the full title but others seem to have resorted to the shorter “Cutless Bearing”.

Some insist that the Cutless brand name was a play on words – that they “cut less” than other rubber bearings as the grooves let grit out like the flutes of a drill. Perhaps.

Why do engines need air intake silencers?

Surely, you might ask, the noise of an engine comes from the exhaust output and not from drawing air in. Well, imagine for a moment the air being drawn into the cylinders of your marine diesel engine. The column of air flowing into your cylinder moves along the pipework rapidly while the valve is open during the intake stroke, and then abruptly stops in its tracks when the valve shuts. The moving column of air suddenly stops flowing in to the engine, compresses a little and bounces back like a spring. This pressure wave travels backward at the speed of sound until it meets a hard surface in the pipework, and then it bounces back toward the cylinder. The air intake then acts like a loudspeaker pumping out vibrations. The pressure wave actually bounces back and forth two or three more times before the intake valve opens again. Just as in a recording studio, these echoes are trapped by closed cell foam in your air intake silencer.

Engine Belts

The crankshaft, usually the pulley wheel at the bottom of the engine, usually drives the belts. Belts transfer power to other pulley wheels on the engine and drive the alternator, to provide power to the batteries, and the water pump to circulate cooling water around the engine.

If the belt is too loose:

The alternator may be inefficient resulting in uncharged batteries. The water-circulating pump may be inefficient resulting in the engine running hotter. Regularly checking gauges such as the voltmeter and engine temperature will highlight both of these problems.

A slipping or loose belt is often visually indicated by black belt dust around the engine near the pulleys. There are two common types of belt; flat belts or ‘V’ shaped. Consult your owner’s manual about their accurate testing and adjustment, but a common rule of thumb to check adjustment is:

• V Belts can be deflected by about 12mm and no more.
• Flat belts should be able to twist through 90 degrees and no more.
• A belt adjusted too tight can put excess pressure on the pulley wheel shaft and increase wear on the shaft bearings of the pulley wheels.

The Volvo Penta MD2030 is what Perkins calls a 103-10. Perkins in turn imported these 100 series engines from a Japanese company called Ishikawajima Shibaura Machinery, Ltd. ISM is part of Ishikawajima Harima Industries, one of Japan’s largest industrial companies.

Perkins marketed this engine in a marinized version as the Perama M30. They sold the engine to Volvo Penta who marketed it as their MD2030. They also sold the engine to Massey Ferguson, McCormick, Terramite, Textron, Jacobsen, Cushman, Vermeer, Leech Lewis, JCB, Kobelco, and Northern Lights to name just a few. In the US, the engine was distributed thru Detroit Diesel – Allison which is closely tied to the MTU conglomerate. By 1996, Perkins had become so successful at marketing these engines to other equipment manufacturers that they formed a joint venture with ISM called Perkins Shibaura Engines, Ltd. and began assembling the engines at the Perkins facility in Peterborough, UK from parts shipped from Japan. In 1997, Perkins was acquired by Caterpillar. With an added boost from Caterpillar, this little engine has become one of the most popular engines in the world. It’s used in turf equipment, tractors, mini-excavators, brush choppers, compressors, welders, pumps, generators and many other applications. Even Caterpillar uses it in some of their smaller equipment. The “Perkins” name was highlighted on the engine ID plate which is located on a distinctive boss just forward of the injection pump. The 2006 model year’s production of the engine has “Shibaura” highlighted on the ID plate. In 2001, the larger Shibaura 400 series engine was introduced with assembling at Peterborough, UK from parts mostly from Japan, and in June, 2004 assembling of the 400 series engine began at a Caterpillar facility in Griffin, Georgia, USA with production exceeding 100,000 units per year.

How do sacrificial anodes work?

When two dissimilar metals are in contact (electrically connected), and immersed in an electrolyte (such as salt water) they produce a galvanic cell (like a battery). As current (that is, electrons) flows from one metal to the other, the metal donating the electrons changes form and corrodes. This process is called galvanic corrosion and will quickly destroys underwater metals.  The way we counteract galvanic corrosion is to add a third metal into the circuit, one that is less noble than the other two to give up its electrons. This piece of metal is called a sacrificial anode, and in marine engines it is most often made from zinc.

You will, no doubt, be familiar with the shaft anode shown above. The stainless steel of the shaft will cause the Bronze of the propeller to corrode, and so we install a zinc sacrificial anode to prevent this happening. Metals can be placed on a scale – the galvanic scale – to let us know which will sacrifice themselves in any pair. If two metals are in contact in an electrolyte (such as salt water) then the metal which is higher up the Galvanic Scale will corrode. You will notice that Zinc is second from top.

Heat exchangers in marine diesel engines are typically fabricated from a mix of copper alloy and mild steel. These are at risk of galvanic corrosion. To combat this, many heat exchangers are fitted with a zinc “pencil” anode. You will find it usually under a plug or plate within in the exchanger. The pencil is unscrewed from the plug for replacement. Raw water cooled engines have a similar zinc anode inside the cooling-water jacket to protect dissimilar metals in the engine. Determine if your engine and heat exchanger are fitted with internal anodes, and if so, check them at least annually. If they are half depleted then best replace them.

Diesel bug is contamination of diesel fuel by microbes such as bacteria and fungi. Water can get into diesel fuel as a result of condensation, rainwater penetration or adsorption from the air — modern biodiesel (a mix of diesel from fossil fuel and oil from plants) is especially susceptible to water absorption. The presence of water then encourages microbial growth which either occurs at the interface between the oil and water or on the tank walls, depending on whether the microbes need oxygen. Dead bacteria, fungus and their waste products result in a sludge which generally lies at the bottom of the fuel tank. In rough weather, however, this sludge can get mixed up into the fuel and sucked into the primary fuel filter, which can cause fuel starvation and your engine stopping in, remember, rough weather – just when you need it.

There are additive treatments which help to reduce the occurrence of diesel bug.  One example is Hydra FuelPlus biocide which “rapidly eliminates all microbial contamination, bacteria, algae and fungi present in fuel tanks without effecting the fuel quality. It stops microbes and promotes trouble free combustion, reduces smoke emissions and improves engine efficiency.” As with most biological growths, Diesel Bug is a temperature related problem and some areas of the world are more susceptible than others. Speaking to your fellow boat owners will help understand the risk in your area.

Remember, the treatment will kill the bug but it will not clear away the sludge. If you have had an infestation in your diesel tanks, then polishing your diesel and steam-cleaning your tanks is the best way to eliminate the residue.

Diesel School – What is Bio-diesel?

Bio-fuels used today are largely derived from agricultural crops. Sugar cane and corn are used to make ethanol which is mixed with petrol for road vehicles, while bio-diesel is made from vegetable oils like soy. As a renewable resource these products have a positive effect on environmental damage caused by fossil fuel use as they consume carbon dioxide from the atmosphere as they grow. Critics point out, however, that food prices have increased in less developed parts of the world due to increased competition for soy and grain without increased supply.

It is, nevertheless, a reality that the diesel oil available as a fuel today is not the same as that available when our diesels were designed, or even when they were newly installed. Currently regulations vary in different parts of the world but road diesel in the UK is 7% bio-diesel and 93% petroleum diesel.

It is this fraction of your diesel that is prone to absorbing water. Boat owners face a unique set of challenges for storing diesel fuel – at least in comparison to car owners. Typically, cars use their diesel supplies quickly and store it in relative dryness. On boats, the diesel might be months or even years old and any air that enters the tank via the breather pipes will invariably be damp. Damp air in tanks will condense on the walls at night and run into the fuel. In the past, this would sit at the bottom of the tank until the tank was eventually cleaned out. Now, the water can be absorbed into the fuel and transported to the engine. Damp fuel can lead to moisture in the cylinder causing white smoke and corrosion in the engine head.

Bore Polishing in Diesel Engines

Think for a moment about the inside of a cylinder in a diesel engine. The inside cylinder wall needs to allow the piston to move up and down at great speed, forming a vacuum and a high pressure by turns. Although the piston is a tight fit in the cylinder, it must not allow the pressurised gases (or the fuel) to leak into the body of the engine and contaminate the lubricating oil. This is achieved by piston rings, which slide up and down between the piston head and the cylinder wall. We obviously want to lubricate the cylinder wall with oil to help the piston rings do their job.

When modern diesel engines are manufactured, the cylinder bores are honed (machined) to produce a “crosshatch” appearance with fine grooves from both directions at about 22 degrees from the horizontal (see below).

The crosshatch pattern is required to retain oil to ensure proper lubrication and to form a seal between the piston rings and cylinder bores. Bore polishing is characterized by a clearly defined area of bright mirror-like finish on the cylinder bore where the crosshatch pattern is worn away (see below).

Bore polishing is caused by a build-up of carbon deposits in the piston top ring land area, i.e. the part of the piston above the top ring. Poor combustion of diesel fuel leads to these hard carbon deposits, which are highly abrasive and scrape away the honing grooves on the cylinder bores. Bore polishing leads to increased oil consumption (blue exhaust smoke) and loss of combustion pressure and performance. This is because the oil film trapped in the honing grooves that maintains the piston ring seal and combustion pressure, is no longer there. Unburned fuel and combustion gases then leak past the piston rings and contaminate the lubricating oil.

There is evidence that operating older style diesel engines with modern high-spec synthetic and semi-synthetic oils, and also running them at constant revs for long periods of time (i.e. in the hours) without varying the throttle can caused bore polishing.

15W40 means that at cold temperatures the oil has the same viscosity as SAE 15W oil. At high temperatures it has the same viscosity as SAE 40 oil. The 15W40 designation means that the oil is a multigrade oil. It has the viscosity of 15W when cold and the viscosity of SAE 40 when hot. This means that one type of oil works in all temperatures. The Society of Automotive Engineers or SAE has established an arbitrary scale for the viscosity of motor oils. The scale ranges from 0 to 60. The numbers from 0 to 25 have the letter W added. This means that they are “winter” viscosity, for use at lower temperatures. The viscosity of a liquid is its resistance to flow. High intermolecular forces between the molecules cause a high viscosity. As the liquid warms up, the added kinetic energy overcomes some of the attractive forces. The viscosity decreases. Hot molasses flows more readily than cold molasses. A single grade oil like 15W or SAE 40 oil has a high viscosity when cold and a lower viscosity when hot. The first number 15W is the viscosity of the oil at cold temperature, and the second number 40 is the viscosity at 100 °C. The 15W40 designation means that the oil is a multi-grade oil. It has the viscosity of 15W when cold and the viscosity of SAE 40 when hot. This means that one type of oil works in a range of temperatures.

Thanks for visiting SaltWaterDiesels. This page isn’t quite ready for the water yet do but check back soon to see updates – we’re launching new articles all the time.

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Engine for small sailboats

Engine for small sailboats : how to choose it.

My projects are focussed on small sailboats, up to about 30 feet, boats on which the choice of the engine often has a heavy impact on the project from several points of view: costs, reliability, weights, effectivness of the propulsion system, safety at sea.

There are currently three main options to power a small (or not so mall, hirundo 750, Petrel 28) sailboat:

inboard diesel engine , both with shaft line transmission or saildrive one

outboard engine on transom bracket

electrical engine, both transom mounted as an outboard engine or fitted in bilge as a traditional diesel inboard one

until two decades ago this triple choice was simply not existent: small 2 strokes outboards were definitely unreliable (on J24 we used to go in and out of the harbour with sails mostly because we were SURE that our engine would have quitted working in the worst possible moment, providing us a plenty of adrenalinic moments…), electrical marine engines simply didn’t exist , so the only possible choice for a small cruising sailboat was a small inboard engine.

Nowadays things are definitely changed: small 4 strokes outboard engine are really sturdy and reliable , electric engine are becoming a reality year after year, and on the other side small diesel engines are stucked on models , technology , costs and weights not so different from 20 years ago, but nonetheless they are really workhorses;

Let’s now consider some of the choice parameters.

idea 19: a 5 HP outboard engine fitted  on transom in central position, a good choice for smallest sailboats

engine has a dramatic impact on total budget on a small homebuilt boat that can be built with a budget of 20-23k euros (hirundo 750, very much less for idea 21 or 19)  , let’s see how much dramatic:

10 hp 4 strokes outboard engine: it costs around 2700 € vat included , and can be purchased basically in each marina, additional costs to fit it on board are low: a plastic gasoline tank with pipes and plugs, a sturdy transom bracket, few bolts and nuts , and you have your powerplant ready to work ; if you want to go for a smaller 5 HP (perfect for Saltaspiaggia or idea 21), engine cost is about 1600 € ; there’s a wide market of second hand engines (pay attention, there are a lot of  “archeological” outboard engines on the market…)

inboard diesel engine : thing become serious for our wallet: a fresh new 10 HP diesel engine fitted with saildrive costs around 7500-8500 €, a shaft line version, just engine and transmission, is about 5600-6500 € , in this case you have to buy also complete shaftline, bracket, propeller, and fitting all the system on board properly requires a good amount of experience (or hiring a mechanic, other costs) , and you even have to buy and fit ancillary systems: fuel tank and filters, exhaust system, and a dedicated engine battery and related wirings ; upgrading from 10 to 20 HP (good choice for bigger boats like Petrel 28) engine cost raises of about 2000 €, both for saildrive models and for shaftline one;

electrical engines: small electric outboards , perfect to power idea 21, costs around 2000€, and other 600-800 € are required  for a spare battery pack, which IMHO a mandatory upgrade; for bigger boats, an electric engine that can be compared to a 10 hp endothermic one , in the 4kW class, can be purchased for around 3700 euros, with no batteries; if you add an AGM battery pack for at least 2 hours of endurance total cost raise to 5700-6700 euros , definitely more expensive than an outboard gasoline engine and not so far from the workhorse diesel inboard engine

use related costs: that is to say , mainly, fuel: given the fact that we’re talking about small engines, fuel rates goes in the order of few liters/hour (tipically 2-3 for 10 Hp diesel inboard and 1.5 for 5Hp outboard engine), so there are not big differences between diesel inboard and gasoline outboard ; of course as long as you have an electric plug in your marina, recharging an electric engine battery pack is free

• servicing: small outboard engines can be taken away form the boat quite easily, so you can service them by yourself in your house’s garden or take them to a mechanic, same thing for electric engine, both outboard and inboard, since they’re quite light ; on the other side, removing a diesel inboard engine is a quite complicated task, and you’ll be forced to service it on board, they’re normally removed only for major overhauling or to be replaced

Reliability

sailboats are made to set on sails and turn off the engine, any sailor know the magic of this moment , but there are several situations in which you have to turn on the engine, even for a good amount of time, and sometimes you have even to push hard your engine: strong tidal streams, strong upwind seas, total zero wind situations (and sometimes even no wind and big waves which is quite annoying), emergency situations due to failures on board and so on; so we need a reliable engine in which we can trust ; I’m not considering here the normal situation in which you’ll turn on the engine for 10-20 minutes to sail away from your marina in calm-normal seas, for this standard purpose each engine is fine; in case of smallest boats, up to 23 feet, I’d anyway go for a electric engine (first choice) or a outboard 4 stroke engine, taking care that our battery pack for the electric engine gives us enough endurance, which may be a decent headache ; for bigger boats, from 23 to 30 footer, diesel inboard engine can be a good choice, and becomes the best choice if you normally need to motor for a quite long amount of time; in this last situation electric engine might be also a good choice but it’d be hugely compromised by the weight of a properly sized battery pack , or by its cost if you go for a light hi tech Litium-polymeric battery pack (they may cost a huge percentage of your entire boat) ; outboard engines often suffer from poor installation on a high placed transom bracket, that may lead the propeller to jump out of the water in short and steep waves with boat pitching, so the engine may be reliable in itself but its set up lead to a unreliable usage (a small well in the cockpit will make the outboard work veeeeery much better, see picture below)

in my humble opinion this is definitely the smartest way to fit an ouboard engine if you have a roomy cockpit : engine is on the centerline, propeller is in a very good position and it can work even with the boat heeled , and if you have to work on the engine you can do it in the cockpit, not hanging outboard in the weirdest positions

inboard diesel engine: Lombardini 20 HP fitted with saildrive on Petrel 28,  an uncompromised classical choice for a mediterranean cruiser

a very neat and clean inboard electric engine fitted on a wooden boat : will this be the future ?

we’ll keep on with this subject in the next part of this post, stay tuned !

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16 HP - 11,8 kW 3600 RPM

27 HP - 20 kW 3600 RPM

31 HP - 23,1 kW 3000 RPM

42 HP - 30,9 kW 3000 RPM

50 HP - 36,8 kW 3000 RPM

59 HP - 43,4 kW 3000 RPM

60 HP - 44 kW 2700 RPM

64 HP - 47 kW 2500 RPM

82 HP - 60,3 kW 2500 RPM

94 HP - 69 kW 2500 RPM

103 HP - 75,8 kW 2500 RPM

160 HP - 118 kW 2300 RPM

196 HP - 143,9 kW 2300 RPM

272 HP - 200 kW 2300 RPM

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Marine Diesel Engines

At VETUS, we understand the profound connection between boat owners and their vessels. Consequently, we take immense pride in having developed a complete range of certified marine diesel engines to suit the most diverse and stringent of expectations.

With almost 60 years of experience in engineering onboard systems, we embrace the importance of delivering products that embody innovation, reliability, and exceptional quality.

Our commitment to excellence is reflected not only in the products we develop but also in the support and services available to you.

VETUS: Marine Diesel Engine overview

Our marine diesel engines' design transform your boating experience into one of joy and comfort. Whether you're seeking the power to cruise swiftly or the tranquility of a quieter voyage, VETUS has a solution.

M-Line Engines - Excellence in Small Marine Diesel Engines

The M-Line marine diesel engines excel in power, torque, reliability, and fuel efficiency. Developed with pioneering and exclusive technologies, including a water-cooled aluminum top cover and a new air inlet filter housing to reduce heat and noise levels. They are ideal for launches, yachts, canal boats, and cabin cruisers up to 52 HP.

The M-line engines are available in twelve different models, with or without sail drive and power output ranging from 12 hp to 52 hp.

H-Line Engines - A perfect balance of reliability and performance

Keeping durability and reliability at the forefront, our H-Line engines are meticulously crafted. Consequently, they emerge as the preferred choice for diverse applications, ranging from cabin boats and small fishing vessels to sizable canal boats. Due to their sturdy construction, the H-Line engines exhibit remarkably low noise and vibration levels, ensuring a comfortable and enjoyable boating experience.

H-line engines, with superior power-to-weight and fuel efficiency, come in 63 HP and 80 HP versions.

F-Line - High-performance common-rail diesel engines for fast semi-planing and planing boats

For those seeking exhilarating speed and high performance, our F-Line engines are the answer. Designed for fast semi-planing and planing boats, these engines combine torque, power, and fuel efficiency in a compact and lightweight design. Due to a variable geometry turbocharger, the F-Line engines will ensure you with a thrilling and efficient boating experience.

The F-line engines are available in three different models with power output ranging from 145 HP to 190 HP.

D-Line Engines: Common-rail technology for heavy displacement boats

The D-Line engines power heavy displacement boats, offering an exceptionally smooth performance at low rpm. Featuring a unique water-cooled top cover and a range of innovative features and optional such as a second high-output alternator, our D-Line engines are ideal for long-distance cruising.

The D-line is available in four different versions, with:

Peace of mind and performance - The best output of an engine

VETUS marine diesel engines are more than just products; they represent a commitment to quality, innovation, and customer satisfaction. Whether you're seeking efficiency, durability, or high performance, our engines will do the job.

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Home | Blog | Choose the Best Small Cummins Engine for Your Yacht

Choose the best small cummins engine for your yacht.

Your yacht needs a high-quality engine to perform at its best and give you the greatest boating experience possible. This need is where Cummins boat engines come into play. They’re some of the most fuel-efficient, best small marine diesel engines around, and many yacht owners trust them to power their vessels. Before you invest in a Cummins yacht engine , you should know how to decide on the best one for your yacht.

As you look for your new Cummins diesel engine, take a moment to consider your power options, maintenance requirements, space needs and other crucial factors. These will ensure you get an engine that fits your yacht and gives you the performance you require.

Power Options

Evaluating your power needs is one of the first things you need to do before investing in a Cummins engine. Depending on your boat model, you can look up recommendations for the appropriate amount of power your yacht needs. You can also figure out which specific power range to choose from.

What is the best diesel marine engine? The answer comes down to your yacht’s requirements. Differences in horsepower, even in the appropriate power range, can dramatically affect your speed and handling, so consider a few options before you buy.

Calculating Horsepower

When you choose a Cummins engine, one of the first factors you should look for is horsepower. While some people think they should get the engine with the highest horsepower, this isn’t the best approach. Your yacht will have a maximum horsepower it can safely handle, and it should have a capacity plate stating the boat’s weight and horsepower limit. Exceeding these limits can cause your yacht to tip, so pay close attention to the recommendations for your boat.

To calculate  your needed horsepower , you can use a basic equation. Since it takes 1 unit of horsepower to transport 40 points of boat weight at 20 miles per hour (mph), you should first determine your yacht’s weight. For example, if you have a 1,000-pound boat that needs to get up to 20 mph, you’ll need to choose an engine with 25 horsepower.

As you choose your engine, consider what you’ll be using your yacht for. If you plan to go on pleasure cruises, relatively low horsepower and a small Cummins engine may be the right option. However, if you plan to tow other boats or want to travel at faster speeds, you should likely go with a larger and higher horsepower engine. In addition to the yacht’s potential applications, try to account for the weight of potential passengers, equipment, and supplies.

Along with these weight factors, evaluate what appliances you’ll hook up to your yacht. For instance, fridge compressors or alternators will draw power away from propelling the yacht and use it for their own applications.

As you calculate your engine’s horsepower, list out your added drives to see how much horsepower will go elsewhere instead of to your propulsion. A 70-amp alternator may need somewhere between 2 to 3 horsepower, while a higher output 175-amp alternator will require somewhere between 6 to 7 horsepower. Always think about these extra drives when you select your engine’s horsepower.

Maintenance Requirements

All engines will need maintenance at some point. Most engines today are electronically controlled and computerized. These models are referred to as common rail engines, and they’re made to meet the Environmental Protection Agency’s (EPA)  Tier 4 emission standards .

In the past, small cummin diesel engines for boats were much easier to maintain and troubleshoot. As engines have improved and become more complex to meet new emission requirements and perform better, there are now fewer maintenance tasks a yacht owner can perform safely. While you can still likely do cooling system repairs and change oil and fuel filters on your own, most other tasks will require specialized training and tools. Older engines had mechanical fuel injection systems, which were much easier for a regular boat owner to troubleshoot.

Today’s high-powered engines are often too complex for the average yacht owner to repair on their own. Attempting to perform maintenance tasks on these engines can even be dangerous, so it’s best for a professional to handle them. New engines will also need electronic diagnostic scanners to figure out what could be wrong with the motor.

As you search for a Cummins boat engine , check its maintenance requirements to see what you can handle yourself and what you should leave to a professional. To meet EPA requirements, you’ll likely need to purchase a newer engine, and as such, you should expect to invest in contemporary models.

Besides making sure you purchase a yacht engine that meets EPA requirements, you’ll also want to research the average costs of repairs for your potential small Cummins engine . By doing this research, you can better predict future costs and narrow your options to a motor that fits your financial needs.

Carrying Spare Components

A weather incident like a lightning strike can knock out your Cummins boat engine’s controls and electronics. When a lightning strike hits a yacht, it’s common for the engine’s onboard electronics to be destroyed or heavily damaged. In this situation, a trained technician must examine the damaged engine and replace the necessary parts.

When you’re out at sea, replacement parts can sometimes take a long time to get to you. Those who want to get back on the water quickly should carry backup components that can be used to repair their engines in the event of damage.

By having the parts available, a qualified technician can usually repair your engine faster and get you back up and running. Before you take your yacht out, you should work with a trusted supplier that will outfit you with spare small Cummins engine parts.

Space Considerations

What size motor should you put on your boat? Understand that you can’t plug any engine into your yacht and expect it to work. The space you have available for your new Cummins engine will determine the type you choose. Too small of an engine can lead to it lacking enough power for the yacht, while too large of an engine can take up too much space in the engine bay.

As you consider your diesel marine engine options, check out the following tips for choosing an engine:

Ensure accessibility:  When you select an engine, pick a size that won’t make it difficult for people to access it. The most important thing is the ability to easily reach the engine’s service items, such as belts and filters and the impeller and dipstick. An engine that doesn’t fit in your engine bay or that has to be awkwardly placed can make it difficult to access these items, causing maintenance and inspections to take much longer.

Select flexible engines: Your Cummins powered boat may have some space constraints in your engine bay that make it challenging to find a proper engine. Sometimes, some suppliers can “hand” engines, which means the supplier swaps service items from one side of the motor to the other. If you have space constraints, moving these items around the engine can help fit it in the bay and facilitate easy access.

Check for proper ventilation: An engine needs the right ventilation to work efficiently. Installing one that’s too large for the yacht can lead to inefficiencies in ventilation. For example, much of a motor’s fuel is used up in heat losses, with the fuel going through the block cooling system. As such, a yacht engine shouldn’t be larger than the cooling system can handle, and the bay should allow for proper ventilation.

Examine insulation:  Usually, engine bays feature insulation to keep the engine noise away from passengers. When you purchase an engine, you should consider how thick the insulation is and how the motor’s size will fit with it. Thick insulation can make it difficult for people to access the engine if the motor is too large. Since boat owners will often move items in the engine, such as filters and raw water strainers, it should be small enough for them to access easily.

Marinization Standards

There aren’t many reliable engine providers for boats, especially those designed to work with yachts. Some non-marine engine manufacturers have converted their motors for marine uses. However, when you choose an engine for your yacht, you should check that the engine manufacturer has marinized their motors appropriately to draw the best performance from them.

Some marinization standards to look for in small Cummins diesel engines include fuel economy, reliability, maintenance cost, quick-start ability, and operational sound. These standards will directly relate to your experience with the engine and its safety. Discounting them can lead to your motor performing inadequately and running into more frequent maintenance needs.

One major marinization standard you should check for is the motor’s ability to handle corrosion. Often, there’s a small temperature gradient between the water surrounding the boat and the engine temperature. If the small Cummins engine is properly marinized, it should handle these temperatures efficiently. A high-quality engine that meets marinization standards will have corrosion-resistant parts that slow down the need to replace them.

Companies with a long history of producing marinized engines are often a buyer’s best bet. Subsequently, a Cummins boat engine is an excellent choice. The company has been producing some of the best small marine diesel engines since 1919 , with the tradition continuing to this day.

Since they know boats have many different needs, Cummins provides everything from small marine diesel engines to larger models, with power ranging from 5.9 to 95 liters. Due to the range of power, you can find a Cummins engine that meets your yacht’s requirements. Cummins’ high marinization standards also mean you get reliable and durable parts.

Drive Train Types

Your small Cummins engine’s efficiency relies on the type of drive train you choose. Your drive train directly affects where the engine goes in your yacht, freeing up more space for other purposes and affecting the boat’s performance. Some popular drive train types that yacht owners can choose from are below:

V-drive:  V-drive inboard systems sit in a yacht’s bottom rear and feature forward-facing motors. One benefit of a V-drive inboard system is that its rear placement enables the yacht to have more space in its interior cabin. This model is generally used by cruising yachts that don’t need high performance, as there’s some power loss due to the complexity of the V-drive’s transmission.

Direct drive:  A direct drive system sits close to the center of the boat. It’s a great engine for boats that prize energy efficiency and simplicity, as it can usually be easily serviced and loses minimal horsepower. If you want more space in the center of your yacht, the central location of the direct drive may make you want to choose a different drive train.

Saildrive or outdrive: Saildrives and outdrives are popular on smaller yachts and are fairly easy to install. These transmission systems are used when the inboard yacht engine uses a horizontal output shaft. Since you can steer outdrives, boat owners often use them on catamarans. Some benefits of saildrives include their smaller, horizontal size, reduced vibrations, lessened propeller walk and greater efficiency. One recurring concern with saildrives is their tendency toward corrosion and greater maintenance needs.

Hydraulic drive: A hydraulic drive allows your small Cummins engine to be placed at a distance from the drive train or offset from it. Instead of using a traditional gearbox and engine to power the propeller shaft, this type of drive train uses a hydraulic motor. A hydraulic drive train will have a hydraulic vane pump installed on the engine’s side, which helps power the propellor shaft. These engines aren’t exceptionally efficient, which keeps some yacht owners from relying on them.

Diesel-electric drive: Diesel-electric drives are gaining popularity since they can be installed far away from the yacht engine. The diesel and electric systems can be used independently, giving boat owners more flexibility in their power options. An advantage of these drives is that they can power multiple propellers from one engine. These are a great backup for a single-engine yacht, as they can use the engine power from the yacht’s genset to propel the boat if needed.

Perkins range of marine diesel engines

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Small Four-Stroke Outboards

Today's small four-stroke engine is still heavier than its two-stroke cousin, but at least prices have come down across the board. consider this field if you need dinghy power or main propulsion..

As most of us know by now, 4-stroke engines are cleaner, quieter, and more fuel-efficient than their two-stroke cousins. A 4-stroke is like an automobile engine—it carries its lubricating oil in a tank, and burns straight fuel in the cylinders. A 2-stroke engine needs oil mixed in with its fuel, and a substantial part of that oil, burned and unburned, is exhausted into the environment. The government has rightly clamped down on such pollution, and marine outboard makers are being forced to move into the 4-stroke arena in order to meet the Environmental Protection Agency’s 2006 emissions standards. The sooner the better, the lighter the better, the more fuel-efficient the better, and the more reasonably priced, the better.

With the ramp-up engineering costs long-since absorbed by the manufacturers, the price of small 4-strokes has dropped within reasonable range of same-power 2-strokes. (The same is not yet true of the bigger four-strokes, unfortunately.)

The bad news is that weight remains a major issue with the small 4-stroke outboard. Some are so heavy they don’t qualify as “portable” engines, in our view. (A 9.9, for example, carries an additional 20 pounds and considerable bulk compared to its 2-stroke counterpart.) The extra weight and size makes the 4-stroke harder to move around, and it can’t be stored as easily—it has to be stored vertically or on one specific side to prevent oil from migrating to the wrong places and damaging the engine.

Still, weighing one thing against another, the future is here, and 4-strokes are the way to go. For sailors considering which 4-stroke might be best suited to their needs, here’s a quick guide.

The Field We rounded up data and specifications on all 4- to 9.9-horsepower 4-strokes currently on the market or slated to be released in 2003.

In total, we collected information on over 30 engines from seven manufacturers. The reality though, is that some engines built by one company are then marketed by another as their own products. This narrows the apparent choices in engines somewhat, but also gives you the option to buy the same engine from a different manufacturer for a lower cost, or from a trusted local dealer.

We looked at and gathered data on most of these small engines at boat shows or dealer showrooms and shops, and also talked with people currently using a small 4-stroke on a dinghy or tender. A few engines, however, were simply not available to us for hands-on inspections, and all data on these were supplied by the makers.

Evaluations consisted of collecting and comparing data from various sources. We did not put any of these engines on boats and run them. Instead, our goal was to build a database of small 4-strokes for the marine consumer to use when selecting an engine for a dinghy, tender or as a kicker—in other words, a “market scan.”

We chased down specifications on all engines and rated each on how easy it is to change the oil. This is an important issue, because even though you don’t have to add oil to the fuel with a 4-stroke, you do have to change the reservoir oil and filter periodically.

Portability is Key We also looked at the portability of these engines. Which ones can a person move more easily and safely from big boat to tender and back; in and out of the station wagon, on and off the kicker bracket on the transom, and so on? Are any easier to store than others, either at home or on board? We look at this from a 4-stroke vs. 2-stoke perspective. In our view, any engine over about 60 pounds is really not very portable, unless you’re an NFL lineman or professional weightlifter.

While inspecting engines at a local marine store, one of our editors had a conversation with a local skipper who has owned and operated a 5-hp Honda 4-stroke on his inflatable for over a year. He travels, and does move the engine off and on the dinghy. He finds the engine quite heavy and unwieldy, making it difficult to transfer from boat to boat. It has nearly gone in the drink on more than one occasion, and since it’s a single-cylinder engine, it doesn’t run as smoothly as he expected.

A number of other boatowners expressed similar opinions about how heavy and cumbersome these engines are to move from boat to boat or dock to boat. We based our portability ratings on these impressions as well as our own findings in hefting these engines at shows and dealerships.

A Word On Pricing No knowledgeable marine consumer would ever pay list price for an engine, but discount prices can vary regionally. They can also vary according to a dealer’s relationship to the manufacturer. For example, with the Bombardier Corporation (makers of Johnson and Evinrude engines), dealers who carry small engines in stock get a 10% discount on the cost of a small engine, versus non-stocking dealers. This means that you will probably be able to make your best deal on the purchase of a small Johnson engine by visiting a dealer that stocks that engine rather than from a dealer that has to order it for you. When we asked Bombardier about this policy, Senior Public Relations Coordinator, Ann Stawski responded, “As to stocking small engines, as always, consumers should consult their local Evinrude and Johnson authorized dealers for the best prices. The factory can only provide MSRP—what price the dealer sells each engine at is up to each individual dealership.”

Accordingly, in this article we’re forced to use MSRP to compare engines on a national basis.

Honda 5 Built by Honda, this single cylinder 4–stroke is the heaviest in its class at 60 lbs., and carries the highest list price as well. Its cowling lacks the curvaceous styling of its bigger siblings, and like all the engines in this review can only be stored vertically or on one specific side.

The Honda 5 has two “rests” built into the case to facilitate storage on that side of the engine. An oil change on this engine is simple, but the oil drain is located on the bottom of the engine block, which makes for oily hands when removing this plug. We’d like to see an oil drain located on the side of the engine like the plug for changing the lower unit oil. No oil filter is installed, so there’s no filter to change or clean.

Bottom Line: A little heavy and a little pricey compared to the competition.

Honda 8/9.9 This Honda two-cylinder carries either an 8 or 9.9 horsepower rating. When purchased as an 8, this engine ranks a little high on both weight and list price as compared to other 8-hp competitors. However, as a 9.9 this engine is the lightest in its class. In either hp rating the engine is quite stylish, with a smoothly rounded appearance. Like the 5, two rests are built into the case to allow for one-sided storage. We like the spin-on oil filter, but don’t like the fact that removal of an engine lower side panel is required for access to service this filter. Even though this engine is the lightest in its class when rated at 9.9 hp, at 87 pounds it’s still virtually impossible to handle over water, especially in a tippy boat. It rated poorly on portability.

Bottom Line: If you must have this much power on your inflatable, this engine, with its good looks and light weight (for its horsepower) should fit the bill. But it’s better as a transom auxiliary on a fair-sized sailboat.

Johnson 6 This Bombardier-built 4-stroke totes the heftiest list price in the 6-hp class. The Johnson 6 comes equipped with a front-mounted shifter and is available with a 15″ or 20″ shaft. For a 6, it is bulked up at 68 pounds—over our weight limit for portability. An oil change on this engine requires the removal of an engine lower side panel to access the filter.

Bottom Line: This engine is so heavy for its horsepower we would not give it serious consideration for purchase. But it’s a lot lighter than the Yamaha 6.

Johnson 8 This two-cylinder built by Bombardier ties with the Yamaha F8 for the lightest weight in the 8-hp class at 83 pounds. It is available in 15″ or 20″ shaft lengths and has a front-mounted shifter. We prefer front or tiller mounted shifters to side mount simply because it puts a critical engine control within easy reach. An oil change on the 8 requires the removal of a lower side panel to access the filter for servicing. This procedure is recommended by Johnson for each oil change, we would like to see the filter relocated to alleviate this added step.

Bottom Line: This is the most competitive engine in the Johnson line, tied for lightest and priced within $90 of the lowest.

Mercury 9.9, Mercury 9.9 Bigfoot, Mercury 9.9 Sail Mercury builds this 2-cylinder engine by combining its own lower unit with a Yamaha power head. It’s available in several standard configurations; we looked at the basic 9.9, the 9.9 Sail, and the 9.9 Bigfoot. The basic 9.9 with a 15″ shaft ranks as the least expensive in its class, with the only MSRP under $2,000. It displaces nearly 20 cubic inches and tips the scales at a hefty 111 lbs. in the basic configuration.

The Bigfoot is bulked up even more with electric start, a higher ratio gearbox, and a 20″ shaft, giving it an all-up weight of 128 lbs. The higher 2.42:1 gear ratio allows it to use a larger slower-turning prop than the basic engine, making it better suited for a larger, heavier boat. The Sail version has all the features of the Bigfoot, plus a 25″ shaft, making it the heaviest engine listed at 133 lbs. We just can’t view these engines as portable. It really is a two-person job to remove or install one of these bruisers. Service on the Mercury 9.9, however, is as easy as it gets: The oil filter is a spin-on that is accessible without removing anything except the engine hood.

Bottom Line: Although this engine is in our opinion not portable, we like the optional configurations available, each designed for specific uses.

Suzuki 4/6, Johnson 4 This single-cylinder 4-stroke built by Suzuki (new for 2003) can be purchased with a 4- or 6-hp rating. Bombardier markets the 4-hp version as the Johnson 4. All 15″ shaft versions weigh in at 55 lbs., which is manageable for the average boater. We like the 6-hp version best. It has a 50% increase in power without any increase in weight. Suzuki achieves this with a slightly larger carburetor, some engine timing changes, and a higher maximum rpm. It is available in 15″ or 20″ shaft lengths, has a side-mounted shifter, and resting pads on the port side for storage. The integral fuel tank has a capacity of just under a half-gallon. An optional external fuel tank can be added. No oil filter is installed, making an oil change a simple drain and fill.

Bottom Line: We prefer this engine in the Suzuki 6-hp configuration; it ranks among the best in class for power to weight.

Suzuki 9.9, Johnson 9.9 Suzuki’s twin-cylinder 4-stroke is marketed as the 9.9 and the 15 (not covered here). Bombardier also markets this engine as the Johnson 9.9. It’s one of the lighter 9.9s we reviewed at 97 lbs. It’s available in 15″ or 20″ shaft lengths and with or without an electric starter. Each oil change on this engine requires the removal and cleaning of the oil screen, and the change requires the removal of a lower side panel.

Bottom Line: The extra work of removing and replacing a side panel on every oil change is a hassle, and it’s difficult to know why one would buy the Johnson version over the Suzuki version—it’s priced over $500 higher. The only reason we can think of would be better dealer access.

Tohatsu, Nissan,Mercury 4/5/6 This Tohatsu-built single cylinder is marketed as nine different engines by three makers. All the 4s have integral fuel tanks and the bigger versions don’t. Tohatsu increases the horsepower from the base engine with changes to the carburetion and boosts in the maximum rpm. This engine, when marketed by Tohatsu, carries the lowest MSRP in each horsepower range it fills. In place of the pair of resting pads most engines carry, this engine lies on the tiller handle and a single pad. All weigh in at 55 lbs. with a 15″ shaft, and a 20″ shaft is also available. The oil drain plug is located on the bottom of the engine block and is not as accessible as we’d like. There’s no filter though—just a drain and fill oil change.

Bottom Line: If you can find a Tohatsu dealer in your area we think you’ll get the best price on this engine. The upside when buying from Mercury is its extensive dealer network.

Tohatsu 9.9, Nissan 9.9 The Tohatsu-built two-cylinder, also marketed as the Nissan 9.9, displaces a whopping 20 cubic inches and weighs in at an equally impressive 114 lbs., the heaviest 9.9 surveyed. It is available in 15″ and 20″ shaft lengths and is within $50 of the lowest cost 9.9 on the market—the Mercury. An oil change on the Tohatsu is a drain and fill, plus a readily accessible spin-on filter.

Bottom Line: This heavyweight lives by the racer’s credo: When you need to make horsepower there is no substitute for cubic inches. We like the price and online manuals. But unless you’re in monstrously buff shape, or have a friend around at moving time, this heavyweight is best for permanent mounting.

Yamaha 4 This single-cylinder by Yamaha is by far the lightest in its class at 48 pounds. It’s available in 15″ or 20″ shaft lengths and carries about one-third of a gallon of fuel in an integral tank. Its list price is at the upper end of the range in the four horsepower class—only the Johnson 4 is priced higher. An oil change is a simple drain and fill; no filter is installed.

Bottom Line: If your boat is particularly weight-sensitive , or you need to take the engine off frequently for racing or security, or if you have a small dinghy used strictly as a putt-putt utility boat and don’t care about speed, this would be an engine of choice.

Yamaha 6/8 Yamaha markets its newest twin cylinder 4-stroke as both a 6- and 8-hp engine. In the 6-hp class it’s a behemoth, weighing almost 30 pounds more than the lightest of the group. But, as an 8 it’s tied for lightest with the Johnson 8, at 83 lbs.

This is the latest entry in the 4-stroke field for Yamaha and sports several ergonomic improvements. The tiller handle is significantly longer than others and the shifter is on the tiller and is very close to the throttle. This allows an operator to stand up in a dinghy for better visibility and still reach the throttle with ease. It is also equipped with a freshwater flush adapter to rinse the engine of saltwater without using “ears” over the raw-water intakes.

Both versions are available with a 15″ or 20″ shaft and have an electric-start option. A high-thrust version of the 8, suitable for bigger, heavier boats is also available. It has a beefier transom bracket, a 20″ or 25″ shaft, and an increased gear ratio of 2.92:1. This enables it to use a larger, slower-turning propeller. The oil drain on this engine, which is readily accessible, is located on the lower unit. No serviceable filter is installed.

Bottom Line: We like the features added by Yamaha to improve the ergonomics of this engine. In the 8-hp class the weight is as good as currently available. As a 6-hp, it’s quite heavy.

Yamaha 9.9 The 9.9 from Yamaha doesn’t have all the recent improvements of its little brother. It is a straightforward engine available in two configurations, the standard and the high-thrust. The basic 9.9 is available in 15″ or 20″ shaft lengths with or without an electric starter and weighs in at 91 lbs. in its short-shaft version. One thing we don’t like is the side-mounted shifter. It’s the only engine in this class that still sports this design, which just isn’t as convenient as the front- or tiller-mounted shifts. Oil changing this engine requires the replacement of a spin-on filter, accessible by removing the engine’s hood.

Bottom Line: Like the Mercury 9.9, this engine is available in a geared-down version suitable for larger boats. It’s lighter than the Mercury but costs a bit more.

Conclusions There’s no question that we should make the move to 4-stroke technology to support the environment. Thenew engines are reliable, quiet, fuel-efficient, and simple to run. You don’t have to carry a separate oil supply or go through all that tedious measuring at the gas dock. And these days the price is right, or right enough.

At least for the foreseeable future, though, a 4-stroke engine will be considerably heavier than its 2-stroke cousin, and more difficult to stow. In trying to figure out which engine to buy, consider your power-to-weight requirements. If you replace the 60-pound 2-stroke hanging on your transom with a 4-stroke that weighs 20 pounds more, what will happen to your trim? Will you still be able to muscle it on and off? Can you afford a reduction in horsepower to offset the added weight?

For dinghy power the choice becomes more complex, involving not only the weight and trim of the boat and how often you contemplate having to move the engine on and off the transom, but your intended uses of the dinghy. It’s always a balancing act. Some sailors who anchor out like to have the biggest outboard they can fit, because a 1/2-mile ride through a headwind and chop in an underpowered dinghy can be unpleasant at best and impossible at worst. Larger engines can be raised and lowered with the help of a sling rigged to a halyard-supported boom—but try it in rough water, and the idea of being able to step aboard the dink with a nice light engine in one hand becomes more appealing.

There’s also the consideration of engine repair in far-flung places. Some long-distance cruisers tell us they’ll stick with 2-strokes until outboard mechanics everywhere know how to fix the 4s, and have parts available.

Also With This Article Click here to view “Value Guide: 4-6 HP Four-Strokes.” Click here to view “Value Guide: 8-10 HP Four-Strokes.” Click here to view “Ratings: 4-10 HP Four-Strokes.”

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Sailboat Diesel Engine Analysis

• By Scott Fratcher
• Updated: May 1, 2019

Working as a marine engineer I get called to inspect a lot of boats and have to make quick determinations of whether the engine is ready for sea or, in the case of a pending sale, to check the condition of the motor before purchase. To do this, I have devised what I call a fast Engine Technical Analysis to help find problems before they become a sea story.

In essence, an ETA is an advanced system of inspecting a boat’s drive train. I perform these tests to help catch pending mechanical issues before leaving the dock. An analysis is much more comprehensive than a typical pre-departure peek at the engine. I visually inspect the motor, of course, but also take a series of measurements that become a base line for future tests. The goal is to use this analysis to help make easily informed decisions about the boat’s drive system now and in the future.

The steps to performing an engine analysis include a visual inspection, a more detailed evaluation of fluids and equipment, a load test, and analyzing the numbers. Here’s how to perform one:

Visual Inspection

To get started, give the engine a general once-over, looking for blatant flaws and obvious irregularities. Open the engine room door (or compartment) and look, feel and smell.

Look for oil drips or other fluid under the engine . Inspect for obvious cracked hoses. Give a gentle bend where a hose attaches to the engine and look for cracks in the rubber cover or for loose hose clamps. Check the fan belts for cracks. Grab the alternator pulley and see if it will spin free on the belt. If it does, the belt is worn or loose.

Feel the inside of the engine room for any oil film. Touch the areas of the engine you can’t see, feeling for rust or salt trails.

Smell for any burned chemical type odor. Smells can be difficult to locate, but in general any burnt, oily or diesel aromas should be searched out. A good nose can tell the difference between a hot belt and a hot wire.

Look over the complete engine for oil leaks. Pay particular attention to the front seal and the area between the gearbox and engine. Leaks in the front or rear seal can mean a coming engine rebuild. Inspect the head-to-block seam, looking for oil or water trails.

Take photos from all angles as a base line for later use.

This is the point where many an engine inspection ends, but in an analysis, we want better scrutiny to help build confidence in the propulsion system.

Details, Details

The next series of checks focus on fluid inspections. With the engine off and cold, check the following:

Oil: Oil is the lifeblood of any engine. It lubricates, cools and quiets the diesel. But how do you know if the oil is doing its job? Begin by pulling the dipstick. Smell for signs of diesel odor. Diesel could mean a leaking fuel pump or an injection pump about to go bad. Pinch a small dab of oil between your index finger and thumb, and expand slowly to see how far you can spread the oil before the gap opens. Take a photo and compare this gap to new oil. Write down the difference in your notes. This is a crude method of checking viscosity and diesel intrusion.

Open the oil fill and look inside the cap for water droplets, condensation or worse, a gray gooey substance indicating water penetration into the oil. Put a small drop of oil on a paper towel and compare it to a new oil drop. Note the amount of black carbon and any shiny deposits.

A more careful inspection can include a look inside the oil filter. Next time you change the oil keep the old filter. Leave it to drain upside down, then cut open the canister and look at the inside of the element. It’s the inside of the paper filter that will contain any heavy deposits or metal shavings. Take photos for later comparison.

Later, when you start the engine, note the oil pressure. Do so again during the load test, and a third time immediately upon reducing throttle after the test. Does the gauge needle dip? It should remain steady from start up and through all the following tests. A dipping gauge can indicate a faulty oil pressure release valve or worn engine bearings.

Coolant: A typical yacht engine has two cooling-water loops: fresh water and raw water. Let’s look at the freshwater loop first. With the engine shut off and cool, open the expansion-tank cover or the heat-exchanger cover — or where you check the coolant water. The reservoir should be filled almost to the top with green or red coolant. A low coolant level can mean a small air leak in a hose.

Check the coolant for oil. It should look new and fresh. The underside of the cap should be clean. No brown, gooey oil or rust should be present under the cap. Any strange colors can mean a failing heat exchanger or failing head gasket.

Next, find the saltwater pump. Inspect for leaks, green drips and rust streaks. Pay particular attention to the area between the pump and the engine. Any leaks mean the internal seals are failing; or maybe a clogged heat exchanger or exhaust elbow is causing back-pressure on the pump.

Diesel Filter: Locate your diesel filters. Most boats have two sets, one on the engine supplied by the manufacturer and a pre-filter or set of filters, often marked with the Racor brand. On the bottom of most fuel filters, there will be a drain valve. Drain off a small amount of fuel into a container and check it for water, dirt and any long, stringy algae. If you find more than a couple teaspoons of water or much dirt, then it’s probably time to change the filters. If your filter system has a vacuum gauge, note the readings in the log for future comparison.

Once you’re done with the engine’s fluids, move on to other critical elements.

Control system: The engine control system, which includes the shift and throttle levers at the helm, directs a skipper’s intentions to the motor. Inspecting the components is critical. For example, if the engine fails to engage in reverse during docking, the results can cause boat damage or worse. Begin by feeling for play in the shifting linkage. Move the controls through all possible motions, feeling for hard spots. Does neutral have a detent or click to signal the transmission is in neutral?

Inspect where the cables attach inside the shifting housing. Look at each split pin for wear. Wiggle and physically inspect each connection looking for cracks, breaks or anything that looks amiss. Pay particular attention to where the cable cover end attaches to the shift housing. If this point slips, you can lose engine control immediately.

Inspect the cable at the engine, and look for chafe from engine vibration. An area where the plastic cable cover has chafed through can let in small amounts of water that will corrode the cable in areas where it can’t be seen, causing an unpredictable loss of engine control.

Gearbox and coupling: Inspect the gearbox-to-prop-shaft flange bolts. Attempt to tighten the Allen setscrews in the coupling. Loose bolts or setscrews indicate a vibration or misalignment. Pull the gear-oil dipstick. Check the level and perform the viscosity check again. Look for water under the cap and smell for any burned odor. Most yacht gearboxes don’t have a filter so any bits of crud or metal keep getting recirculated. It is imperative to keep an eye on the oil.

Starter load test: Place the jaw of a clamp-amp electrical meter over the positive cable leading to the starter. Clip your voltmeter to the starter’s positive and negative. Hold the stop button or manual engine shutdown while turning the engine over for 10 seconds. (The engine should not start.) Note the starter amp draw. On a typical 40- to 75-hp engine the starter draw reading should be 225 to 275 amps. Look at the starter for a rated amperage or wattage (watts/volts=predicted amps). They should match what you see on your meter. Drawing more amperage could indicate poor cables, a bad armature in the starter or failing batteries. The voltage during this test should remain above 9.5 volts at the starter. The results of this test should be placed in your notebook and the vessel’s log for future reference. This is an important test as it indicates if the starter is about to fail, and helps troubleshoot later problems.

Air inlet: In order for a diesel to run efficiently, it needs a large amount of good, clean, cool air. Here are some tests to help ensure the quality of the air feed to the engine: With the engine off, pull the air filter and inspect for cleanliness. Reach a finger down the air inlet. It should be lightly covered in dirty, but not gritty, oil. Any loose oil or splatter indicates a possible faulty intake valve, or maybe a turbo failing. Check for grit, indicating a leaking air filter. If the engine has a turbo charger, you might be able to reach inside and give the blades a spin. They should move easily with no restrictions or any bumps.

The next series of tests duplicate the engine working under load, such as when motoring hard into a head sea. If the engine is relatively small, the idea is to pull against the dock lines while safely tied in the marina. This test can also be performed at anchor, if the boat is run hard in reverse. Larger yachts with more horsepower will need to conduct these tests while underway. Note: It is imperative to inspect the dock lines, anchors and cleats for suitability for such loads. Double up your spring lines and inspect the dock cleats before beginning. Be sure to locate the manual shutdown on the injection pump, and be ready to shut the engine off if you suspect any problems.

Start up check: Start the engine; wait a couple seconds for oil pressure to build. First off, before the batteries have had time to charge, test to see if the alternator is operating at full output. With the engine in neutral, increase the throttle to 1,000 rpm. Set your clamp amp to DC and place the jaws around the positive/red cable at the back of the alternator. Check the output and note the DC reading. Change the meter to AC and again note the reading. The AC reading should be around 3 amps. If the AC reading is near half of the DC reading, this indicates a faulty alternator diode. Return at the end of your testing and verify the voltage has stabilized around 14.2 volts.

Seawater flow test: While the engine continues to warm up, increase the throttle to about 2,000 rpm in neutral. Take a bucket to the engine exhaust and time how long it takes to fill the bucket with exhaust water. This number will come in handy if you ever suspect a failing impeller, clogged raw-water loop or clogged exhaust riser.

RELATED: Quieting Your Boat’s Engine

Load tests: Wait for the engine to warm up (maybe 5-10 minutes). Put the engine in forward (reverse if this is a test at anchor). Slowly bring the engine up to half throttle. Check all the dock lines one more time. If all the lines and cleats appear strong then slowly increase the engine to full throttle.

From wide open, reduce the engine speed by 200 rpm for testing. For example, a Yanmar might have a max rpm of 3,600, but would only reach 3,400 pulling against the dock lines, and thus should be run at 3,200 rpm for the remaining tests. In other words, run the engine hard, but not overloaded. A motor in good condition, properly installed, should be able to run under this type of load for hours without overheating or causing other problems.

Before proceeding, recheck the dock lines and cleats.

Look at the engine mounts. They will be taking the thrust of the engine and should be compressed forward. Inspect them for any indication they are “rolling out,” indicating a coming failure. Even under strain, the rubber-mounted studs should still sit vertically. Look for metal-to-metal contact in the base. At the end of the load test, run the engine hard in reverse and recheck the mounts. Take photos of the mounts under load for later comparison.

Check for smoke: As the engine comes up to full temperature, you might see signs of smoke. Determine if it is escaping from the engine, or simply gassing-off an old film of oil and dirt. Pay particular attention for smoke escaping from between the engine and exhaust manifold. This may be combustion gas that could contain carbon monoxide, which is a dangerous, colorless, odorless gas that can kill when released into confined spaces.

Inspect the back of the engine. Take a good look at the shaft-to-gearbox coupling. It should be sitting almost perfectly smooth. Any pumping, fore-and-aft motion could indicate failing engine mounts. A circular motion could indicate a bent shaft, out-of-alignment engine or possibly a damaged prop.

Look at the packing gland for the amount of water dripping past the shaft into the boat. Compare this to the manufacturer’s recommendation. Be sure to note the amount of drips over a 60-second period and write this in your notebook and the ship’s log, as it’s a common point of worry for crew. Remember, when motoring at speed there will normally be less dripping at the packing gland due to the forward motion of the boat causing a low pressure or suction at aft end of the hull. (This is why some skiffs will self-drain when they get up to speed.)

Temperature tests: By now the complete engine should have stabilized in temperature. It’s time to do a few tests with an infrared thermometer. If you have not used a laser temperature gauge before, you’ll be amazed at how much information this simple tool will show you about an engine. They can be purchased from most tool shops for about $30.

Begin by scanning the ­gearbox looking for hot spots. Pay careful attention to the gearbox output bearing and the area of the case around the clutch packs. If any part of the gearbox is going to build up heat, it will be during this pull test. The complete gearbox temperature should be under 170 degrees F; 120 degrees F is typical.

Continue checking with the laser gauge along the cylinder head. It should read about 180 to 190 degrees F everywhere. There should be no hot spots. A hot spot would be more than a 10-degree change in the head from one end to the other.

Note: Testing the area ­directly around the exhaust ­manifold does not count as a hot spot, as this will be hot due to the high temperature of the exhaust. Concentrate on the area around the injectors.

Check the oil temperature by shooting the oil filter and oil pan. The oil temp should be about 7 to 15 degrees above the head temperature, but not above 220 degrees F.

Check the inlet ­temperature to the saltwater pump and the outlet of the final heat ­exchanger. The difference should be less than 15 degrees. A higher difference can indicate an engine producing too much heat (perhaps because of a failing head gasket) or too little water flow (a clogged saltwater system).

Next, check the freshwater temperature inlet and outlet of the heat exchanger. The difference should be about 15 to 20 degrees F and stable. You should be able to move the thermometer beam along the body of the heat exchanger and show the cooling effect of the heat exchanger. By this method you can see how much reserve cooling you have left in the heat exchanger.

If your engine is turbocharged, the pre-turbo exhaust temperature should be about 550 to 750 degrees F.; the after-turbo should be between 150 and 250 degrees F lower. Check the exhaust gas temp between the cylinders and exhaust manifold for each cylinder. Normal for a loaded non-turbo engine is ­between 450 and 550 degrees F. A ­cylinder temp lower than the rest shows a clogged injector or perhaps a dead cylinder. A high cylinder temp possibly shows a leaking exhaust valve or poor injector spray pattern.

Turbo pressure test: If the engine has a turbo, then before the load exam, find a test port on the intake manifold and install a pressure gauge to check the turbo boost. Compare this to the manual. Many turbochargers boost up pressure to 30 psi. Most Yanmar engines boost to around 18 psi. The pressure should be stable. Any cycling of pressure, or coughing, can indicate after burning, blow-by or a clogged air inlet.

Reduce throttle tests: Take one last look for any signs of problems and slow the ­engine down to an idle. With the ­infrared thermometer, check to see if the engine cools quickly, in less than five minutes.

Verify the alternator is ­producing 14.2 volts. Less means the voltage regulator needs to be inspected, and more may mean the batteries could fail early.

Shut the engine down and be sure to take careful notes on ­every reading you took.

For most yacht owners, an in-depth inspection of the ­vessel’s drive system can mean more confidence, safer travel and a much easier time of finding trouble spots in the ­future. Remember to keep careful records and photos of all the tests performed so down the road when something has changed, you can repeat the tests, make short work of the ­troubleshooting and get back to enjoying time on the water.

Scott Fratcher is a licensed marine engineer who has acquired over 300,000 sea miles. He was a critical member of Earthrace that currently holds the Union International Motonautique Round The World speedboat record. Scott now lives in New Zealand with his wife, Allison.

• Here is the equipment needed to perform a thorough engine analysis:
• Clamp amp meter; ­electrical multi meter
• A 0- to 45-psi pressure gauge (if your engine has a turbo)
• An infrared thermometer
• Any other special tools the engine might require
• A few rags and spray cleaner
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