Part 1 - The best diesel engine

All the engines listed in this guide were designed for displacement boats, and powering these hulls requires considerable forethought to gain the right blend of power and fuel efficiency.

While there's no point overpowering a displacement hull, as it cannot exceed its hull speed, the engine should be able to reach this speed without overloading, so that when throttled back to cruising revs a substantial amount of engine load is removed and fuel efficiency increases.

For example, a 100hp diesel that has a "C" or medium-duty output rating (up to 3000 hours of usage per year) and achieves 2800 revs at Wide Open Throttle (WOT) may use 19-20lt/h, but throttled back to 2600 revs the consumption will drop to around 12lt/h. Based on a standard prop power curve, at 2600 revs the prop is absorbing less than 70 per cent of the engine's maximum output, allowing the engine to run under relatively light load.

Smaller "A" or pleasure-duty rated diesels must be operated continuously at far lower revs than "C" rated engines, particularly turbocharged units that develop substantially more power than their naturally aspirated counterparts. For example, although Volvo Penta's turbocharged D2-75 revs to 3000, the maximum continuous cruising revs should be 2600, where based on a standard prop power curve the fuel consumption drops from 17.7lt/h at WOT to 12.3lt/h.

However, as none of the engines listed in this guide have electronically controlled fuel injection, when used for extended periods they should be operated at or slightly above the rev range at which maximum torque is produced.

All diesels with mechanical fuel injection suffer from advanced fuel-injection timing below the maximum torque range and retarded timing above it, and should not be operated for extended periods at low revs, as cylinder glazing through fuel over-supply may result. Glazing can occur even faster in turbocharged diesels, which normally have lower compression ratios than their naturally aspirated counterparts - particularly in direct-injection diesels where the fuel/air mix is not mixed as thoroughly as in indirect-injection or pre-combustion diesels.

To meet upcoming EU and US exhaust emission standards, most mechanically controlled diesels now have indirect injection, which can handle periods of low-rev operation better than direct-injection units. And to ensure that emission levels are further reduced, all small diesels (apart from some marine-specific Bukh and Yanmar engines) have heat-exchanger or closed-circuit cooling systems to maintain relatively constant cooling-water temperatures across a wide range of operating conditions.

Also, heat exchanger engines can easily be fitted with hot-water takeoffs for showers and galley sinks.

DETERMINING THE POWER NEEDED
Over the past couple of years I have discussed at length the steps required in repowering displacement hulls and how to choose the right type of prop, so there's no need to cover these steps in detail again.

Before repowering your boat, I recommend talking to a qualified naval architect who can specify almost exactly the amount of power you require. This is achieved by combining information such as the hull's waterline length, waterline beam, draft amidships, the volume of water displaced (marginally different for fresh and saltwater) and block coefficient to calculate the hull displacement.

The block coefficient is a combination of the three main hull sections: the entry or forward third of the hull, the midships third and the aft third. Block coefficients vary according to the fullness of the forward and aft thirds and whether the hull has a transom or canoe stern, whereas the midships third remains relatively constant for every hull. Displacement-hulled cruisers have a block coefficient of about 0.4.

An example is a 12.2m cruiser with a waterline length of 11m, a beam of 3.35m and a draft amidships of 0.9m excluding keel and so on. The block coefficient is 0.4, and by combining these figures and dividing by the volume of water, the displacement works out to 13.4 tonnes.

The next step is to calculate the speed-length ratio, which is generally the square root of the waterline length in feet times a figure of 1.3 to 1.8 depending on the run aft and transom broadness. The full displacement cruiser model I've chosen has a hull speed of 8.0kt.

When a displacement hull moves through the water, a series of waves is formed. The action of the hull entry in pushing water aside creates a bow wave that runs along the waterline of the vessel as it passes through the water. The distance between one wave crest and the next at the waterline is proportional to the vessel's speed.

Hull speed is achieved when a second series of waves emanates from the stern, and if the speed is such that the bow wave is peaking (highest) while the stern wave is dragging (lowest), the waves will cancel out one another and eliminate stern drag.

The third step to finding out the power needed is by determining the Admiralty coefficient, which is a hull-resistance factor approximated from known values for vessels of about the same waterline length. By using a standard formula the coefficient for the 12.2m cruiser is 77.5.

The final step is determining the shaft horsepower (SHP) using another formula that takes into account the vessel's displacement, hull speed, Admiralty coefficient and number of engines needed to achieve hull speed.

The minimum amount needed for the 12.2m cruiser is 37.7 SHP from one engine, but as a reasonable headsea can absorb twice the output, I would fit at least 75 SHP continuous. Bear in mind that the SHP figure applies to commercially rated diesels that develop maximum continuous power at around 2000rpm and produce much greater torque than recreational-application engines running at around 3000rpm.

Had the cruiser been built in the 1960s to early '80s it may have been powered by a diesel such as the naturally aspirated direct-injection 3.9lt Perkins 4.236, which developed 76hp continuous (a crankshaft rating and 5–10 per cent lower than SHP) at 2250 revs. It produced 252Nm at 1600 revs and with a 2:1 reduction hydraulically actuated gearbox weighed 519kg.

But nowadays to get a comparable torque output (which is more relevant to pushing displacement hulls than horsepower) a higher-revving diesel, geared down at the prop, may be needed. If you have a choice between turbo and turbo-aftercooled, always pick the latter as fuel efficiency is better and exhaust gas temperatures are lower, creating fewer problems if you choose keel cooling and a dry exhaust.

An example of a modern counterpart is Volvo Penta's turbo-aftercooled direct-injection TAMD31S, which develops 99.2hp at 3000 revs at the crankshaft from its 2.4lt powerhead. And with its flat torque curve it's more than a match for the Perkins, producing 252Nm at 1400 revs out to a maximum of 285Nm at 1800 revs - yet with a hydraulic HS25A gearbox it weighs only 370kg. At 2250 revs the 2.71:1 gear ratio ensures lower prop revs than the Perkins.

THE ENGINES
To avoid reader confusion in this guide, all engine outputs are in horsepower, where 1.0hp = 746W. US manufacturers still use this measurement, whereas European and Japanese companies prefer 1.0hp = 735.5W (PS), which gives the impression that horsepower outputs are higher than they really are.

And although kilowatt ratings could be mentioned, most boaties still think in terms of horsepower, hence the standardisation on the traditional conversion.

Apart from Bukh, all the engines listed below have "A" ratings, where normally a maximum operating time of 300 hours is allowed with less than 10 per cent of this at Wide Open Throttle. In the versions listed they are strictly for pleasureboat usage, and should higher yearly operating hours be envisaged owners should contact the respective engine distributors for alternative output ratings.

BETA
Beta Marine is the only UK-based diesel engine manufacturer in this guide and marinises Kubota tractor and industrial engines to its own standards. All models are naturally aspirated and have indirect injection and heat-exchanger cooling (keel cooling is optional across the range) to comply with current and upcoming EU and US exhaust emission regulations.

Cylinder blocks and heads are cast iron and the camshafts and raw or saltwater cooling pumps are gear driven for reliability. Brass sump drain pumps are standard and the starter motor and alternator are mounted well above the engine beds. All models will handle up to 15° of static fore and aft inclination and 25° of heel when underway.

The twin-cylinder BZ482 develops 13.3hp at 3600 revs and weighs only 89kg while its three-cylinder BD722 counterpart develops 19.7hp at the same revs and weighs 106kg. The most powerful three-cylinder engine in the range - the BD1005 - develops 27.6hp at 3600 revs and weighs 150kg.

All models from 31.5 to 49.3hp have four cylinders and subsequently the smaller engines are very smooth-running for their outputs. The 31.5 and 34.5hp models rev to 3600, the 37.0 revs to 3000 and the 42.4 and 49.3hp models reach 2800.

The BV1205 develops 31.5hp and weighs 170kg, the BV1305 is 34.5hp and BV1505 37.0hp and both weigh 170kg. The heavier-duty BV1903 and BV2203 develop 42.4 and 49.3hp respectively and both weigh 267kg.

The most powerful and only five-cylinder engine in the range, the BF2803, develops 61.1hp and weighs 350kg. A hydraulically operated Newage PRM gearbox is standard, whereas all the other engines have mechanical Hurth boxes.

For more details, contact Beta Diesel Australia, tel (02) 9525 1878.

BUKH
Bukhs are the only diesels in this guide that have maximum continuous rating outputs instead of "A" or intermittent, as do the competition, and are equipped with twin counter-rotating balance weights in all models.

These factors place the engines at a disadvantage because they appear very heavy for their output, but then they are very smooth-running and their intermittent ratings are much higher.

For example, the least powerful of the range, the DV24 ME, develops 23.6hp at 3600 revs from 964cc but weighs a hefty 210kg, whereas the 1001cc Beta Marine BD1005 develops 29hp intermittent at the same revs but only 23hp at the maximum continuous rating. However, the Beta engine weighs 150kg.

But weight is only part of the story and Bukh says its ME series is built to the same standards as its SOLAS-rated lifeboat engines. This means they're over-engineered for their output and have features not found in the direct competition. Yet Bukh claims the engines comply with EU and US emission laws, despite them having direct injection and raw or saltwater cooling (heat-exchanger systems are optional).

A couple of decades ago, most small diesels (Lister, Volvo Penta and so on) had direct injection, which meant lower compression ratios and much easier hand-starting. The Bukh DV24 ME and DV32 ME have raised hand-starting as standard in addition to electric starting - a feature I consider very important on engines fitted in cruising yachts.

Other long-forgotten features include forward-mounted flywheels, enabling the engines to be mounted deeper in the bilges of yachts where the keel is part of the hull and not simply bolted on to it - Hulls such as the H36 and older Holman & Pye transom-sterned cruising yachts. And if you're repowering an old timber launch, the forward flywheel concentrates engine weight further forward to compensate for passenger load in the cockpit.

Another feature is the high mounting of the starter motor and alternator, which is made possible by the forward flywheel location. The starter motor is located halfway between the engine bed and rocker cover, while the alternator is mounted on a bracket that's level with the cylinder head.

Bukh has two twin-cylinder and two three-cylinder models, the increased power of the up-rated version of each being achieved by fitting a small turbocharger. The range comprises the naturally aspirated DV24 ME, turbocharged DV32 ME developing 31.5hp at 3600 revs and weighing 218kg, the naturally aspirated DV36 ME at 35.5hp and the same revs weighing 265kg, while the turbocharged DV48 ME develops 47.3hp at 3600 revs and weighs 273kg. Heat-exchanger cooling adds 16kg to the DV24/32 ME and 20kg to the DV36/48 ME.

IVECO
IVECO is one of the largest diesel engine manufacturers in the world and in January underwent a name change from IVECO AIFO to IVECO MOTORS.

The Italian-based company has been making engines since 1975 utilising the experience of Fiat, which has been an engineering company for a century. It has built 15 million engines of which apparently four million are still in service, and the company produces more than 460,000 diesels annually.

Currently, IVECO's recreational marine-diesel range covers engines from 16.1 to 1182hp.

In the recreational engine range below 100hp there are six diesels, all built in association with Lombardini Marine - the marine division of a long-running specialist manufacturer of industrial diesels. All models have indirect injection and heat-exchanger cooling, but unusually, the three smaller models have belt-driven overhead camshafts while the larger units have gear-driven camshafts and overhead valves actuated by rockers.

The smaller model range comprises the 4021 M20, which develops 19.7hp at 3600 revs, the 4031 M30 developing 29.6hp at the same revs from its three-cylinder 1028 powerhead and the 4041 M39, 38.5hp at 3600 revs from a four-cylinder powerhead. Complete with mechanical Technodrive gearboxes, the dry weights are 98, 111 and 137kg respectively.

The larger models are the naturally aspirated four-cylinder 4241 M41 and 4141 M48, developing 40.2 and 47.7hp at 3000 revs and each weighing 230kg. A turbocharged-aftercooled version of the 4141 M48 is available and develops 80.4hp at 3000 revs and weighs 258kg. All weights include down-angle gearboxes.

IVECO also offers these engines in saildrive versions, the three smaller models having the "Mini" S-Drive with 2.18:1 gear ratio and weighing 125, 141 and 160kg respectively. The larger models use the "Maxi" S-Drive with 1.92:1 ratio and 253, 253 and 274kg. Both saildrives have bronze clutches with double-cone engagement.

For more details on these engines, contact Stewart Butler at Lees Industries, tel (07) 3390 5522 or email mailto:leesinda-sales@powerup.com.au

NANNIDIESEL
Marinised in France, all Nannidiesel models up to 130hp are based on Kubota tractor and industrial diesels and have indirect injection and heat-exchanger cooling to comply with current and upcoming EU and US exhaust emission requirements.

According to owners interviewed, the engines are very reliable, and with their Kubota bases most parts are easily sourced throughout Australia and Asia and are reasonably priced. The standard turbocharging on the "85" model engine gives considerable power and torque output increases without significantly increasing weight, while the five-cylinder models are much smoother-running than four. The five-cylinder "62" model base engine is also used by Beta Marine.

Saildrive versions of the engines are available from 9.9 to 61.1hp and the drive unit adds about 25kg to the weight of the shaftdrive engines.

For more details, contact Peter Collins at Nanni Diesel Australia, tel (02) 9319 5222 or email mailto:sales@collinsmarine.com.au

VOLVO PENTA
Volvo Penta has been a world leader in marine engine technology for several decades, and the company has been making its presence felt in Australia since the 1960s when it introduced its innovative Aquamatic range of sterndrive engines.

Its compact yacht diesels were released in the 1970s, while its Duoprop sterndrives appeared in the '80s followed by the throttle-responsive KAD supercharged and turbocharged diesels in the '90s. Last year the electronically managed D3, D4 and D6 diesels were released.

Volvo Penta is a wholly owned subsidiary of the Volvo Group, which along with its truck manufacturing Renault and Mack acquisitions produces up to 175,000 diesels per year. The recreational shaftdrive diesel range covers 10.7 to 759hp and Aquamatic diesels 129 to 305.6hp.

In the sub-100hp range, Volvo Penta offers seven different models, six available with saildrives. Naturally aspirated models include the MD2010, MD2020, MD2030, MD2040 and D2-55, all of which have indirect injection and heat-exchanger cooling. The MD series is based on Japanese industrial engines and all develop maximum power at 3600 revs, while the homegrown D2-55 and D2-75 reach 3000 revs. Outputs and cylinder numbers are 10.7hp and two for the 2010, 18.8 and three for the 2020, 28.2 and three for the 2030 and 38.9 and three for the 2040. Volvo Penta doesn't supply weights for the smaller models.

The D2-55 develops 55.0hp and the turbo-aftercooled D2-75 73.7hp, both from four-cylinder powerheads. Including gearboxes, the weights are 243 and 258kg respectively. Top of the sub-100hp lineup is the 99.2hp TAMD31S mentioned in the "Determining the Power Needed" section of this guide.

For more details, contact Graeme Avers at Eastern Engine, tel (07) 3902 5444 or email mailto:graeme.avers@volvopenta.com.au

YANMAR
Australia's best-selling brand of marine diesel engine is in the process of several model upgrades, and models such as the new ones offer more power for less weight and fewer exhaust emissions.

Through its association with Scania, Yanmar Marine has moved into higher-horsepower engines, which have electronic fuel injection to compete with the offerings from Cat, Cummins, IVECO and Volvo. But currently all engines under 500hp still have mechanical fuel injection, which Yanmar prefers for reliability. All the weights listed below include gearboxes.

In the sub-100hp range there are currently seven models, those up to 75hp available in either shaftdrive or saildrive form, which adds about 25kg to the weights listed here. All engines have pushrod-operated overhead valves with gear-driven camshafts for reliability.

Smallest of the range is the popular single-cylinder 1GM10, developing 8.9hp at 3600 revs, and at 76kg with raw or saltwater cooling it's the lightest marine diesel on the Aussie market.

Next up is the twin-cylinder 2GM20, which develops 17.7hp at the same revs and weighs 106kg with raw water or 114kg with heat-exchanger cooling. This is followed by the three-cylinder 3GM30, developing 26.9hp at 3600 revs and weighing 129kg with raw water or 137kg with heat-exchanger cooling, although this model is in the process of being replaced by the 3YM30.

Having heat-exchanger cooling as standard, this three-cylinder engine develops 28.6kg at the same revs but weighs only 133kg or 157kg in saildrive form. Like the 1GM10, 2GM20 and 3GM30, the 3YM30 has indirect injection to comply with upcoming US and EU exhaust-emission regulations.

There are four more powerful engines in the sub-100hp range, all direct injection and heat-exchanger cooled, and like the GM series utilising common parts. Smallest is the three-cylinder naturally aspirated 3JH3, 39.4hp at 3600 revs and weighing 186kg. Next is its naturally aspirated four-cylinder counterpart, the 4JH3, developing 55.2hp and weighing 223kg. The turbocharged version of this engine is the 4JH3-TE, which develops 73.9hp at the same revs, while the turbo-aftercooled 4JH3HTE develops 98.6hp at 3800 revs.

The weights are 249 and 258kg respectively, the latter being very light for its output and ideal for powering maxi racing yachts where every extra kilo counts.

Upcoming Yanmars under 100hp include a new 40hp engine. For more details, contact Michael Blair at Power Equipment, tel (03) 9764 0711 or email power.equipment@yanmar.com.au.

CHOOSING THE RIGHT BRAND
My advice to potential owners is to contact members of different yacht clubs or marinas and find out what operating and/or servicing problems they've encountered with their engines - bearing in mind that many owners won't admit they've invested in a troublesome package.

Compare the engine warranties, the availability and price throughout Australia and the Pacific of parts such as cooling waterpump impellers, fuel pumps and injectors and so on to make an informed decision on your OEM or repowering investment.