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

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.

Beta Marine, marine diesel engine

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.

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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.’

More power

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.

Volvo diesel marine engines

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 Motors

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

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.

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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.

Dual fuel

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

Best Marine diesel engines: Volvo Penta D2-50: Easy to get NMEA data from the engine

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
  • RPM: 3,000
  • Fuel consumption at max rpm: 11.5lt/hr
  • Dry weight: 249kg
  • D2-50: £9,360
  • Dealers:;

Yanmar 3JH40

Best Marine diesel engines: Yanmar 3JH40: An award-winning 40ho engine

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
  • RPM: 3,000
  • Fuel consumption at max RPM: 8.8lt/hr
  • Dry weight: 192kg
  • £9,704

Beta 43

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

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
  • Cylinders: 4
  • Displacement: 1,999cc
  • RPM: 2,800
  • Fuel consumption at max RPM: 9.3lt/hr
  • Dry weight: 238kg
  • £8,916

Nanni N4.43

Best Marine diesel engines: Nanni N4.43: Low rpm makes for a quieter unit

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)
  • Cylinders: 4
  • Displacement: 2,197cc
  • RPM: 2,800
  • Fuel consumption at max RPM: 10.3lt/hr
  • Dry weight: 228kg
  • £8,916

Solé Mini-44

Best Marine diesel engines: Solé Mini-44: Decent torque at mid range

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, freelance marine journalist and former magazine editor

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)
  • Cylinders: 4
  • Displacement: 1,758cc
  • RPM: 3,000
  • Fuel consumption at max RPM: 10.4lt/hr
  • Dry weight: 175kg (w/o gearbox)
  • £6,652

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