2024 diesel outboard engine buyer's guide

Using diesel fuel tends to be a big-boat thing, using inboard engines to turn a sterndrive or a screw. These bigger boats tend to be better suited to diesel technology than those that want to hang an oil-burning engine off the transom.

There are downsides to diesel engines, particularly for smaller boats. The main one is that they tend to be a lot heavier, and more top-heavy than their petrol-powered equivalent, so you’re going to need to need a wide transom to support them.

They also tend to struggle a bit at wide-open throttle, but that’s more of a problem relating to the fact that the engines need to run a prop optimised for a higher-revving petrol engine rather than one made to give its best performance at slower diesel engine speeds.

The worst bit, though, is their cost, with a diesel outboard engine generally priced at around twice the price of their petrol-fuelled equivalent.

But the diesel engines are very good at saving fuel. It’s why they tend to be more viable in high-running hour commercial applications than for recreational boats that tend to hit the water every now and again.

They’re also good at recharging batteries, producing around 25 percent more charging amps compared with conventional petrol-powered equivalents.

If you want to go down the diesel outboard engine path, there are not too many choices on the table.

British diesel outboard engine specialist Cox has hit the market with a single engine, the 300hp Cox CXO300.

The CXO300 was designed from the ground up as an outboard engine. It features a twin-turbocharged vertical crankshaft 4.4-litre V8 with a low 3700rpm to 4000rpm propping speed that produces a meaty 650Nm between 2250rpm to 3000rpm – it could produce more torque at lower revs, but risks cavitating the propeller.

This is an engine that was designed to be lightweight, featuring a unique 60-degree bank alloy block, quad cams and two turbochargers.

The Cox CXO300 isn’t a lightweight engine, tipping the scales at 393kg in its lightest form. On that, it’s not a particularly skinny engine either, measuring 693mm wide and standing 1997mm tall.

The engine is available in three shaft lengths: 25-inch (635mm), 30-inch (762mm) and 35-inch (889mm).

The benefit of the Cox engine is that it has a strong focus on the recreational boat market. That means you get a five-year warranty on the engine, much better than the 18 months or 1500 hours offered to commercial users.

Swedish diesel outboard engine specialist OXE is leading the oil-burning charge with four outboard engines available.

Three of the engines, rated to 150hp, 175hp and 200hp, are based on a GM-sourced single turbocharged 2.0-litre in-line four-cylinder engine with the crankshaft lying horizontally instead of vertically – most outboard engines lie the cylinders on their side.

A range-topping 300hp version uses a BMW-sourced twin-turbocharged 3.0-litre in-line six-cylinder engine, again with the crankshaft laying flat on top.

A unique feature is OXE’s system of using a hydraulic gearbox mounted below the engine and an oil bath-cooled belt to turn the lower leg instead of a driveshaft.

This makes it easy to turn an engine into a counterrotating unit – all you need to do is swap over a couple of hydraulic hoses and tweak the software to suit.

OXE is more open about its engines’ performance, with plenty of low-down torque. The smaller engines can be optimised for high speed (producing just 380Nm at 2800rpm) or high torque (producing 607Nm at 2800rpm) by changing the gear ratio from 1.73:1 to 2.17:1. 

The engines all weigh significantly more than their petrol-fuelled equivalents – around 100kg heavier for the 150hp version – so factor that into any plans to repower. 

The range-topper is a monster, producing 945Nm of propeller-twisting grunt from its mid-range. 

OXE is more limited on its shaft lengths, with just 25-inch (635mm) and 33-inch (838mm) legs on offer.

OXE has big plans for the future, teasing a 450hp diesel-electric hybrid outboard engine that it plans to launch in the next few years. 

The engine will feature a full silent mode allowing it to silently sneak out of and into the marina under battery power alone, with the electric motor assisting the diesel engine when more performance is needed.

The electric motor is attached to the OXE engine’s drive belt, making it easy to fit and operate.

Mercury does build a diesel outboard engine, but buying one looks to be a pretty difficult affair.

The 175hp-rated Mercury 3.0-litre diesel outboard engine features a unique spark ignition system so that it can run at a very low compression ratio and turn at higher revs. 

Normally, a diesel engine will use very high compression to heat the air in the cylinder, which in turn gets hot enough to ignite the diesel fuel. Using this method of igniting the fuel means the engine turns at much lower revs at wide-open throttle than compared with a petrol-powered equivalent.

However, using a spark plug combined with a glow plug greatly improves the diesel engine’s cold-weather starting, as well as allowing the engine to have an operating range of 5000rpm to 6000rpm.

Little information is available on the Mercury 3.0L diesel outboard engine, but it is based around a significantly undersquare 3044cc V6. Diesel engines are all about the torque they produce, not the power, and unfortunately, Mercury doesn’t disclose this number.

The engine is largely based on a petrol engine, meaning parts can easily be swapped, although the engine's cylinder head is specific to the diesel application.

You can only get this engine with a 25-inch (635mm) shaft length.