After many years of R&D, this engineering masterpiece will soon grace our shores.

This year we will see a new turbocharged diesel outboard hit the water in earnest. Built by Stratford-upon-Avon-based company Caudwell Marine, the passion of British billionaire John Caudwell, this looks a truly impressive engine. Like its two main competitors, the Cox Powertrain CXO300 and the OXE300, it has a 300hp power output. By the nature of the beast, this diesel V6 weighs in at 475kg – quite a bit more than the 260 to 300kg you would expect of a 300hp petrol outboard. However, this power plant puts out an impressive 590Nm (435ft/lb) of torque at just 2200rpm, with peak power around 4000rpm – easily a 50% increase over a 300hp petrol engine. This torque output is nearly identical to the 594Nm put out by Mercury’s 270hp 3.0TDI sterndrive diesel V6, and the weight is almost the same too. In terms of petrol outboard comparisons, although they do not release torque figures, Yamaha’s V8 XTO 450 would be very similar as regards torque output, though its delivery would be further up the power spectrum.   

A lot of technological development has gone into this innovative outboard. The powerhead features a robust cast iron long block, capable of withstanding intense duty cycles, along with an advanced dual-circuit closed-loop cooling system. It is a long-stroke engine, meaning its stroke is longer than its bore, in this case by 9mm – quite a generous difference that certainly helps in producing torque. It is, of course, turbocharged, and in this case it is assisted by an air cooler. Caudwell Marine invested a lot of time in rigorous functionality and validation testing before its launch to the media – notably over 19,000 hours of sea trials, as well as 4,000 hours of dedicated lab-based testing – so this engine is past that prototype phase. The Design Validation Programme included structural testing, log-strike tests, rough-weather sea trials, hot/cold environment testing, impact assessments and corrosion resistance analysis.

Caudwell V6 - This engine has had 19,000 hours of all-weather sea trials.

This engine has had 19,000 hours of all-weather sea trials.

In terms of its innovative transmission system, there is the ZF dimension. With a long legacy of transmission engineering in the marine industry, ZF’s technical partnership with Caudwell brings extensive engineering prowess to the project. As part of this agreement, ZF have developed a highly sophisticated twin-clutch transmission system that seamlessly integrates with Caudwell’s V6 diesel powerhead and the Axis Drive steering system. ZF will also be actively involved in the assembly of the Caudwell-designed lower unit.

Fuel figures

In terms of economy, the test team recorded fuel figures on a commercial 9m 9000 Libra RIB. From this, they concluded that this engine will burn about 58L/h (13gph) at wide open throttle, so cruising speeds of around 35 knots will return about 4.5nmpg. In comparison, a 300hp petrol outboard on a similar boat at this speed would fall quite a bit short at around 2.5nmpg. Caudwell Marine state that ahead of its official launch, they will publish final performance data, ‘demonstrating real-world savings to be expected across a range of different vessels and conditions’. It may be a heavy engine, relatively speaking, in outboard terms, but its torque output punches well above its weight. Clearly suited to the commercial market at which it is aimed, it will also be an ideal alternative to diesel sterndrives for high-end pleasure craft and superyacht tenders.

caudwell marine V6

Only the lower section of the leg steers.

Engineering and design features

Axis Drive steering system

Caudwell feature a patented integrated steering system known as ‘Axis Drive’. This electro-hydraulic steering system articulates steering of the lower part of the outboard leg only, with the powerhead and upper leg section remaining stationary – which is similar in concept to the Mercury V12 Verado. The Axis Drive steering system operates at a 120-degree angle to the horizontal, as opposed to the vertical rotation of a conventional outboard. This has its advantages, as the bevel gear connecting the main shaft to the drive shaft does not have to transmit at a 90-degree angle, instead driving through a more gentle 120-degree connection. Right-angle bevel gears have their limitations in diesel engines, as transmitting large amounts of low-end torque through 90-degree gear sets has been the death of quite a few sterndrives. It is stated that ‘the propellers are deployed to effectively deliver power throughout the turn, providing full control at both high and low speeds and in all conditions’. From this you can suppose that the trim angle is altered slightly in the turns to combat the ‘trimming-in effect’ caused by the steering rotating through a 120-degree angle.

Dual propeller

The Axis Drive steering system is further enhanced by a contra-rotating dual-propeller set-up, providing added grip and balanced torque, while at the same time increasing fuel efficiency. For twin-engine applications, single props would be more efficient at high speed, but this high-output engine will have a single-installation appeal.

Twin-clutch gearbox

Featuring a sophisticated twin-clutch system, this gearbox provides smooth gear engagement between forward and reverse. The proportional hydraulic technology within the gearbox allows independent control of two clutches, leading to seamless directional transitions at high and low speed, and features full crash-stop functionality.


The engine’s cowling features a service hatch for ease of access for daily checks and maintenance. The ‘user-focused’ cowling design aims to ensure easy maintenance of key components, such as the water pump, along with the ability to drain and replace both engine oil and lower-unit oil from the back of the vessel.

caudwell marine V6

The water pump, water strainer and fuel filter sit at the top under the cowling hatch.


The Vessel Control System is ergonomically focused. Supplied with electronic stop/start, anti-theft functionality, MFD outputs and multi-helm station options, the VCS is equipped comprehensively and designed for a straightforward user experience. A joystick control package with dynamic positioning is available as an additional option, enabling you to hold position at the press of a button, and dock with relative ease.

Cost and availability

We have not been given a specific UK cost, but we are told it will be compatible with the Cox Powertrain and the OXE300, and the engine will be available later this year.

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