Diesel or Petrol Marine Engines?

Diesel engines are more common in Europe, while petrol (or ‘gas’) are more common in the US.

So which is best?

Diesel engines are generally regarded as more robust in a marine environment. They have a longer life, as the nature of diesel is that it provides an extra degree of lubrication to the engine. This is often regarded as the main advantage. They are also somewhat safer than petrol, simply because diesel is less combustible than petrol, and less liable to explode. Although petrol explosions are very rare indeed, the engine bay of petrol engines should always have a blower which is necessary to blow away petrol fumes. This blower must always be switched on before a petrol engine is started. Typically you should switch the blower on a minute before firing up your engines (but always check your manual for what is recommended by the manufacturer).

Diesels are also typically more economical to run, at least at UK and European petrol prices. However, this running cost advantage may be over-turned by possibly higher costs for diesels parts, maintenance and repairs.

Most marinas have diesels pumps; however, not all have petrol. If you are thinking of getting a petrol driven boat, check that your chosen marina can supply petrol. Our advice is to never cart petrol to your boat in petrol cans – it is simply too dangerous.

Diesel, however does have some disadvantages. They put out less power than an equivalent size petrol engine, and tend to be somewhat smokier. Older diesels are worse in this respect. Also, a diesel engine is more expensive at initial purchase than an equivalent sized petrol. However, the price differential in older boats may be quite small.

There are some other factors to take into account.

Exhaust gases from petrol engines contain carbon monoxide, which is deadly. Under normal running, with the boat moving along, any fumes may be blown away. However, if the boat is not moving, and a petrol engine is running, then there is a risk that carbon monoxide could be blown back into an enclosed space. Therefore, never run a petrol engine for any period while a boat is at a standstill. Also, invest in a few carbon monoxide detectors – one for each enclosed area on the boat. They are cheap, and you are worth the investment!

If you want a generator on board (e.g. to provide mains voltage electricity while away from shore power), our opinion is that you should never get a petrol driven one owing to the risk.

There is a more ‘technical’ factor to take into account, related to horsepower and torque. Torque is the twisting power of an engine, and of course a prop needs torque. This is especially true for larger, heavier boats. A diesel engine puts out more torque than a petrol one. Therefore, the heavier the boat, the stronger the argument for diesel compared to petrol.

So – let’s answer the question we asked at the beginning.

As a very general rule, diesel is on balance better. But don’t let that put you off buying a petrol engined boat that is suitable in other respects. For the smaller sports cruisers, often of US origin, petrol is more common. For the larger yachts, diesel is the only practical choice.

You should always be aware that specific engines may have specific characteristics or even flaws. For example, with one engine, if the gasket on the riser fails, this could destroy your engine by allowing water to seep into the cylinders. On another engine, there are known occasional issues when the installed turbocharger takes over from the supercharger (or, to be more precise, fails to take over correctly, resulting in flat spot in the torque and horsepower curve). For more specific information, feel free to contact us.

[A necessary legal note: All the above advice is given in good faith, but the entire responsibility for the safety of you, your crew, passengers, third parties and the boat is yours, not that of Go Earth Ltd or its directors or employees.]

If you want more information and specific advice, then please contact us on contact us now, and we can help you with your boat search now.