Boats: what’s better? Petrol or diesel engine? The ultimate guide to making an informed choice

If the choice is essentially fixed in the case of outboards—since they are practically only available in petrol—some careful consideration is needed when it comes to sterndrive units.

The starting point for a proper evaluation is estimating the number of engine hours you expect to log each year.

This figure, multiplied by the difference in fuel consumption (which tends to favor diesel), and compared with the difference in purchase cost between petrol engines and the significantly more expensive diesel units, allows you to assess which option is more cost-effective.

From our experience (as well as data provided by some engine manufacturers), average annual usage by owners is around 40 hours. While this may seem low, it’s important to remember that these are engine hours, not overall boat usage time (after all, with fast boats, even long distances can be covered in just a few hours).

The difference in fuel consumption at an average speed of 30 knots for an MX-11—as well as for an MX-13—with diesel versus petrol engines is about 30 liters per hour more with petrol engines. This means that over the course of a season—based on 40 engine hours—and even assuming a constant speed of 30 knots, an additional 1,200 liters of fuel would be consumed. Considering that fuel prices are now fairly similar, this would amount to roughly €2,300 per year at most.

On the other hand, the difference in purchase cost between the two types of engines is around €40,000 + VAT more for diesel engines compared to equivalent petrol units.

Needless to say, this would mean breaking even only after about 20 years… far too long.

Unless, of course, the choice is driven by other factors—chief among them often being concerns about having petrol on board. It’s worth clarifying that modern engines no longer have the issue of petrol evaporation from carburetors, as they are now all fuel-injected. In addition, our boats are equipped with proper engine room ventilation systems using dedicated air extractors (“blowers”), as well as special “sniffers,” sensors that detect (and trigger an alarm for) any presence of petrol vapors.

Heavier use of the boat can (or should) naturally shift the preference toward diesel engines. If at 40 hours per year the balance leans toward petrol, things clearly change when usage approaches—or exceeds—100 hours annually.

Another need that is often highlighted is the ability to share fuel with a “mother ship” when using  RIBs as tenders. In this case as well, for practical reasons, the choice may fall on diesel propulsion.

Lastly, it’s also worth considering the better value retention and resale appeal on the used market of boats equipped with diesel engines compared to petrol ones—especially when there’s a high number of engine hours.

In conclusion, all of the above suggests that petrol engines are advisable for moderate use (which, to be clear, is no different from the typical usage of outboards), whereas diesel engines become the preferable choice when a high number of engine hours is expected.

For boats, is a petrol or diesel engine better? Summary table