Are Inboard or Outboard Motors More Fuel Efficient?

Whether it's inboards or outboards, there are many factors that influence a boat engine’s fuel economy. Displacement, running conditions, tuning, weight and propeller pitch are just a few of them. 

Inboard vs outboard motor fuel efficiency

But what about how the engine is configured? Which is more fuel efficient, an inboard or an outboard engine? Outboards are far more popular on small boats, but are they winning the fuel economy battle?

How is Fuel Efficiency Calculated on a Boat?

Fuel efficiency is driven in large part by the motor’s power output, so that’s a good hint into any outboard’s fuel consumption. Here’s a good equation to guide you:

  • Horsepower/10 = GPH (Gallons Per Hour)

Start with the horsepower on the engine, divide by 10, and that’ll give you how many gallons of fuel you can expect an outboard motor to burn per hour. 

Outboard motor fuel efficiency

So, say you have a 40HP outboard engine. Forty divided by 10 gives you 4, so you can expect about four gallons of fuel burned for every hour the engine runs. This isn’t an exact calculation, since there can be variance in how a motor is operated during a one-hour period. But this number is a good guide for what sort of fuel consumption you’ll get if you’re running at full speed that whole time. Slowing down increases fuel efficiency.

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Outboard vs. Inboard/Outboard

So which is more fuel efficient, an outboard motor or an inboard? It’s really close, and some people see modest gains in fuel efficiency in an inboard/outboard setup versus an outboard

Inboard motor fuel efficiency

On the other hand, a pure inboard loses a lot because of prop angle and drag, but can make up for that by using diesel fuel, which is heavier to carry on the boat, but burns more efficiently.

Inboard vs outboard engine fuel efficiency

In the end, there are many advantages to outboard motors for small boats, but an inboard/outboard (sterndrive) might be better-suited for you, depending on the type of boating you do regularly. 




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