Tips for estimating fuel consumption

Pump it up

Fuel costs account for a large proportion of total running costs so it is worth knowing how much you are likely to use.

Estimating consumption is difficult as there are many variables. Sea state, hull shape, boat type and weight, engine size and other factors all influence the amount of fuel a boat uses.

That said, the main factor is engine revs - the higher the revs the more fuel the engine will burn. A graph from This is Motor Boating illustrates this clearly:

Graph of fuel consumption vs engine revs

At 100% engine revs - usually referred to as Wide Open Throttle (WOT) - fuel consumption is at its highest. Reduce revs to around 70% of maximum and fuel consumption is halved.

The book also provides a simple formula to estimate fuel consumption at WOT for boats with a planing hull.

Estimating consumption

Take the rated engine output in horsepower and apply the multiplier below to estimate fuel consumption in litres per hour at wide open throttle (WOT)

Type Multiplier
Petrol 2-stroke 0.4
Petrol 4-stroke 0.3
Diesel indirect injection 0.25
Diesel direct injection 0.2

So, for my 2-stroke outboard rated at 150HP it’s 150 * 0.4 = 60 litres per hour at WOT. I seldom run anywhere near maximum revs so typical consumption is more like 20-30 litres per hour running at around 25 knots.

Reference

This is Motor Boating, Ramon Gliewe. Adlard Coles Nautical (1992). ISBN 0713634588