Work out boat fuel consumption instantly with clear inputs, formula shown and shareable results.
A petrol outboard uses roughly 0.21 litres per hour per horsepower at full throttle, and load falls steeply as throttle is reduced. Cruising at 70% throttle uses only about 40% of wide-open-throttle fuel, which is why the most economical cruise is well below maximum.
Boat fuel
L/h = hp x (throttle fraction)^2.5 x 0.21 for petrol; diesel is roughly 0.17
Because propeller load rises roughly with the cube of speed while throttle position maps non-linearly onto that. An empirical exponent near 2.5 fits observed consumption well.
Plan one third of fuel for the outward passage, one third to return, and one third in reserve. It is standard practice for offshore boating.