A diesel engine with the correct propeller will burn around 0.05 gallons of fuel per hour per horsepower. But consumption is not linear: as this graph shows, at 80% maximum speed, consumption is halved. It pays to ease off on the revs.
A boat’s optimum cruising speed naturally depends on its design, displacement or weight, propeller size and the power of its engine. The speed of a displacement, as opposed to planing, hull is subject to a mathematical formula based on its waterline length (see below).
Pushing any displacement boat even a little bit faster than its design speed will require disproportionately more power, although alowance must be made for strong winds and head seas.
Similarly an inland waterways boat needs enough power to overcome currents and strong winds.
With the maximum speed established it is generally better to cruise at 20 per cent below the maximum under normal conditions as this will work the engine reasonably hard, give an acceptable boat speed and low fuel consumption – as low as half the top-speed consumption.
