Most boaters know roughly what their boat burns and almost none know what it burns on the trip they are about to make. Those are different numbers, and the gap between them is where people run out of fuel.
The starting estimate
For a rough figure, a gasoline outboard at cruising speed burns approximately 0.1 gallons per hour per horsepower — in metric terms, roughly 0.4 litres per hour per kW. A 200 hp engine at cruise is therefore in the region of 20 gallons (about 75 litres) per hour.
Diesel engines are more efficient, typically around 0.05-0.06 gallons per hour per horsepower at cruise.
Treat these as a starting point only. Hull shape, load, propeller and trim change them considerably.
The number that actually matters: miles per gallon
Gallons per hour tells you how fast you are spending fuel. Nautical miles per gallon tells you whether you get home. Divide your speed over ground by your consumption per hour, and you have it.
Do this at several speeds and you will find something useful: most planing boats have a clear efficiency sweet spot, usually just above the speed at which the hull settles onto the plane. Below it, you are pushing water; well above it, consumption climbs far faster than speed. The difference between cruising at the sweet spot and cruising ten knots faster can be thirty or forty percent of your range.
What ruins the calculation
- Head seas. Punching into waves can double consumption. This is the single biggest factor and the one people forget, because the trip out was fine.
- Current. Two knots of current against you on a 20-knot boat costs ten percent of your speed over ground, and every gallon buys ten percent less distance.
- Weight. Full water tanks, gear, people. Load has a real effect on a small boat.
- A fouled hull. A season's growth can cost a substantial part of your efficiency, and it happens gradually enough that you do not notice.
- Wrong propeller or bad trim. An engine that cannot reach its rated rpm range at wide-open throttle is working outside its designed operating point.
The rule of thirds
One third out, one third back, one third in reserve. The reserve is not a nicety: it is what covers the head sea on the way home, the current that turned, the detour around weather and the fact that fuel gauges on boats are famously optimistic.
If you plan a trip that uses more than two thirds of your tank, you are planning to rely on everything going right.
How to measure your own numbers
- Fill the tank completely and note the reading.
- Run a known distance at a steady cruising speed, in calm conditions.
- Fill again and record exactly how much went in.
- Divide distance by fuel used: that is your real miles per gallon at that speed.
- Repeat at two or three speeds to find the sweet spot.
Then keep a log. Consumption that quietly rises over a season is telling you something about the hull, the propeller or the engine — usually before anything else does.
A fuel flow meter makes all of this continuous rather than occasional, and on a boat used regularly it pays for itself in avoided guesswork.
Domande frequenti
How much fuel does a boat engine use per hour?
As a rough starting point, a gasoline outboard at cruising speed burns approximately 0.1 gallons per hour per horsepower, and a diesel around 0.05 to 0.06. Hull shape, load, propeller and trim change these figures considerably.
What is the rule of thirds for boat fuel?
One third of the tank to get there, one third to get back and one third held in reserve. The reserve covers head seas on the return, adverse current, weather detours and the fact that boat fuel gauges are optimistic.