Litres per nautical mile is the right comparison for a trip
Divide fuel consumption in litres per hour by speed in knots. That gives you litres per nautical mile. Multiply by the trip distance and the fuel price to work out the trip cost. This is a calculation, not a forecast for a specific boat. Use measured figures with a normal load and keep a fuel reserve for changing conditions.
Do the calculation directly in the fuel consumption calculator. Engine power alone isn't enough to tell you what your boat burns.
A clear worked example
Assume a measured consumption of 20 litres/hour at 20 knots. That gives 20 divided by 20 = 1 litre/nautical mile. A 30-nautical-mile passage then requires 30 litres, and at an assumed price of 22 kr/litre the cost comes to 660 kr. All figures are illustrative inputs and calculations, not market prices or manufacturer claims.
| Assumed consumption | Assumed speed | Calculated |
|---|---|---|
| 20 l/hour | 20 knots | 1 l/nautical mile |
| 12 l/hour | 8 knots | 1.5 l/nautical mile |
The example shows why lower hourly consumption doesn't always mean less fuel over the same distance. The slower boat is also out on the water longer.
Measure your own boat
- Start from the trip computer's fuel flow reading if fitted, and check its accuracy against refuelling over time.
- Note speed, rpm, load and trim during steady running.
- Repeat in comparable conditions. Current and wind affect GPS speed.
- Compare litres per nautical mile across suitable cruising speeds.
- Check that the propeller lets the engine reach the manual's wide-open-throttle range when conditions allow a safe test.
Keep a log rather than trying to remember a single instrument reading. Note whether the fuel is also used by a heater or other equipment.
Why two identical engines can burn different amounts of fuel
Hull, weight, load distribution, propeller and trim all affect drag and engine load. Mercury describes these relationships in its manual under factors affecting performance. So an engine's spec sheet doesn't tell you how much fuel a given installation will burn. Be careful when comparing the same engine in different boats.
Find basic specs in the boat catalog and the manual in the engine catalog. Check whether a published test covers the same engine, propeller, load and speed. If not, the figure is at best a rough point of comparison.
Cut consumption without compromising safety
Keep the hull bottom and propeller in good condition, follow the service schedule and use the right trim. Read about choosing a propeller before you try to cut fuel consumption with a different pitch. An engine that labours isn't automatically economical.
Plan fuel stops around your real range and the harbours available. Don't assume the tank's full rated capacity is always usable. The fuel consumption tool is no substitute for seamanship or a reserve for headwinds, detours and idling while you wait. Also note down the price you actually paid, and your next budget will be more accurate.
Frequently asked questions
How do I calculate litres per nautical mile?
Litres per hour divided by speed in knots. Use measurements from the same running conditions.
Is there a reliable rule of thumb per horsepower?
Not for any specific trip. Load, hull, speed and propeller must be taken into account. Measure your own installation.
Is the lowest hourly fuel consumption the cheapest?
Not necessarily per distance travelled. A lower speed can mean more engine hours for the same trip, so compare litres per nautical mile.
Sources
- Mercury 15/20 EFI FourStroke – faktorer som påverkar prestanda servicelit.mercurymarine.com
- Mercury Marine – propellerns påverkan mercurymarine.com