Compare trips in litres per nautical mile
Divide the consumption in litres per hour by the speed in knots to get litres per nautical mile. Multiply by the trip length and the fuel price and you have the cost of the trip. Use figures you've measured yourself with a normal load, and carry fuel for a change in the weather.
Do the sums directly in the fuel consumption calculator. Engine power alone doesn't tell you what your boat burns.
A worked example
Say you have measured 20 litres an hour at 20 knots. 20 divided by 20 gives 1 litre per nautical mile. A 30 nautical mile trip then uses 30 litres. At a made-up price of 22 kr a litre, it costs 660 kr.
| Assumed consumption | Assumed speed | Calculated |
|---|---|---|
| 20 l/hour | 20 knots | 1 l/nautical mile |
| 12 l/hour | 8 knots | 1.5 l/nautical mile |
So lower consumption per hour doesn't always mean less fuel for the same distance. The slower boat is out longer.
Measure your own boat
- If you have a trip computer, read the fuel flow. Compare it with what you actually fill up over a period to see how accurate it is.
- Note speed, rpm, load and trim when the boat is running steadily.
- Repeat the measurement in similar conditions. Current and wind affect GPS speed.
- Compare litres per nautical mile at different cruising speeds.
- When it is safe to run at full throttle, check whether the engine reaches the WOT rpm in the manual with the propeller you have.
Keep a written record. A figure you remember from the display is not much to go on. Also note whether a heater or anything else draws from the same tank.
Why two identical engines can burn different amounts of fuel
Hull, weight, how the load is distributed, propeller and trim all affect resistance and how hard the engine has to work. Mercury covers this in its manual under factors that affect performance. So the engine's data sheet tells you nothing about what a particular installation will burn. The same engine in a different boat can give completely different figures.
Basic specifications are in the boat catalogue and the manual is in the engine catalogue. When you read a boat test, check whether the engine, propeller, load and speed match yours. If they don't, treat the figure as a rough comparison.
Burn less fuel without compromising safety
Keep the hull and propeller clean, follow the service schedule and trim correctly. Read about choosing a propeller before changing pitch to save fuel. An engine that labours rarely saves anything.
Plan your refuelling around the range you actually have and where fuel is available. You usually can't use the tank's full volume. Always keep a reserve for headwinds, detours and idling while you wait. Note down what you paid per litre, and your next budget will be better.
Frequently asked questions
How do I calculate litres per nautical mile?
Litres per hour divided by speed in knots. Take both values from the same run.
Is there a standard figure per horsepower?
Not one that holds for a particular trip. Load, hull, speed and propeller all play a part. Measure your own boat.
Is the lowest hourly fuel consumption the cheapest?
Not always per distance. At lower speed the engine runs more hours for the same trip. Compare litres per nautical mile.
Sources
- Mercury 15/20 EFI FourStroke: faktorer som påverkar prestanda servicelit.mercurymarine.com
- Mercury Marine: propellerns påverkan mercurymarine.com