Marine fuel planning

Why boat fuel planning is a different problem to car fuel planning

A car’s fuel economy is roughly stable across the speeds you actually drive. A planing boat’s is not. Hydrodynamic drag on a planing hull rises with roughly the square of speed, and the power needed to overcome it with roughly the cube — so a boat that burns 20 litres an hour at 18 knots can burn close to 35 at 24 knots while covering only a third more distance. The fuel you need is not proportional to how far you are going; it is proportional to how fast you insist on getting there.

There is also no fuel station between you and the destination. A car that misjudges its range coasts to a stop on a verge. A boat that misjudges its range is adrift, usually in a rising sea state, often out of VHF range of anyone who can help. Everything in this section is built around that asymmetry: plan conservatively, and treat the last third of the tank as though it does not exist.

Hull type sets the shape of the fuel curve

Planing hulls

Bowriders, centre consoles and sportscruisers lift onto the water surface once they pass the transition “hump”. Fuel consumption spikes hard through that transition, then settles once the boat is up and running. Counter-intuitively, dawdling just below planing speed is one of the most expensive ways to travel — the hull is pushing water aside without the lift that makes planing efficient. If you must go slow, go properly slow at displacement speed rather than sitting on the hump.

Displacement hulls

Trawlers, full-keel yachts under motor and lobster-style cruisers push through the water rather than over it. Their maximum practical speed is set by waterline length — hull speed in knots is about 1.34 × √(waterline length in feet) — but within that limit they are dramatically more efficient. A 40-foot trawler at 8 knots can achieve fuel economy comparable to a planing hull travelling twice as fast, which is why long-range cruising boats are almost all displacement designs.

Pontoons and tritoons

Parallel-tube hulls carry high drag at speed and are usually most economical in a narrow cruise band well below wide-open throttle. Adding a third tube and a larger engine buys speed at a steep fuel cost. Where a planing monohull rewards finding the efficient cruise RPM, a pontoon punishes leaving it more severely.

What a speed change costs you

The chart below shows the pattern the planner models: fuel burn against cruise speed for a typical mid-size planing hull. Note that the burn rate keeps climbing after the point where the extra speed stops meaningfully shortening the trip.

Indicative of the cubic power relationship the planner applies, not a measurement of a particular vessel. Your engine’s published consumption curve is always the better input.

The one-third rule, and why the planner assumes 85% usable

The long-standing convention is to divide usable fuel into thirds: one to get out, one to get back, one untouched. That gives you a genuine reserve for a headwind on the return leg, an unexpected detour, a fouled prop or a slower passage than planned.

Separately from the one-third rule, tank capacity on the spec sheet is not fuel you can actually use. Pickup height, trim angle and the practical impossibility of running a tank dry mean the last portion is unavailable. The Boat Trip Fuel Planner applies an 85% usable-capacity factor before applying any one-third guidance, so the range it reports is already the conservative number rather than the theoretical one.

Getting a burn rate you can trust

In descending order of reliability: a flow meter reading at your actual cruise RPM; your engine manufacturer’s published consumption curve at that RPM; litres burned divided by hours run across several logged trips; and — only if you have none of those — a horsepower-based estimate. The planner supports the last of these but labels it as approximate, because the same horsepower on a different hull with a different load can burn materially differently.

Whatever source you use, add margin for load. A boat carrying full water tanks, a full crew and dive gear needs more power for the same speed and will burn 10–15% more than the same hull running light.

Fuel price at the marina

Marine fuel is usually dearer than road fuel, and the gap is not constant. Before pricing a trip, check what the road price is doing — our Australian fuel price data page carries current capital-city averages and the 2026 excise timeline, which moved pump prices twice this year. Then add the marina premium you actually pay locally rather than a generic allowance.

Related sections

For trailering the boat to the ramp, see towing and caravans. For comparing a boating trip against a driving one, see trip planning.