Two peaks, and only one of them is useful
Take a set of measured points off an ordinary planing boat and work out distance per gallon at each:
| Rpm | Speed | Burn | nm per gallon | Gallons for 40 nm | Time for 40 nm |
|---|---|---|---|---|---|
| 1,000 | 5.5 kt | 1.2 gal/hr | 4.583 | 8.7 | 7 h 16 |
| 2,500 | 9.0 kt | 3.4 gal/hr | 2.647 | 15.1 | 4 h 27 |
| 3,500 | 22.0 kt | 9.0 gal/hr | 2.444 | 16.4 | 1 h 49 |
| 4,500 | 29.0 kt | 14.5 gal/hr | 2.000 | 20.0 | 1 h 23 |
| 5,500 | 36.0 kt | 23.0 gal/hr | 1.565 | 25.6 | 1 h 07 |
The best figure in the table is the slowest one, by a wide margin, and that is the normal result rather than a curiosity. At 5.5 knots the hull is in displacement mode and the power required is very small, so the miles per gallon is enormous. Nobody crosses a bay at 5.5 knots in a boat that will do 36.
The useful number is the best economy among the speeds you would actually run. Set a floor at 18 knots and the answer becomes 3,500 rpm and 2.444 nm per gallon — not the peak of the curve, but the peak of the part of it you use. That is why this page asks for a minimum speed instead of just reporting the maximum.
What the shape is doing
Between the displacement peak and the top there is a hump, and it is the expensive part of the range. As a planing hull is pushed past its displacement speed it climbs its own bow wave: the boat is bow-up, dragging, and burning a lot of fuel to go not very fast. Distance per gallon bottoms out somewhere in there.
Once the hull comes over the hump and settles onto plane, wetted surface drops sharply and economy recovers. That recovery produces a local peak just above the clean planing speed, and it is usually the best compromise the boat has. Past it, drag climbs with the square of speed and the power to overcome it faster still, so economy falls away again toward the top.
The frequently quoted idea that best cruise sits somewhere around two thirds to three quarters of wide open throttle is a description of that shape rather than a rule. Where the second peak actually falls on your boat depends on the hull, the load, the prop and the trim, which is precisely why it has to be measured rather than looked up.
Running the trial so the numbers mean something
Two things ruin a speed trial and both are avoidable. The first is current. Speed over the ground with a two knot tide behind you is two knots of pure fiction, and it does not cancel out by itself. Run each setting on reciprocal courses over the same piece of water and average the two, which removes the along-track component. The second is conditions. A trial run into a head sea and back is a different boat from the same trial on a flat morning, and comparing points taken in different conditions compares the conditions rather than the settings.
The other variables to hold still: fuel level, because a full tank is a couple of hundred pounds; people and gear; trim, whether by tabs or by engine angle; and the bottom, which grows. Let the boat settle at each rpm for long enough that the speed and the flow figure both stop moving before writing anything down — a minute or two, not ten seconds.
If there is no fuel flow meter, the measured-tank method works: fill, run a known distance at one setting, fill again, and divide. It takes a day to get five points that way, and it also checks the flow meter if you have one, which is worth doing because on many installations the flow figure is calculated from injector timing rather than measured, and calculated figures drift.
What running fast actually buys
The comparison worth making is not economy against economy but the exchange rate between fuel and time. In the table above, 3,500 rpm covers 40 nautical miles on 16.4 gallons in 1 hour 49. Wide open covers it on 25.6 gallons in 1 hour 07. Nine gallons buys 42 minutes, which is about 13 gallons per hour of trip time saved.
Some days that is an easy trade and some days it is a poor one. A weather window closing, a bar that has to be crossed near slack, children on board, a long way to go — these are all reasons the fuel is worth spending. What the arithmetic does is make the trade explicit instead of leaving it to the throttle hand.
Questions people ask
What is the most fuel efficient speed for a boat?
On a planing hull, the highest distance per gallon is almost always at displacement speed — a slow idle, where the power needed is tiny. That figure is real but it is not usable for getting anywhere. The number people actually want is the best economy among speeds they would run, and on most planing boats that sits just above the speed where the hull settles cleanly onto plane, once wetted surface has dropped but before drag starts climbing steeply. Frequently that lands somewhere in the region of two thirds of wide open throttle, but the spread between boats is wide enough that the figure is only worth having if you measured it on your own boat, with your own load and prop.
Why is my distance per gallon worse in the middle of the rev range than at the top or the bottom?
You are on the hump. Between displacement speed and clean planing, a hull is climbing its own bow wave: bow up, a lot of wetted surface dragging, and a great deal of power going into making waves rather than going forward. Distance per gallon reaches its worst point somewhere in that transition. Once the boat comes over and settles, wetted surface drops sharply and economy improves even though the speed went up, which is the counter-intuitive part. It is also why sitting at hump speed to save fuel is the one thing guaranteed not to.
Do I need a fuel flow meter to do this?
No, but it takes longer without one. The measured-tank method is to fill the tank, run a known distance at a single steady setting, fill again, and divide gallons by distance. One setting per fill, so five points is a project rather than an afternoon. It has one advantage over a flow meter, which is that it measures fuel that actually left the tank. On many installations the flow figure shown on the display is calculated from injector on-time rather than measured by a sensor, and calculated figures drift with wear and with fuel density. Checking a flow meter against a measured tank once is worth the fill.
Why average the speed on reciprocal courses?
Because a GPS reports speed over the ground and the ground is not moving but the water is. A two knot current behind you inflates every reading by two knots and there is nothing in the number to reveal it. Running the same setting in both directions and averaging cancels the along-track component of the current, which is nearly all of the error on a run laid along the stream. It does not cancel a cross-track current, which is another reason to run the trial in slack water on a flat day rather than trying to correct for conditions afterwards.
How much do sea state and load change these numbers?
Enough to make a trial taken in different conditions worthless for comparison. A boat punching a short head sea sees the burn go up and the speed go down at the same throttle setting, and both move in the direction that hurts, so distance per gallon falls faster than either. Load does the same more quietly: a full tank on a small boat is a couple of hundred pounds, and so are three passengers and their gear. Bottom growth is the slow version — a fouled hull costs speed continuously and nobody notices because it happens over months. None of that means a trial is useless; it means the trial describes the boat on that day, and it is worth repeating when the boat has materially changed.