Every hull has a sweet spot
Ask what the most fuel-efficient speed for a boat is and you'll get the answer that follows most boating questions: it depends. But this one depends in a predictable way. Every hull has a speed where it covers more miles per gallon than at any other — a sweet spot — and where that sweet spot sits is set almost entirely by what kind of hull you're driving. Find it and you buy range with nothing more than a throttle setting.
The reason there's a sweet spot at all is that two things move against each other as you speed up. Gallons per hour climb steeply and non-linearly, because drag doesn't grow in a straight line — while the miles you cover per hour grow only linearly. Plot fuel per mile against speed and you get a curve with a low point. That low point is the sweet spot; the rest of this guide is where it lives on each hull type and how to measure yours.
The yardstick: nautical miles per gallon
First, agree on the yardstick. Gallons per hour is what a flow gauge shows, but economy is a per-mile question, so the number to compare throttle settings with is nautical miles per gallon: nmpg equals your speed in knots divided by your burn in gph. Cruise at 24 knots on 12 gph and you're making 2.0 nmpg. Push to 40 knots and let the burn climb to 30 gph and you've dropped to 1.3 — far faster, and a third fewer miles from every gallon.
Comparing gph alone will steer you wrong, because throttling back almost always cuts gallons per hour. It does not always cut gallons per mile. An engine idling in gear burns very little per hour and gets you almost nowhere, and a planing hull dragged just below plane burns less per hour than it did on plane while covering far fewer miles for it. If you're new to putting a burn number next to a speed, our guide to how much fuel a boat uses covers reading it from a flow gauge or working it out fill-to-fill; from here on we'll assume you can.
Planing hulls: pay for the climb, then cruise the plateau
A planing hull lives two lives. At low speed it sits in the water like any displacement boat, supported by buoyancy. To go fast it has to climb over its own bow wave and up onto the surface — and the climb is the expensive part. In the transition the hull runs bow-high and plowing, dragging the most wetted surface at the worst attitude. That's the hump, and it's the worst fuel-per-mile territory the boat has.
Once over the hump, the hull flattens out, wetted surface shrinks, and economy recovers sharply. The efficient band starts just above the speed where the hull cleanly holds plane and usually runs as a broad plateau through the lower middle of the planing range — the exact peak varies boat to boat, which is why you measure. Push toward the top of the range and drag climbs steeply again, so each extra knot costs outsized fuel. Illustrative numbers for a 24-foot center console with a single 300-hp gas outboard:
- 8 knots, bow-high on the hump: about 8 gph, which is 1.0 nmpg — the worst per-mile fuel the boat makes.
- 17 knots, just settled on plane: about 10 gph — 1.7 nmpg.
- 24 knots, mid-band cruise: about 12 gph — 2.0 nmpg. The sweet spot.
- 30 knots: about 18 gph — back down to about 1.7 nmpg.
- 40 knots, near wide open: about 30 gph — 1.3 nmpg.
The trap just below plane
Notice the shape those numbers make: similar economy at 17 knots and 30 knots, the best of it in the middle, and by far the worst just below plane. That shape is why "slow down to save fuel" needs a caveat on a planing boat. Easing back from the sweet spot only helps until the hull falls off plane; pull back past that and you land on the hump — bow up, plowing, burning more per mile than you did at cruise. If you genuinely need to go slower than the efficient band, come all the way down to a level-trim displacement speed instead of wallowing in between.
In fairness, at a true displacement crawl — six knots or so — a planing hull can post per-mile numbers that rival or beat its planing sweet spot, because drag at those speeds is small. But it turns a one-hour run into a four-hour one, which is delivery pace, not cruising.
Displacement hulls: the wall at hull speed
A displacement hull never climbs out of the water, so its story has no hump — just a wall. As the boat speeds up it digs a wave system whose length grows with speed. At a certain point the wavelength matches the waterline length, the boat sits in the trough of its own wave, and pushing harder mostly digs the hole deeper. That point is hull speed, and the classic estimate for it, in knots, is 1.34 times the square root of the waterline length in feet.
Work it for a trawler with 36 feet on the waterline: the square root of 36 is 6, and 6 times 1.34 is right about 8 knots. A sailboat or launch with 25 feet of waterline: the square root of 25 is 5, times 1.34 is 6.7 knots. That's where the hull meets the wall — not a hard limit, but the point past which burn climbs viciously for almost no speed.
Unlike a planing hull, a displacement boat has no interior sweet spot: slower is more economical all the way down, right to bare steerage. The practical answer is a speed-to-length ratio around 1.1 to 1.2 — about 6.6 to 7.2 knots on our 36-foot waterline — where you keep most of the economy and still make a passage at a useful pace. Push the last knot toward hull speed and you're buying the most expensive fuel the boat sells: the burn can nearly double for about a tenth more speed. Here's that collapse in illustrative numbers for the trawler:
- 6.5 knots: about 1.7 gph — 3.8 nmpg.
- 7.2 knots: about 2.4 gph — 3.0 nmpg.
- 8.0 knots, at hull speed: about 4.5 gph — 1.8 nmpg.
Semi-displacement: in between, on a sliding bill
A semi-displacement hull splits the difference: it climbs partway out of the water and runs past hull speed without fully planing, and its drag curve sits in between. Past hull speed the burn climbs hard, and by around twice hull speed the penalty is back to displacement-wall steepness.
In practice a semi-displacement boat is two boats. Run it below hull speed and it returns near-trawler economy. Run it in the teens and it gets you there at a cost that climbs with every knot. The most fuel-efficient speed is almost always the slow mode; the honest question is what your schedule can afford, and the useful move is to measure both modes so the cost of the fast one is a number instead of a surprise.
How to find your sweet spot in one morning
Everything above tells you where to look; only measurement tells you where yours actually is. You need a calm day with light wind and as little current as you can find, the boat loaded the way you normally run it, and a way to read burn — a fuel-flow gauge, or the engine data most modern EFI outboards and diesels broadcast to a helm display.
No flow meter? You can still get there. Fill the tank, run a known distance at one fixed RPM, fill again, and divide miles by gallons — tedious, one setting per outing, but it's the same number. Failing that, cruise a little above the speed where the bow drops and the hull holds plane without mushing — on most planing boats that's near the plateau even before you've measured it.
With a burn readout aboard, the test itself is a short ladder of steady-throttle legs:
- Pick three or four RPM points across your usable range. On a planing boat: just above where the hull cleanly holds plane, a low cruise, your normal cruise, and a fast cruise. On a displacement or semi-displacement boat, bracket the range from well below hull speed up to it.
- Hold each setting steady for two or three minutes, until speed and fuel flow both settle, then log speed in knots and burn in gph.
- If there's any current at all, run each setting on reciprocal headings — out and straight back — and average the two GPS speeds. GPS reads speed over ground, and a knot of current will otherwise flatter half your runs and slander the rest.
- Divide knots by gph at each point. The highest nmpg is your sweet spot, and the points around it show how wide the plateau is.
Current moves the target: over the ground vs through the water
One wrinkle matters once you leave the test course: your burn is set by your speed through the water, but the trip only pays off in speed over ground. The economy that actually empties or spares your tank on a passage is SOG divided by gph — miles made good per gallon — and current drives a wedge between the two.
A fair current shifts your optimum slower: every hour under way, the current hands you free miles, so it pays to let it do more of the work. A foul current shifts the optimum faster, which surprises people — crawling into a 2-knot current means spending hours going almost nowhere while the engine burns anyway.
Run the numbers for the trawler from earlier pushing into 2 knots of foul current and both failure modes show up: crawl and you make almost nothing good per gallon; power through at hull speed and the wall eats everything the extra speed bought. The best over-ground economy now sits in the middle, well above the boat's still-water optimum. On a planing boat cruising at 24 knots, 2 knots of current barely moves the target — the effect scales with the current's size relative to your speed, which makes it a slow-boat problem first and a coming-home-against-the-tide problem always:
- 3 knots through the water on about 0.6 gph: 1 knot made good — about 1.7 nmpg over the ground.
- 6 knots through the water on about 1.5 gph: 4 knots made good — about 2.7 nmpg over the ground.
- 8 knots through the water on about 4.5 gph: 6 knots made good — about 1.3 nmpg over the ground.
What the sweet spot is worth (and what moves it)
Why go to the trouble? Because on many boats the sweet spot is worth a third of your range. Take the center console from earlier with 90 usable gallons aboard: at the 24-knot sweet spot's 2.0 nmpg, that's 180 nautical miles of theoretical range; at 40 knots and 1.3 nmpg, about 120. Same boat, same tank — the throttle setting is the difference. Turning that range number into a safe plan, with a reserve held back, is its own exercise; see our guide on how to calculate your boat's cruising range.
One honest caveat before you tattoo a number on the dash: the sweet spot is a band, not a point, and it drifts with how the boat is set up and what the day is doing. Re-measure when things change:
- Weight. A heavy load of fuel, water, crew, and ice raises the burn at every speed and raises the speed the hull needs to hold plane — the sweet spot shifts up the RPM range and the economy at it drops.
- Trim. Bow-high plowing and over-tabbed dragging both cost you. If you have a flow gauge, trim while watching it: the gph responds in real time, and minimum burn at a steady speed means the trim is about right.
- Bottom growth. A season of slime and barnacles cuts nmpg across the board, and can leave a marginal hull struggling to hold plane at the RPM that used to be the sweet spot.
- Sea state. Chop and head seas raise the burn and often force you off the calm-water optimum entirely; the most efficient speed in a seaway can be a different, slower number.
