Speed decides most of your fuel bill. On a planing boat, dropping from 20 knots to 16 can take a third off the burn in those hours, and nothing else comes close. The next two levers are a clean underwater body and correctly pitched propellers: fouling alone can add 10–20% to fuel consumption, and props that never reach rated rpm cost you at every throttle setting. Here is where the fuel goes, how to check each item, and what the fixes are worth on a 55 ft boat running 250 engine hours a year.
Why the fuel curve is steep, and where your boat sits on it
Fuel consumption does not rise in a straight line with speed. Push a hull faster and it makes bigger waves, and making waves is where the energy goes. IMO's energy-efficiency guidance puts ship power at roughly the cube of speed in the design range, which is why a small pull-back on the throttle produces a large cut in fuel.
The curve bends because of hull speed — the point where the wavelength of your bow wave matches your waterline, usually estimated at 1.34 × the square root of waterline length in feet. A 49 ft waterline gives about 9.4 knots. Below that, the hull slips through its own wave easily. Above it you are driving uphill, and power climbs much faster than speed. A planing hull escapes by climbing onto the surface, but the transition zone in between — the hump — is the most expensive water your boat will ever travel through.
You can see the shape of it in owner-reported figures for a 55 ft semi-displacement motor yacht with twin 550 hp diesels:
| Engine rpm | Speed | Total burn | Distance per gallon |
|---|---|---|---|
| 1,100 | 8.0 kn (9.2 mph) | 8 gal/h | about 1.0 nm/gal |
| 1,400 | 9.9 kn (11.4 mph) | 15 gal/h | about 0.66 nm/gal |
| 1,600 | 11.1 kn (12.8 mph) | 22 gal/h | about 0.50 nm/gal |
| 1,800 | 14.4 kn (16.6 mph) | 28 gal/h | about 0.50 nm/gal |
| 1,900 | 15.6 kn (17.9 mph) | 32 gal/h | about 0.49 nm/gal |
Read the middle two rows together: between 1,400 and 1,600 rpm the boat gains 1.2 knots and burns 47% more fuel an hour. That is the hump. The last two rows are the opposite — 1.8 knots more for 4 gallons an hour — because by then the hull is carrying its own weight on the surface.
So there is no universal "most economical speed". It is the speed just below the shoulder of your own curve, and you find it with a fuel log — gallons added against engine hours — plus a GPS speed reading. How hull shape and material set the curve in the first place is covered in our guide to yacht hull materials and construction.
The cheap speed and the expensive speed
On a planing boat the gap between displacement running and full plane is larger than most new owners believe. Captain-reported numbers for a 60 ft planing sport cruiser make the point: at 12 knots it burns about 40 litres an hour, or about 0.88 gallons per nautical mile. At 28 knots it burns about 180 litres an hour (47.6 gal/h) — about 1.7 gallons per mile. Twice the speed, twice the cost per mile, and at top speed it can pass 280 litres an hour.
At 2026 prices that gap is what empties the tank: US fuel docks averaged around $4.85 a gallon through 2026, and Mediterranean marina pumps ran roughly €1.55–1.85 a litre in spring 2026.
The rule for a planing boat: either be on the plane properly, or do not try to half-plane. Sitting at the top of the hump — bow high, stern squatting, engines working hard for a knot or two — is the most expensive way to move a boat. On a displacement hull the advice inverts: stay just under hull speed instead of fighting past it. What is under the floor changes the arithmetic too, and prop matching is covered in our yacht propulsion systems guide.
Hull and propeller: the 10–20% you can win back
This is the part of fuel efficiency owners actually control, and the numbers are brutal. Published fouling research puts a lightly fouled hull at roughly 10% more fuel and a heavily fouled one at up to 35%; a slime layer just 0.5 mm thick covering half the hull has been measured at 20–25% more energy to move. A US naval trial found fouling on propellers alone required about 10% more fuel to hold a given speed. FAO figures for small working vessels suggest fouling can add up to about 7% after one month in the water and up to 44% after six.
Against that, IMO's own estimate of what the fixes deliver is: hull cleaning 1–10%, propeller polishing 3–8%, better fouling-control coatings 1–5%. Those are ranges, not promises — where you land depends on water temperature and how much you actually run the boat.
- Time the lift-out to the season, not the calendar. A clean hull in June is worth more than a clean hull in October, because June is when you cover the miles.
- Treat the propellers as a separate maintenance item. Polishing is among the cheapest efficiency jobs in boating and carries the highest return per euro spent.
- Match antifouling to your pattern of use. A boat that sits in a warm marina for weeks needs a different product from one that moves daily. Paint choice, prep and timing are set out in our bottom care and antifouling guide.
- Record a reference point. Note GPS speed at a fixed rpm (say 2,000) with a fixed load on relaunch day, then repeat it six weeks later. The gap between the two numbers is your fouling cost, measured in your own water.
Weight, trim and the tender you stopped noticing
Displacement is the other half of the resistance equation, and cruising boats accumulate it quietly. On a 50–55 ft motor yacht, 1,200 US gallons of diesel weigh close to 3.8 tonnes and 750 gallons of fresh water about 2.8 tonnes, so leaving with everything full means carrying 6–7 tonnes more than lightship — and sitting the hull deeper, which increases wetted surface. The items owners forget to weigh: the tender and outboard on davits, a second anchor and chain, spare fuel in cans, tools, diving gear, water toys. Together, 500 kg of "nice to have" on a 20-tonne boat is 2.5% of the displacement, carried high and far aft, which is the worst place for trim and drag.
Trim is the free adjustment. Bow-down, the boat ploughs its forefoot and makes extra spray; bow-up, it squats and drags its stern. Tabs or an interceptive stern system set the running angle for the load and sea state of the day. The check: at a fixed rpm on flat water, note GPS speed with the tabs at zero, then with the boat trimmed to its natural best angle. If the second number is higher, keep that setting.
Is your boat propped right? Run the wide-open-throttle test
Propeller pitch is the most commonly wrong setting on a used boat, and it is invisible until you look for it. A prop that is too coarse — over-propped — holds the engine below its rated speed, which raises cylinder load at every rpm, burns more fuel and makes the boat sluggish to accelerate. The test takes ten minutes:
- Fuel and water at normal cruising level, bottom reasonably clean, flat water, no current.
- Open the throttles fully and hold long enough to settle — a minute is plenty.
- Read the tachometers against the rated maximum on the engine plate; marine diesels commonly rate at 2,400–2,800 rpm depending on model.
- If the engines fall more than about 50 rpm short, the props are too coarse. If they exceed the rating, they are too fine and you are leaving speed and efficiency behind.
- Re-check at the end of the season with a fouled bottom. If the boat can no longer reach rating at all, that is the fouling penalty showing up as lost rpm.
Why it matters: a turbocharged diesel's specific fuel consumption — fuel burnt per unit of power — is best in the upper-middle of the load range, commonly quoted as 70–80% of rated rpm, and it degrades quickly below about 30–40% load. One published diesel curve puts the minimum at around 70–80% of maximum rpm, roughly 3% better per horsepower-hour than at 60%. A boat cruising at 60% load because it is over-propped sits permanently in the wrong part of that curve, and no throttle technique fixes it. Light-load running leaves deposits too, and on some engines glazes the cylinder bores. Service intervals are covered in our yacht engine maintenance guide; if consumption has risen suddenly rather than gradually, treat it as a fault and start with our diesel engine troubleshooting guide.
What it adds up to over a season
Take a 55 ft planing motor yacht doing 250 engine hours a year at an average 24 gallons an hour — 6,000 gallons, or a $30,000 fuel year at $5.00 a gallon. What the measures above are typically worth against that baseline:
| Measure | Typical effect | Fuel saved | Value at $5.00/gal |
|---|---|---|---|
| Cut cruise speed by 3–4 knots for half your hours | about 30% of the fuel burnt in those hours | about 900 gal | about $4,500 |
| Clean hull and props at the start of the season, not the end | 5–15% of all fuel | 300–900 gal | $1,500–4,500 |
| Correct an over-propped boat | 3–8% of all fuel | 180–480 gal | $900–2,400 |
| Take off 500 kg of gear and the tender | 1–3% of all fuel | 60–180 gal | $300–900 |
Two cautions with numbers like these. The percentages are not additive — a clean bottom already delivers part of the speed-reduction saving, because you get the same speed for less throttle. And a 15-knot headwind on an exposed passage can cost 20–30% of fuel efficiency at the same throttle setting, so one rough day can wipe out a week of careful running. Log wind and sea state next to the fuel numbers, or you will chase ghosts.
Frequently asked questions
What is the most fuel-efficient speed? On a displacement or semi-displacement hull, just under hull speed — 1.34 × the square root of waterline length in feet. On a planing hull, either be properly on the plane or stay at displacement speed. Cutting 2–4 knots off your normal cruise typically saves 15–30% of the hourly burn.
How much does a dirty bottom actually cost? Published research puts a lightly fouled hull at about 10% more fuel and a heavily fouled one at up to 35%. Propeller fouling alone has been measured at around 10% more fuel to hold a given speed. A mid-season hull clean and propeller polish is usually the best-value efficiency work you can buy.
Should I re-prop for a better cruising speed? Only after the WOT test says you have a problem. A correctly matched propeller lets the engines reach rated rpm at full throttle with a clean bottom and a normal load. If yours does, wanting a lower cruising rpm is a different conversation.
The order of operations is the same for every boat: start a fuel log this week, find your shoulder speed with three half-hour runs, set a cruise policy you will actually keep, clean the running gear before the season rather than after it, and run the wide-open-throttle test before you spend money elsewhere. If you are shopping for a boat, ask for twelve months of fuel receipts and the rpm at wide-open throttle — the second number says more about how the boat has been set up than any broker's description.