Estimate the engine power required for your boat's target speed
Typical cruising sailboat or trawler
Lightweight, high-speed boat
Balances speed and efficiency
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Download NowSpeed and power are not linear partners; they are closer to a wager where each extra knot demands disproportionately more horsepower. Understanding that curve is what separates a boat that cruises efficiently from one that is permanently flat out, and it is the reason a hull that needs 40 hp for 20 knots may need 120 hp for 25.
A displacement hull is governed by hull speed, beyond which power is largely wasted pounding through its own bow wave. A planing hull pays for speed with the Crouch relationship, where required horsepower grows with the square of speed times displacement, so weight and target knots together set the engine you need.
Enter waterline length or displacement, pick the hull type and the speed you want, and the calculator shows the horsepower required and the point where the cost per knot starts to climb steeply.
Hull Speed = 1.34 × √LWL; Planing Power (Crouch) = Displacement × (Speed ÷ Crouch C)²| Target | Relative Power | Note |
|---|---|---|
| Hull speed | 1.0× | Efficient for displacement hulls |
| 1.2 × hull speed | ≈ 2.5× | Over the hump for a planer |
| 20 knots | moderate | Comfortable cruise for many hulls |
| 30 knots | ≈ 3-4× | Getting steep on the curve |
| 40+ knots | ≈ 6×+ | Diminishing returns dominate |
Approximate power multiplier to reach each speed band.
Formula verified against standard naval-architecture references · Updated October 2026
It depends on hull type, weight and the speed you want. A planing hull is sized with the Crouch relationship, where required power rises with displacement times the square of speed divided by a hull-quality constant, so heavier and faster both push hp up steeply. A displacement hull is governed by hull speed instead. Enter your numbers here and the calculator shows the shaft power and a realistic engine size to reach it.
Because resistance grows far faster than speed. For a planing hull the power needed scales with roughly the cube of speed once on plane, so going from 20 to 30 knots can demand more than double the horsepower. For a displacement hull the wall is hull speed, where the bow wave trains and further power barely adds knots. That non-linearity is the entire point of choosing cruise speed wisely.
Hull speed, about 1.34 times the square root of waterline length, is the point where a displacement hull's wave drag rises sharply. It still governs cruisers, trawlers and sailing yachts under power, where exceeding it costs fuel for almost no gain. Planing hulls escape it by climbing onto their own wake, which is why they need the Crouch power relationship instead.
It is a single number that captures how well a hull turns power into speed. A fine, light, well-planing hull sits high, around 190 to 200, and needs less power for a given speed; a blunt or heavy hull sits lower, near 150 to 170, and needs more. Choosing the right value is the main judgement in the calculation, so pick it from your hull form and adjust to match sea-trial numbers.
Profoundly. Required planing power is directly proportional to displacement, so a twenty percent heavier boat needs roughly twenty percent more power for the same speed. Fuel, water, gear and passengers all count, which is why sizing to dry weight underpowers real boats. Always enter the loaded displacement you actually cruise at.
Mildly over-powered is safer than under-powered, because an under-powered boat lugs the engine, drinks proportionally more and struggles in a headwind or with a load. But grossly over-powering adds weight, cost and the temptation to run dangerously fast. The aim is an engine that reaches rated rpm and cruise speed comfortably with a little reserve for heavy conditions.
They give a sound first estimate of the power a hull form needs, not a guarantee for any one boat. Trim, prop, gear losses, fouling and loading all shift the real curve, so use the result to narrow the engine choice, then confirm against owner reports of the same model and a sea trial before committing.