Calculate optimal engine power for your boat
Planing hull, high speed
Displacement hull, auxiliary power
Heavy displacement, long range
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Download NowEngine power sizing answers a blunt question: how many horsepower do you need to push your hull to the speed you actually want? Buy too little and the boat is frustrating and unsafe in a head wind or current; buy too much and you pay for weight, fuel, cost, and noise you will almost never use.
The right answer depends on hull type more than anything else. A displacement hull is governed by wave-making drag and is content at a modest S/L speed, while a planing hull must lift onto a plane and needs a large power-to-weight ratio to get there. This calculator applies the accepted formulas for each regime and adds a safety margin.
Enter your displacement, target speed, and waterline length (plus a Crouch constant for planing hulls) to get the required horsepower and a recommended rating with headroom built in.
Displacement: HP = D ÷ (10.665 ÷ (V ÷ √L))³| Hull type | Power rule of thumb | Typical S/L |
|---|---|---|
| Displacement sailboat | 2 - 3 HP per long ton | up to 1.30 |
| Heavy trawler | 3 - 5 HP per long ton | 1.20 - 1.50 |
| Semi-displacement | 6 - 12 HP per long ton | 1.50 - 2.20 |
| Light planing runabout | 3 - 5 HP per 100 lb | 2.50+ |
| High-performance | 8+ HP per 100 lb | 3.50+ |
Rough power guide by hull type and duty.
Formula verified against standard naval-architecture references · Updated October 2026
It depends on hull type and target speed. A displacement sailboat is fine at 2 to 3 HP per long ton and rarely needs more, because extra power cannot overcome wave-making drag. A planing runabout needs several horsepower per 100 pounds just to get on the plane. Enter your figures here and the calculator applies the standard formula plus a 25% margin to give a practical recommendation.
Required power is the minimum shaft horsepower to hold your target speed in calm water with a clean hull. Recommended power adds a margin — here a factor of 1.25 — to cover a fouled bottom, head wind or current, extra load, and general aging. Sizing to the recommended figure means the engine is never run at full throttle in ordinary conditions, which improves economy, noise, and longevity.
Crouch's formula estimates horsepower for planing and semi-displacement hulls: HP = (D ÷ (C ÷ S/L)²) ÷ efficiency, where D is displacement in tons, S/L is the speed-length ratio, and C is the Crouch constant. Lower C values mean a more efficient hull: about 130 to 160 for smooth-water racers, 190 to 230 for runabouts, and 230 to 300 for heavier utility hulls.
Because resistance climbs steeply once you approach hull speed. A displacement hull at S/L 1.34 needs roughly 50% more power to reach S/L 1.5, and the wake grows dramatically. Beyond a point the extra horsepower is mostly wasted making waves instead of speed. If you truly want more knots, the right tools are a longer waterline or a hull form that planes.
Compare like with like. Manufacturer ratings are often flywheel horsepower, but what reaches the propeller is shaft horsepower after the gearbox, cutless bearing, and shaft losses — typically 3 to 5% for a straight single-shaft line, more with angle drives. Size the engine so the derated shaft figure meets the calculator's recommendation.
Only marginally and expensively. Once a displacement hull is at its natural S/L, most additional power is swallowed by wave-making. You will gain a small speed bump at a large cost in fuel, weight, and noise. The smarter upgrades are trimming drag, a better propeller, or accepting a slower, more economical cruise speed.
A rough, fouled, or badly painted bottom can raise friction drag enough that the same engine no longer holds its design speed. This is exactly why you size to the recommended, margin-included figure rather than the bare requirement — that headroom is what lets a slightly fouled hull still make the speed you planned on.