What is the Crouch formula?

The Crouch formula estimates the horsepower required for a planing hull: HP = (Displacement × Speed²) / C², where Displacement is in pounds, Speed is in knots, and C is a hull performance constant. C typically ranges from 150 (heavy cruisers) to 220+ (high-performance race boats). The formula was developed by George Crouch in the early 20th century and remains a practical rule of thumb for powerboat design.

What hull type should I choose?

Displacement hulls travel at or below hull speed (roughly 1.34 × √LWL knots) and push through the water. Examples: trawler yachts, full-keel sailboats, long-range cruisers. Planing hulls rise up on top of the water at speed and require significantly more power to reach planing speed. Examples: runabouts, center consoles, sport boats, bass boats.

Why does my displacement hull show a feasibility warning?

Displacement hulls have a practical speed range tied to waterline length, commonly about 1.3–1.4 × √(LWL in feet) knots. Near this range resistance rises steeply. A target above the upper check is rejected because weight and LWL alone cannot predict the power required to exceed it.

How accurate are these estimates?

These are planning-stage guidelines, not precision engineering figures. The displacement estimate follows a commonly published engine-selection rule of thumb of about 4 bhp per loaded metric tonne, with up to 30% allowance for heavy sea or tide conditions. Actual selection also depends on hull form, propeller, gearing, fouling, sea state, auxiliaries, and duty cycle. Consult a naval architect, engine dealer, and propeller specialist for a final specification.

What does the Crouch C constant represent?

C reflects hull efficiency and type. Higher C means the hull needs less power for a given speed. Common values: 150 = heavy cruisers; 170 = average runabouts; 190 = light sport boats; 210 = high-performance V-bottoms; 220+ = race hulls. This calculator uses selectable presets to guide the estimate.

Marine Engine Power Calculator

Enter hull type, loaded weight, and target speed to estimate planing power or a displacement-boat installed-engine guideline in HP and kW.

Boat Parameters

Hull Type

Hull Performance (Crouch C)

Loaded Displacement Weight

Target Speed (knots)

Crouch C Constants

Higher C = more efficient hull for a given speed.

C	Hull type
150	Heavy cruiser
160	Semi-displacement cruiser
170	Average runabout
190	Light sport boat
210	High-performance V-bottom
220	Race hull

Crouch formula: HP = (Displacement_lb × Speed_kts²) / C²

Displacement Hull Speed Limits

Hull speed = 1.34 × √LWL_(ft) knots. Exceeding this requires exponentially more power.

LWL (ft)	Hull speed (kts)

Selection guideline: About 4 installed bhp per loaded metric tonne; allow up to 30% more for heavy sea or tide conditions. Target speed is checked against 1.3–1.4 × √LWL, not used as a resistance model.

Summary

Enter hull type, loaded weight, and target speed to estimate planing power or a displacement-boat installed-engine guideline in HP and kW.

How it works

Select your hull type: displacement (sailboats, trawlers, full-keel cruisers) or planing (powerboats, runabouts, sport boats).
Enter the boat's loaded displacement weight in pounds or kilograms.
Enter your target speed in knots.
For planing hulls, the Crouch formula calculates HP from the speed-to-power ratio (C) for your hull type.
For displacement hulls, waterline length checks whether the target speed is practical; it is not used to invent resistance from missing hull geometry.
The displacement result uses a 4 bhp per metric tonne installed-engine rule of thumb and shows a 30% heavy-service allowance.

Use cases

Size an outboard or sterndrive engine for a new powerboat build.
Verify whether an existing engine can push a boat to its target cruise speed.
Compare engine requirements between a displacement trawler and a planing runabout.
Estimate the power needed to bring a heavy cruiser to hull speed.
Evaluate the trade-off between added boat weight and engine power requirements.
Help a buyer understand why a heavy displacement boat needs fewer horsepower than a planing hull at the same speed.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu