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 theoretical speed limit tied to waterline length (hull speed). If your target speed exceeds 1.3 × √(LWL in feet), resistance rises dramatically and enormous power is wasted. The calculator warns you when your target speed pushes past the practical limit for displacement mode.

How accurate are these estimates?

These are planning-stage estimates, not precision engineering figures. Real-world results depend on hull efficiency (Cp, beam, draft), propeller selection, drive losses, sea state, and loading. Use the result to narrow engine size options, then consult a naval architect or engine dealer for a final spec.

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

Name: Marine Engine Power Calculator
Availability: InStock
Author: Nham Vu

Enter your hull type, boat weight, and target speed to get estimated engine power 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)

Power method: Shaft HP estimated via Froude SLR and empirical resistance ratio (Rt/Δ ≈ 0.004–0.012 depending on speed/hull-speed ratio).

Summary

Enter your hull type, boat weight, and target speed to get estimated engine power 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, the Froude speed-to-length ratio checks feasibility and estimates shaft HP from an assumed resistance curve.
Results appear in both HP and kW.

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: 2026-07-22 · Reviewed by Nham Vu