Planing Speed Calculator (Crouch's Formula)
Past the wall: Crouch's planing formula for speed from power and weight — with the hull-constant table for runabouts, cruisers and racers.
On plane, weight per horsepower is destiny: Crouch's whole formula is a hull-quality constant over its square root. Note what's absent — length! Once planing, dynamic lift, not waterline, carries the boat.
Formula
⚠️ For planning and education only — verify with your vessel's documentation, naval-architecture data and official sources. Not for navigation or stability decisions on real voyages without professional data.
Past the wall: Crouch's planing formula for speed from power and weight — with the hull-constant table for runabouts, cruisers and racers.
About Planing Speed Calculator (Crouch's Formula)
Once a hull climbs onto plane, displacement-era rules die: waterline length drops out entirely, and George Crouch's 1930s formula — still the industry's first estimate — predicts speed from just power loading and a hull-quality constant. This calculator runs it with the standard constant table, from heavy cruisers (C=130) to race hulls (C=220), answering the eternal dock questions: what will she do, and what would another fifty horses add?
How to use Planing Speed Calculator (Crouch's Formula)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula Crouch: speed (mph) = C ÷ √(lb per hp) — planing speed scales with √power substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Planing Speed Calculator (Crouch's Formula)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula Crouch: speed (mph) = C ÷ √(lb per hp) — planing speed scales with √power with sources cited on the page
- ✓On plane, weight per horsepower is destiny: Crouch's whole formula is a hull-quality constant over its square root. Note what's absent — length! Once planing, dynamic lift, not waterline, carries the boat.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why doesn't boat length appear in Crouch's formula?+
Because a planing hull rides on dynamic lift, not buoyancy: the wave-making physics that made √LWL king in displacement mode is left behind (literally — the bow wave is behind the boat). What carries the boat is pressure on the planing surfaces, governed by weight and the power driving it: hence speed = C/√(lb/hp), length nowhere in sight.
What does the constant C physically represent?+
Everything the simple formula omits: bottom loading and deadrise, appendage and air drag, propulsive efficiency. Flat-bottomed race hulls with surface drives earn 210–230; clean outboard runabouts ~150; heavy deep-V cruisers with shafts and struts 120–140. It's calibrated hindsight — pick the constant from a similar known boat and predictions land within a few mph.
Why does doubling horsepower NOT double speed?+
The square root: speed scales with √(hp/lb), so 2× power buys 41% more speed — at twice the fuel flow and beyond. Equivalently, each mph gets pricier as the square: a 45-mph boat wanting 50 needs (50/45)² = 23% more power. This is the arithmetic behind every repower disappointment and every race-class power escalation.
What about getting ON plane — does the formula cover that?+
No — Crouch predicts the planing equilibrium, not the hump transition: a boat can have the top-end power for 40 mph yet struggle over the hump when overloaded aft, badly propped (no low-end thrust), or trimmed wrong. Hole-shot is a torque-and-trim problem; top speed is a power-loading problem. Marginal-power boats live or die at the hump, not the top end.
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