Boat Hull Speed Calculator
Compute displacement hull speed (1.34 x sqrt LWL), sail area-to-displacement, capsize screening ratio, and propeller slip from boat dimensions. Free and client-side.
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What is Boat Hull Speed Calculator?
The Boat Hull Speed Calculator runs the classic naval-architecture rules of thumb on your boat specs: displacement hull speed from waterline length, sail area-to-displacement on the standard power curve, the capsize screening ratio for offshore safety, and theoretical propeller speed with slip from pitch, RPM, and gear ratio.
Key Benefits
- Set realistic speed expectations for any displacement hull
- Compare cruisers and racers objectively with SA/D
- Screen offshore stability with the proven capsize ratio
- Diagnose propeller and gear mismatches from slip
- Instant, private, client-side math with no sign-up
Common Use Cases
- •Evaluating a used sailboat listing before a sea trial
- •Tuning propeller pitch after an engine repower
- •Comparing design ratios between candidate boats
- •Teaching seamanship students the classic performance rules
How to Calculate Boat Ratios
- Enter hull dimensions: Fill in waterline length, displacement, and beam from the boat spec sheet - hull speed appears immediately.
- Add sail area: Enter sail area for the SA/D ratio and capsize screening ratio, with typical ranges to compare against.
- Add engine data for slip: Enter propeller pitch, RPM, gear ratio, and measured speed to see theoretical speed and propeller slip.
Key Features
- Displacement hull speed from LWL using the classic 1.34 x sqrt formula
- Sail area-to-displacement ratio on the standard 2/3 power curve
- Capsize screening ratio for coarse offshore safety screening
- Theoretical propeller speed and slip from pitch, RPM, and gear ratio
- Readable errors for zero or negative dimensions
- Instant client-side math - no sign-up, no upload