Hull Speed & Displacement Ratio Calculator
Calculate theoretical hull speed, speed/length ratio and displacement/length ratio for any hull.
Last reviewed by the myCalculator Editorial Team
Input Parameters
Waterline length, not length overall — overhangs do not contribute to hull speed.
Calculation Results
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Formula & How Hull Speed & Displacement Ratio Calculator Works
A displacement hull generates a bow wave whose length grows with speed. At a speed/length ratio of about 1.34 the wave equals the waterline length and the boat sits in the trough of its own wave — the classic hull speed barrier, where power demand rises almost vertically. Only hulls with enough power and the right shape climb over it into semi-displacement and planing regimes. Because hull speed scales with the square root of waterline length, doubling speed requires four times the length.
- Enter Waterline Length LWL (m) (m): Input your target parameter value.
- Enter Displacement (kg) (kg): Input your target parameter value.
- Enter Beam (m) (m): Input your target parameter value.
- Enter Actual Speed (knots) (kts): Input your target parameter value.
- Apply Formula: Calculate using
Hull Speed = 1.34 × √(LWL in feet) | D/L = Displacement (long tons) / (0.01 × LWL ft)³. - Review Results: View exact computed answers, metrics, and worked mathematical proofs.
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Frequently Asked Questions (FAQs)
Can a boat exceed its hull speed?
Yes, but only with enough power to climb over its own bow wave into semi-displacement or planing mode. For a heavy displacement hull the power required rises so steeply that it is usually impractical.
What is a displacement/length ratio?
It normalises weight against waterline length. Below 100 is ultralight racing, 200 to 300 is a typical cruising yacht, and above 400 indicates a heavy traditional workboat.
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Authoritative Standards & Verified Reference Citations
Calculations and mathematical logic on this page are grounded in published statutory rules, regulatory standards, and official government data published by the following bodies:
Open government data, scientific datums, and public sector mathematical reference datasets.
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