OceanCalc

Hull Speed Calculator — Examples

Estimate theoretical hull speed from waterline length using the classic 1.34 × √LWL rule for displacement hulls.

Last updated: March 28, 2026

Author: OceanCalc Editorial Team · Publisher: Albor Digital LLC

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Explore typical numbers with the Hull Speed Calculator. Try the examples in the Examples section below while you vary inputs in the calculator.

Hull Speed Calculator

Result

Approx. hull speed (kn)

7.58

Formula

Hull speed (kn) ≈ 1.34 × √(LWL ft)

Overview

The Hull Speed Calculator gives you precise nautical results from familiar formulas, with an interface suited to deck and chart-table use.

How to use

With the Hull Speed Calculator, type your figures into the inputs above—the output refreshes live, making it easy to sanity-check several cases.

Formula

The relationship behind this tool is: Hull speed (kn) ≈ 1.34 × √(LWL ft)

Classical displacement estimate: V ≈ 1.34√LWL (knots, LWL in feet). Planing hulls and multihulls often exceed this.

Based on Froude number ~0.4 for typical monohull wave-making resistance.

Practical use cases

Typical uses for the Hull Speed Calculator include passage planning, briefing crew, converting instrument readouts to chart units, and double-checking mental math when fatigue or weather make errors more likely.

Tips for accuracy

  • Confirm that the units you enter match your chart, GPS, or instrument readout before relying on the Hull Speed Calculator.
  • In rough weather or poor visibility, cross-check important results with a second method or a crew member.
  • Treat simplified models (wave height, radar horizon, etc.) as estimates—real conditions vary with fetch, refraction, and equipment.

Practical examples

  • 32 ft LWL → ~7.6 kn
  • 40 ft → ~8.5 kn

Frequently Asked Questions

  • Is hull speed a hard limit?

    No. It is a useful rule of thumb for displacement monohulls. Light air, surfing, or planing can yield higher speeds.

  • How accurate is this calculator?

    This calculator uses standard maritime formulas and practical approximations where noted. It is suitable for planning and cross-checks; always verify safety-critical decisions with official references and local conditions.

  • Can I use this on mobile?

    Yes. OceanCalc tools are responsive and work on phones and tablets for quick checks on deck or in the cockpit.

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Results are estimates for educational purposes only and should not be used for real navigation decisions.

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