Twenty-five-foot LWL
At coefficient 1.34, the empirical reference is approximately 6.7 knots.
Sailboat Hull Speed Calculator
Created by: James Porter
Last updated:
Calculate an editable empirical displacement hull-speed reference, speed-length ratio, actual/reference percentage, and travel-time scenarios.
Sailboat Hull Speed Calculator
BoatingExplore an editable empirical speed-length reference without treating it as a maximum or guarantee.
kn
nm
What is a Sailboat Hull Speed Calculator?
This calculator presents the familiar speed-length relationship as an editable empirical reference, not a maximum.
It converts metric LWL to feet, calculates a reference speed, and compares optional actual speed and constant-speed travel time.
How the Sailboat Hull Speed Calculator Works
Reference speed is coefficient times square root of LWL in feet.
Actual speed produces speed-length ratio and percentage of the selected reference.
Reference kn = coefficient × √LWL(ft)
SLR = actual kn ÷ √LWL(ft)
Input Guide and Conventions
Use length at waterline rather than overall length. Verify whether the published LWL represents the loaded static condition or a design reference.
The coefficient is editable because 1.34 is a conventional displacement-hull reference, not a universal constant. Changing it creates a comparison, not a prediction.
Optional actual speed should come from a clearly documented speed-over-ground or speed-through-water observation with current and conditions noted.
Hull mode matters: semi-displacement, planing, surfing, foiling, and multihull behavior can make the classic relationship a poor descriptor.
Examples
How to Interpret the Results
The reference speed marks a familiar wave-making comparison point. It does not identify an abrupt barrier or the power required to exceed it.
Actual/reference percentage can exceed 100 percent without proving an input error. Conditions, current, hull type, and dynamic waterline must be examined.
Travel time assumes the selected constant speed for the entire entered distance. Manoeuvres, calms, waves, current, traffic, and routing are excluded.
Comparisons between boats should use the same coefficient, LWL convention, loading condition, and speed basis.
Recommended Planning Workflow
Start with verified design dimensions, then compare the empirical number with polars, sea-trial records, and logged passage speeds.
Run several coefficients and actual-speed cases to understand sensitivity instead of treating the default as exact.
Use the passage-time result only after checking route distance, weather, current, crew, and safe alternatives.
Common Applications
- • First-pass performance comparison
- • Speed-length education
- • Passage-time scenarios
Planning Tips
Use verified LWL.
Keep hull mode and conditions visible.
Do not treat the result as a safe or guaranteed speed.
Frequently Asked Questions
Is hull speed a maximum speed?
No. The classic coefficient-times-square-root relationship is an empirical reference for displacement-wave behaviour. Hull form, power, sail force, surfing, planing, foils, multihulls, sea state, current, and loading can produce different results.
Why is the coefficient editable?
The familiar 1.34 coefficient is a convention, not a physical constant. Editing it exposes the assumption rather than presenting one number as universal.
What is speed-length ratio?
It is speed in knots divided by the square root of waterline length in feet. It is descriptive and does not establish safety, efficiency, or structural limits.
Which waterline length should I use?
Use the LWL appropriate to the vessel and condition. Dynamic waterline can change with heel, trim, load, and speed.
Sources and References
- World Sailing. Equipment Rules of Sailing 2025–2028, accessed July 16, 2026; https://www.sailing.org/document/equipment-rules-of-sailing-2025-2028/.
- Offshore Racing Congress. Measurements and International Measurement System resources, accessed July 16, 2026; https://orc.org/measurements.
- US Sailing. Speed Guide Explanation, apparent/true wind mathematical relations, accessed July 16, 2026; https://www.ussailing.org/wp-content/uploads/2018/01/Speed-Guide-Explanation.pdf.
- Royal Yachting Association. Speed Charts and sailing-wind resources, accessed July 16, 2026; https://www.rya.org.uk/racing/speed-charts/.
- Empirical yacht-ratio and vector conventions are documented in each calculator.
Model limitation: The empirical reference is not a universal maximum, polar prediction, engine recommendation, structural limit, or performance guarantee.