HS
Theoretical hull speed
Theoretical hull speed estimates the speed where a displacement hull's wave-making rises sharply: about 1.34 times the square root of LWL in feet, result in knots. FairHelm judgment: useful for passage planning bands; modern hulls still exceed it when surfing or with enough power.
Formula
Hull speed (knots) ≈ 1.34 * sqrt(LWL_ft)
LWL in feet. Result in knots. Metric LWL must be converted to feet first.
History
The square-root waterline rule comes from classical naval architecture for displacement craft. Yachting adopted the 1.34 constant widely. Planing and semi-displacement forms break the simple story.
Rule of thumb
A 30 ft LWL boat sits near 7.3 knots theoretical hull speed. Expect cruise RPM targets below that in displacement mode. Downwind surfing in a Baltic swell can exceed the number briefly without magic.
Limits
Not a speed limit for modern fin-keel cruisers with fine entries. Ignores sail power, sea state, and fouling. Using LOA instead of LWL overstates the estimate. Multihulls and planing powerboats need different tools.
Boat examples
- Hallberg-Rassy 36
Displacement cruiser: hull speed frames realistic day runs better than brochure 'max speed' claims.
- Linjett 40
Longer LWL raises theoretical hull speed; confirm LWL from a lines plan or solid brochure table.
- Beneteau First 31.7
Lighter performance hull may surf above theoretical hull speed more often than a heavy long-keel boat.
FAQ
- Why 1.34?
- It is the common imperial constant tying knots to feet of waterline for displacement wave-making. Other constants appear in metric rewrites.
- Does hull speed apply under power?
- The wave-making idea still applies to displacement motoring. Semi-displacement hulls with enough horsepower can exceed it.
- How should I use this on a passage plan?
- Use a fraction of hull speed as a conservative SOG guess in displacement mode, then adjust for weather routing and fouling.