About the Hull Speed Calculator
This hull speed calculator estimates how fast a displacement boat — a typical cruising sailboat, trawler or narrowboat — can go before it starts climbing its own bow wave. Enter the waterline length (not the overall length) and it applies the classic formula hull speed = 1.34 × √LWL, giving the answer in knots, miles per hour and kilometres per hour, plus how long a passage would take at that speed.
It is useful for sailors comparing boats, owners planning passage times, and anyone sizing an engine for a displacement hull, since pushing past hull speed takes rapidly increasing power. As a boat approaches hull speed it creates a wave as long as its own waterline, sitting between the bow wave and a trough at the stern; going faster means climbing that wave, which heavy hulls cannot do efficiently.
Hull speed is a rule of thumb, not a hard limit. Light, narrow boats, multihulls and planing hulls routinely exceed it. If you enter the boat’s displacement, the calculator also estimates a displacement-adjusted maximum using Dave Gerr’s empirical speed-length relationship, which lets lighter boats go somewhat faster than 1.34 and heavier ones a little slower.
With the default inputs, the hull speed is 7.34 knots. Change any value above to recalculate instantly.
How to use the hull speed calculator
- 1Pick feet/pounds or metres/kilograms.
- 2Enter the waterline length from the boat’s specifications (LWL, not LOA).
- 3Optionally enter the displacement to get a weight-adjusted top-speed estimate.
- 4Enter a passage distance in nautical miles to see the time at hull speed.
- 5Use the table to plan at a realistic cruising speed of 80–90% of hull speed.
Formula and method
A displacement hull moving through the water makes a wave system whose length grows with speed. Deep-water wave speed in knots is about 1.34 × √(wavelength in feet), so when the boat’s own bow wave stretches to the full waterline length the boat is travelling at about 1.34 × √LWL knots — the classic hull speed. Metric waterlines are converted to feet first (1 ft = 0.3048 m); the equivalent constant is about 2.43 for metres.
The optional adjustment uses the displacement/length ratio, which compares the boat’s weight in long tons (Δ in pounds ÷ 2240) with the cube of one-hundredth of its waterline length in feet. Naval architect Dave Gerr’s empirical formula, S/L = 8.26 ÷ (D/L)^0.311, estimates the maximum practical speed-length ratio: lighter boats (low D/L) can exceed 1.34, heavy ones fall short. Speeds are converted at 1 knot = 1.852 km/h = 1.15078 mph.
- V
- Hull speed in knots
- LWL
- Waterline length in feet
- Δ
- Displacement in pounds
- D/L
- Displacement/length ratio
- S/L
- Speed-length ratio (V ÷ √LWL)
Worked examples
30 ft waterline cruising sailboat, 11,000 lb
Hull speed is 1.34 × √30 = 7.34 knots, or about 8.45 mph and 13.6 km/h, so 100 nm takes about 13.6 hours. The D/L ratio is (11,000 ÷ 2240) ÷ 0.3³ ≈ 182, a moderate displacement, and Gerr’s formula gives a speed-length ratio of about 1.64 — roughly 9.0 knots.
10 m waterline in metric, displacement skipped
A 10 m waterline is 32.81 ft, so hull speed is 1.34 × √32.81 ≈ 7.67 knots (14.2 km/h). A 60 nm passage at that speed takes about 7.8 hours.
Heavy 40 ft trawler
A 38 ft waterline gives a classic hull speed of 1.34 × √38 ≈ 8.26 knots. At 36,000 lb the D/L ratio is about 292, a heavy displacement boat, and Gerr’s estimate of about 8.7 knots is only slightly above the classic figure.
Frequently asked questions
What is hull speed?+
Hull speed is the speed at which a displacement boat’s bow wave becomes as long as its waterline, so the boat sits in a trough between its bow and stern waves. Going faster means climbing over the bow wave, which requires a steep rise in power. It is roughly 1.34 × √(waterline length in feet) knots.
Can a boat go faster than hull speed?+
Yes. Light, slender hulls, catamarans and planing boats regularly exceed it, and any displacement boat can briefly surf faster down waves. For heavy cruising boats, though, the extra power needed beyond hull speed is so large that it acts as a practical limit.
Why use waterline length instead of overall length?+
The wave the boat makes depends on how much hull is in the water. Overhangs at the bow and stern do not count until the boat heels or squats, so the waterline length (LWL) is the right input. Using length overall would overstate the hull speed.
What is the hull speed formula in metres?+
Convert metres to feet and use 1.34 × √LWL, or use the equivalent metric constant: hull speed in knots ≈ 2.43 × √(LWL in metres). A 10 m waterline gives about 7.7 knots either way.
What is a good cruising speed for a displacement boat?+
Most displacement boats are run at about 80–90% of hull speed for a good balance of speed and fuel use. Fuel consumption rises sharply in the last few tenths of a knot below hull speed, so backing off slightly often saves a lot of fuel.