About hull speed
Hull speed is the traditional estimate of the top speed of a displacement hull — a boat that moves through the water by pushing it aside rather than skimming over the surface. As a boat moves, it generates a bow wave and a stern wave. The faster it goes, the longer the wavelength of that wave system. When the wavelength grows to match the boat's waterline length, the hull effectively sits in a trough between its own bow and stern waves and can no longer climb the wave in front of it without a large jump in power. That speed is called hull speed.
The formula
The classic expression, credited to naval architects and popularized in imperial units, is:
- Hull speed (knots) = 1.34 × √(LWL in feet)
- Metric equivalent: Hull speed (knots) ≈ 2.43 × √(LWL in meters)
The coefficient 1.34 is not arbitrary. Deep-water gravity waves travel at v = √(gL / 2π), where g is gravitational acceleration and L is the wavelength. Setting the wavelength equal to the waterline length and converting feet-per-second to knots yields the factor 1.34. Because the relationship depends on the square root of length, doubling a boat's waterline length raises its hull speed by only about 41 percent, not 100 percent.
Why waterline length, not overall length
Hull speed depends on the length of the hull actually in the water (LWL), not the boat's overall length (LOA). Overhangs at the bow and stern do not count while the boat sits still. However, as a boat heels or is driven hard, those overhangs can immerse and lengthen the effective waterline — one reason many boats slightly exceed their static hull-speed estimate.
Common reference points
- A 16 ft waterline dinghy: 1.34 × √16 = 1.34 × 4 = 5.4 knots.
- A 25 ft waterline cruiser: 1.34 × 5 = 6.7 knots.
- A 36 ft waterline sailboat: 1.34 × 6 = 8.0 knots.
- A 400 ft waterline ship: 1.34 × 20 = 26.8 knots.