# What hull speed is actually telling you
A displacement hull pushes a wave from the bow and another from the stern. When those crests sit about one waterline apart, the boat is travelling with its own wake. Extra power then mostly heaps water up instead of buying speed. That condition is what sailors call hull speed.The calculator uses the common amateur ratio 1.34 knots per square root of waterline feet. Fine, long hulls can sit a little higher; barges sit lower. Treat the result as a teaching mark, then compare a real log speed against it.# Worked waterline examples
| Waterline | Hull speed | Typical boat |
|---|---|---|
| 4.2 m / 13.8 ft | 5.0 kn | Dinghy |
| 6.5 m / 21.3 ft | 6.2 kn | Daysailer |
| 10 m / 32.8 ft | 7.7 kn | Family cruiser |
| 13.5 m / 44.3 ft | 8.9 kn | Bluewater yacht |
| 18 m / 59.1 ft | 10.3 kn | Small workboat |
# How to read the bow wave
- Below the mark the wave is short and the boat is still cheap to drive.
- Near the mark the bow wave steepens and fuel or sail power buys less speed.
- On the mark the wavelength matches LWL. A displacement hull is at the classic barrier.
- Above the mark a displacement hull is climbing its wake. A planing hull may already be up.