Hull Speed

This is a somewhat mythical thing. First let us note that hull speed is not an absolute limit on the speed of a kayak. It is merely a point at which the resistance to motion increases significantly. It is possible to paddle a kayak above its hull speed - Olympic K1 paddlers do it all the time.

Hull speed is related to the waterline length of the hull. The concept was discovered by William Froude (1810-1870) when he observed the behavior of models of similar shaped hulls. Basically, he observed that the hydrodynamic resistance was dependent on the shape of the waves around the hull.

When a hull moves through the water, the water must part to go around the hull. The energy imparted into the water by the hull causes waves to form. A transverse bow wave forms at the front of the hull. If you look at the side of a vessel as it passes by, you can see the bow wave. The back of the wave forms a trough and then a second wave and so on. The stern also creates a wave. The distance between these waves (the wavelength) is a function of the speed of the wave. This of course is the same as the speed of the hull. Froude observed that the resistance significantly increased when the wavelength increased to match the waterline length of the hull. At this point, the bow's second wave and the stern wave coincide at the stern.

The wavelength and speed at which this happens can be calculated easily and gives the equation:

  L  = 2 π v<sup>2</sup> / g

Where: L = length in feet π = 3.14159… v = velocity in knots g = gravitational acceleration, 32.2 ft/sec^2

Which results in:

    L = 0.557 v<sup>2</sup>or v = 1.34√L

Example:

The waterline length of an Olympic K1 kayak is 17.06 ft. The hull speed is therefore:

   v = 1.34√17.06 = 1.34*4.13 = 5.53 kt = 6.37 mph = 10.25 kph.

Now compare Eirik Veraas Larsen's gold medal winning time of 3:25.9 in the K1 1000m event at Athens.

  v = 1km/3:25.9m =  1*60/3.43 = 17.49 kph.

Thus the Norwegian exceeded the kayak's hull speed by over 7kph or by 70%!