We see or hear this term used a lot and unfortunately it is widely misunderstood as it applies to kayaks. The traditional formula for hull speed in knots is 1.34 times the square roof of the waterline length (in feet) of a displacement hull. The concept was demonstrated by William Froude working with models of naval ships for the British admiralty about 150 years ago.
This does not directly apply to kayaks since they are not displacement hulls, but rather semi-displacement hulls. There are other limitations -- catamarans and surf skis for instance where the length to beam ratio is very large; surfing hulls and/or planing hulls and their kin are other examples that don't fit this formula.
An excellent example of a displacement hull is shown here (no, Jason, this is not a Pro Angler) -- a 73,000 ton aircraft carrier, which happens to be temporarily blocking the path of this somewhat lighter kayak.
At the traditional hull speed, boats are sailing in a trough formed by the bow and stern wave and depending on the hull shape, displacement and power available, may be trapped or even (rarely) sailed under or capsized at faster speeds.
For light boats the trough isn't deep enough to trap it if there is enough power to climb out. For a Hobie Adventure for example, "hull speed" is about 5.36 knots or 6.2 MPH, yet sprint speed is closer to 7.5+ mph and sailing speed (Adventure Island) can exceed 10 MPH with no ill effects. So obviously 6.2 MPH is not really the hull speed at all!
Most naval designers have long ago abandoned the traditional formula for hull speed and adopted a more appropriate and realistic Froude number to describe the great variation of hull performances that are achievable in modern boats. As kayak users, it would be desirable for us not to allow our thinking to be limited by outmoded and misapplied theories.
