In a word.. length. However, this is a very misunderstood concept. While it is true that with displacement hulls such as kayaks and canoes, hull speed is a function of waterline length, there is more to the story than that. While longer boats can go faster so long as sufficient power is available, longer boats also carry more wetted surface area, which equals drag. This is why you frequently see longer boats being narrower in beam. The designer is trading stability for speed in an attempt to control wetted surface.
Hull speed is the theoretical maximum speed of a displacement hull. It can be calculated by this formula.
Hull Speed = 1.44 times the square root of the waterline length.
Here are some examples:
Water Line / Hull Speed: 12 feet 4.9 knots, 14 feet 5.4 knots, 16 feet 5.8 knots, etc. etc etc In recreational paddling or touring, average travel speeds are 2-4 knots. So if you’re not racing, how long a boat do you really need?
Remember this: in general shorter boats are lighter and easier to paddle (less wetted surface, less windage).
