Boat Hull Speed Chart
I got tired of looking around for my calculator, so I made up this handy chart in knots, mph and km/h.
Displacement hull speed is an important indicator of how fast a displacement-type boat will go. It is the speed at which a boat begins to climb it's own bow wave, essentially going uphill.
Hull Speed Formula
Theoretical displacement hull speed is calculated by the formula: velocity in knots = 1.35 x the square root of the waterline length in feet.
Example: The Odyssey 18 rowboat has an overall length of 18' and a waterline length of 17'-7". On the chart 17'-7" is about half way between 17 and 18 feet, so hull speed is 6.5 mph.
(Note that this hull speed formula does not apply to needle-like hulls such as racing shells).