Hull Speed calculation and Chart

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. Theoretical displacement hull speed is calculated by the formula: velocity in knots = 1.35 x the square root of the waterline length in feet.

I got tired of looking around for my calculator, so I made up this handy chart. Since I usually think in terms of statute mph, I included that too. I also included kilometers per hour. (Note that this hull speed formula does not apply to needle-like hulls such as racing shells).

Waterline length in feet	Hull speed knots	Hull speed mph	Hull speed kmph		Waterline length in feet	Hull speed knots	Hull speed mph	Hull speed kmph
8	3.8	4.4	7.0		20	6.0	6.9	11.1
9	4.0	4.7	7.4		21	6.2	7.1	11.5
10	4.3	4.9	8.0		22	6.3	7.3	11.7
11	4.5	5.2	8.3		23	6.5	7.5	12.0
12	4.7	5.4	8.7		24	6.6	7.6	12.2
13	4.9	5.6	9.1		25	6.7	7.8	12.4
14	5.0	5.8	9.3		26	6.9	7.9	12.8
15	5.2	6.0	9.6		27	7.0	8.1	13.0
16	5.4	6.2	10.0		28	7.1	8.2	13.2
17	5.6	6.4	10.3		29	7.3	8.4	13.5
18	5.7	6.6	10.5		30	7.4	8.5	13.7
19	5.9	6.8	10.9		31	7.5	8.6	13.9

 

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.