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