Boat Hull Speed Table
I got tired of looking around for my calculator, so I made up this handy table in knots, mph and km/h. 

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

 
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).