The formula is: 1.34 X the square root of LWL = Speed (in knots)
A boat's LWL (Length at Water Line) is equal to the distance between the wave crests created at the bow and stern by the boat's motion through the water.
A boat rides in the trough between the two wave crests when it is at it's theoretical maximum hull speed . In reality this formula was developed for keel boats and many other factors including displacement, beam, and the shape of the bow and stern are also involved.
Planing hulls, such as centerboard dinghies, are capable of exceeding their theoretical hull speed when they achieve a planing attitude. This is accomplished under wind conditions that are strong enough to propell the boat up it's own bow wave. The boat is lifted partially out of the water as it rides the crest. (Yeeeee Ha)
Theoretical hull speed for Holder 14 designs having a LWL of 12'6" = 4.7 kt (5.69 mph)
1 knot = 1 nautical mile per hour
1 knot = 1.2 mph (1.9 km/hr )
How far does a nautical mile get you? . . . 60 nautical miles = 1 degree of latitude
A good discussion on Hull Speed as well as a solid text on sailing can be found
in the book, Colgate's Basic Sailing By Steve Colgate.
Excerpts from this text can be viewed on-line at:
SailNet - Hull Speed Demystified
SailNet - The Balance of Hull and Sails
SailNet - Learning to Sail