Hull speed calculator and sailboat performance ratios
Type in a waterline length to get theoretical hull speed, then add displacement, sail area and beam to see the classic design ratios that sailors use to compare boats on paper before a test sail.
Theoretical hull speed
0 kn
What hull speed really means
Hull speed is the speed at which the wave a displacement hull makes is as long as the hull's waterline. The formula used here is 1.34 times the square root of waterline length in feet. Pushing past it takes a steep increase in power, which is why heavy cruising boats tend to settle just below it.
Light boats with flat aft sections, and especially catamarans with slim hulls, can exceed hull speed when they start to surf or plane. Treat the number as the comfortable top end for a conventional monohull, not as a hard limit.
Reading the ratios
Displacement to length compares weight with waterline length. Below about 120 is ultralight, 200 to 300 is moderate cruising territory and above 300 is heavy. Sail area to displacement measures power. Around 16 to 20 is typical for cruisers, above 20 points to a lively boat that will need reefing earlier.
Capsize screening divides beam by the cube root of displacement. Values under 2.0 have traditionally been used as a rough sign of suitability for offshore work. Ted Brewer's motion comfort ratio estimates how quickly a boat will move in a seaway, higher numbers meaning a slower, easier motion.
Frequently asked questions
How do you calculate hull speed?
Multiply 1.34 by the square root of the waterline length in feet. A 36 foot waterline gives about 8 knots.
What is a good displacement to length ratio for cruising?
Most offshore cruisers fall between about 200 and 300. Lower numbers are faster but more sensitive to cruising weight.
What does a capsize screening value under 2 mean?
It is a traditional rule of thumb suggesting the boat has enough weight relative to beam for offshore use. It is not a safety certification.
Can a sailboat go faster than hull speed?
Yes. Light planing hulls and catamarans regularly exceed it, especially when surfing downwind.