Boat Hull Speed
Calculator
Results
- Hull speed (knots)
- 7.580184
- Hull speed (km/h)
- 14.038502
- Hull speed (mph)
- 8.72312
Aviation and marine results
| Hull speed (knots) | 7.580184 |
| Hull speed (km/h) | 14.038502 |
| Hull speed (mph) | 8.72312 |
formula-map diagram
- Hull speed (knots)
- 7.580184
- Hull speed (km/h)
- 14.038502
- Hull speed (mph)
- 8.72312
Aviation and marine relationship
Formula
Hull speed = 1.34 × √(waterline length in ft) knots= 7.5801846943198
Note
This is a simplified model: it applies a standard navigation, performance or seamanship formula to the numbers you entered. True airspeed and density altitude use the ISA troposphere model for dry air and ignore humidity, compressibility and instrument or position error, so they are not a substitute for your aircraft's flight manual or an air data computer. Wind, climb, descent and fuel figures assume a steady wind and a constant speed and fuel flow for the whole leg; they ignore manoeuvring, holding, taxi and start-up fuel, terrain, turbulence and air traffic control routing. Takeoff distance scales an entered sea level figure by a rule-of-thumb percentage per 1000 ft of density altitude and is not a certified performance chart. Weight and balance results depend entirely on the arms and weights you enter and must be checked against the approved loading envelope. Hull speed is the classic 1.34 × √LWL displacement estimate, boat fuel burn is a horsepower-based approximation, anchor scope is a rule of thumb, and the rule of twelfths is a linear approximation of a sinusoidal tide that does not hold in all locations. Always use official charts, tide tables, the aircraft or vessel manuals and current weather, and treat these numbers as planning estimates only.
More in Aviation and marine
See all →Frequently asked questions
What is hull speed and what does it represent?+
Hull speed is the theoretical maximum speed a traditional displacement hull can efficiently achieve, based on the length of its waterline. Beyond this speed, the boat would need to climb over its own bow wave, requiring disproportionately more power for each additional knot of speed.
How is hull speed calculated from waterline length?+
The common formula is: hull speed (knots) = 1.34 × √(waterline length in feet). This comes from the physics of the wave pattern a displacement hull creates, where wave length becomes proportional to the square of speed.
Why can't a displacement boat just add more power to go faster than hull speed?+
Past hull speed, the boat's own bow and stern waves lengthen until the vessel is essentially trying to climb its own bow wave, and the power required to overcome this increases dramatically — often requiring many times the power for only a small speed gain, which is why most displacement boats cruise near or below hull speed.
Does hull speed apply to planing boats the same way?+
No — planing hulls are designed to rise up and skim across the water surface at higher speeds rather than push through it, effectively escaping the wave-making resistance that limits displacement hulls. The hull speed formula is meaningful mainly for displacement and semi-displacement hulls, not full planing hulls.
What's a common misconception about hull speed?+
That it's a hard physical limit the boat cannot exceed. In practice, many displacement sailboats can exceed hull speed somewhat, especially surfing down waves or with a very powerful engine, but doing so becomes increasingly inefficient and is not sustainable cruising behavior.