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SL Ratio
A simple way to think of theoretical hull speed is that a vessel is limited by the wave it creates. That is, because it is a displacement vessel it doesn’t climb its bow wave as a planning vessel would. Going faster than the theoretical hull speed is possible but the wave moves under the transom so the boat has to plow through the water at an angle, bow up which then requires exponentially more power to move the vessel once theoretical hull speed is reached.
So a displacement boats theoretical hull speed is when the wave length is equal to the waterline length. The important thing here is the length of the wave is relative to the length of the boat.
The SL Ratio is a simple formula for calculating displacement hull speed which is typically 1.34
- Velocity = (g * Length / 2 pi) ^ ½
- g = acceleration of gravity (32.15 ft/sec ^2)
- Convert ft./sec. to Knots and the result is 1.34 * L^½
- Metric V(m/s) = 1.25* LWL^1/2
But of course, not all hulls are the same; depending on other factors like displacement and beam the boat may create a larger or smaller bow wave in proportion to its length. Heavier displacement vessels will have difficulty getting over 1.1 times the theoretical hull speed whereas vessels with lighter displacement and slimmer hulls like catamarans will have less trouble.
David Gerr’s propeller handbook is an excellent reference with some extensions to this formula.
See:
Boat Hull
Boat Hull Types :
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