Displacement Hull Speed and Speed/Length Ratio
The displacement wall only enormous power pushes past.
Example
You enter
- Load waterline length LWL (ft) 25
You get
- Theoretical hull speed 6.70 kn
Checked against published worked examples from: Charles Doane (WaveTrain).
Details, formula, and sources
Scopea displacement hull is trapped by the wave it makes, so hull_speed = 1.34 x sqrt(LWL) knots is a practical ceiling. A 25 ft waterline caps near 6.70 kn no matter the power, until a planing hull breaks free. Enter an actual speed for the speed-length ratio SL = speed / sqrt(LWL) and the regime: SL <= 1.34 displacement, 1.34-2.5 semi-displacement, > 2.5 planing (that same 25 ft hull at 18 kn is SL 3.6, planing, off the wall). A boater reading a prop-based speed without the displacement wall over-predicts a heavy cruiser and over-props the engine. A planning estimate; the hull form, displacement, and power govern.
Formula or tablehull_speed_kn = 1.34 x sqrt(LWL_ft); SL_ratio = actual_speed / sqrt(LWL_ft); regime = SL <= 1.34 displacement / 1.34-2.5 semi-displacement / > 2.5 planing.
Source and editionDisplacement hull-speed relation (Froude speed-length theory), the 1.34 x sqrt(LWL) ceiling, by name; the actual hull form, displacement, and power govern.
Free accessThe speed-length relation is a published empirical result of displacement-hull / Froude theory; the waterline length comes from the boat's measurements.
What governsEstimate. AHJ and licensed professional govern.
API field nameslwl_ft, hull_speed_kn
- Displacement wall 1.34 x sqrt(LWL) is a practical ceiling for a pure displacement hullFroude speed-length theory
- Regime SL <= 1.34 displacement, 1.34-2.5 semi-displacement, > 2.5 planingdisplacement-hull practice
- Approximation the coefficient varies 1.34 to 1.4; hull form and displacement governscope of this tile