Waterline Length for a Target Hull Speed
The waterline a pure displacement hull needs to reach a target speed.
Example
You enter
- Target hull speed (kn) 8
- Speed-length coefficient (~1.34) 1.34
You get
- Required waterline length 35.6 ft
Checked against published worked examples from: Charles J. Doane (Wave Train / boats.com).
Details, formula, and sources
ScopeLWL = (target speed / 1.34)^2. An 8 kn displacement cruiser needs a ~35.6 ft waterline; 12 kn needs ~80 ft. This is the displacement ceiling - a planing hull breaks free with enough power. The coefficient is editable (~1.34-1.4). A planning estimate; the hull form, displacement, and power govern.
Formula or tablewaterline_length_ft = (target_hull_speed_kn / coefficient)^2, the displacement hull-speed relation hull_speed = 1.34 x sqrt(LWL) solved for the waterline; coefficient default 1.34.
Source and editionDisplacement hull-speed relation (Froude speed-length theory), solved for the waterline, 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.
What governsEstimate. AHJ and licensed professional govern.
API field namestarget_hull_speed_kn, coefficient, waterline_length_ft
- Displacement wall LWL = (speed / 1.34)^2 is the waterline for the displacement ceilingFroude speed-length theory
- Approximation the coefficient varies 1.34 to 1.4; a planing hull breaks the ceilingscope of this tile