Waterline Length for a Target Hull Speed
The inverse of the hull-speed tile: the waterline a pure displacement hull needs to reach a target speed, LWL = (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 and source
waterline_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.
Displacement hull-speed relation (Froude speed-length theory), solved for the waterline, by name; the actual hull form, displacement, and power govern.
Audience
This tile is built for mechanics and the adjacent professions in the Mechanic - Auto, Marine, Aviation group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.