Draft and Displacement Calculator

Displacement is the weight of water a boat displaces when floating. By Archimedes' principle, a floating object displaces a volume of water equal in weight to the object itself. A 20,000-lb displacement boat displaces 20,000 lbs of water. Displacement is used to classify boat types (light, medium, heavy displacement).

Draft (or draught) is the vertical distance from the waterline to the lowest point of the hull (keel). It determines the minimum water depth needed for safe navigation. A boat with a 5-foot draft needs at least 5 feet of water plus a safety margin to avoid grounding.

Archimedes' principle states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. A boat floats when the weight of displaced water equals the boat's weight. Displacement (lbs) = Volume submerged (ft3) * seawater density (64 lbs/ft3) for saltwater, or 62.4 lbs/ft3 for freshwater.

The displacement-to-length ratio (D/L) is a measure of a boat's relative heaviness: D/L = (Displacement in long tons) / (0.01 * LWL in feet)^3. A D/L below 100 is an ultralight racing yacht; 100 to 200 is light; 200 to 275 is moderate; 275 to 350 is heavy; above 350 is very heavy (blue water cruising or traditional designs).

Yes. Saltwater is denser (approximately 64 lbs/ft3 or 1,025 kg/m3) than freshwater (62.4 lbs/ft3 or 1,000 kg/m3). The same boat will float slightly higher in saltwater and sit slightly lower in freshwater. The difference is about 2.5%, which can be significant for vessels loaded near their marks.