Hull Speed Calculator
Calculate the theoretical hull speed of a displacement vessel based on waterline length using the 1.34 constant.
Results
What is it?
Hull speed is the theoretical maximum speed of a displacement vessel. The formula is: Hull Speed (kts) = 1.34 x sqrt(LWL in feet), where LWL is the waterline length. Beyond this speed, the boat must climb its own bow wave, requiring exponentially more power. This formula comes from the relationship between wave speed and wavelength in water.
How to use
1. Enter the waterline length (LWL) of your vessel in feet. This is the length measured at the waterline, not the overall length. 2. Enter the displacement in pounds. 3. The result shows hull speed in knots, mph, and km/h.
Example scenario
A classic sloop with a 35-ft waterline. Hull speed = 1.34 x sqrt(35) = 1.34 x 5.916 = 7.93 knots. This is a typical cruising speed for a well-found 40-ft sailing yacht. Pushing beyond 8 kts requires significantly more engine power and fuel.
Pro tip
The 1.34 constant applies to hulls in salt water. In fresh water, use 1.32. Modern light-displacement and semi-planing hulls can exceed hull speed, as can narrow hulls (canoes, kayaks) and multi-hulls (catamarans). A speed-to-length ratio (V / sqrt(LWL)) above 1.34 indicates the hull is operating in semi-planing or planing mode.