Hull Speed Calculator 🚢
Calculate the theoretical maximum hull speed for your boat. Supports displacement, semi-displacement, and planing hulls. Includes Speed-Length Ratio and Froude Number.
Hull Speed Guide
Hull speed is the theoretical maximum speed a displacement hull can travel efficiently. It is determined by the waterline length (LWL) using the formula V = 1.34 × √LWL, where V is in knots and LWL is in feet.
The Froude Number (Fn) quantifies this relationship. At Fn ≈ 0.4 (hull speed), the bow wave and stern wave lock together, creating enormous resistance. Exceeding hull speed requires exponentially more power.
Semi-displacement hulls can push past hull speed (SLR 2.0–3.0) thanks to hull design that allows partial planing. True planing hulls have no theoretical speed limit — they skim on top of the water once they reach planing speed.
FAQ
What is hull speed?
Hull speed is the theoretical maximum speed of a displacement hull, calculated as 1.34 × √(waterline length in feet). At this speed, the boat sits between its own bow and stern waves. Exceeding hull speed requires significantly more power.
How accurate is the 1.34 hull speed formula?
The 1.34 constant is a well-established rule of thumb for displacement hulls. Actual hull speed varies slightly with hull form, displacement, and waterline shape. Heavier, fuller hulls may be slightly slower; fine, light hulls may slightly exceed the formula.
Can a displacement hull exceed hull speed?
Yes, but at greatly increased fuel and power cost. Semi-displacement hulls are designed to operate up to 2–3× hull speed. Planing hulls can exceed hull speed entirely by riding on top of the water rather than pushing through it.
What is the Froude number?
The Froude number is a dimensionless ratio comparing a vessel's speed to the speed of gravity waves of the same length. At hull speed, the Froude number is approximately 0.4. Values above 0.4 indicate the hull is being pushed past its efficient displacement speed.