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.
Boat & Marine Calculator - Introduction
Enter your boat's waterline length (LWL) in feet and select hull type to see the theoretical maximum efficient speed in knots, along with the Speed-Length Ratio and Froude Number. Displacement hulls are constrained by their wave-making resistance; semi-displacement and planing hulls can push beyond those limits with enough power.
How It Works
For displacement hulls the formula is hull speed (knots) = 1.34 × √(LWL ft); semi-displacement hulls use a coefficient of roughly 2.75. The Froude Number Fn = v (m/s) / √(9.81 × LWL m) reaches approximately 0.4 at hull speed. Example: a 36-ft LWL sailboat has hull speed = 1.34 × √36 = 1.34 × 6 = 8.04 knots. Pushing past that requires exponentially more engine power and fuel.
Usage Scenarios
- Passage time estimate for a cruising sailboat: A 40-ft cruiser with 35-ft LWL has a hull speed of 1.34 × √35 ≈ 7.9 knots. Budgeting a 200-nm offshore leg, the crew plans on 26–28 hours at 90% hull speed rather than assuming open-water boat-speed claims.
- Comparing a trawler upgrade: A buyer is deciding between a 28-ft LWL trawler (hull speed ≈ 7.1 kts) and a 36-ft LWL model (hull speed ≈ 8.0 kts). The extra foot per second at cruise saves roughly 1.5 hours on a 100-nm coastal run, a meaningful difference for liveaboard use.
- Understanding when to push throttle: A 22-ft center console with a semi-displacement hull and 20-ft LWL hits a transition zone around 2.75 × √20 ≈ 12.3 knots before planing. Knowing this helps the skipper avoid the worst fuel-burn band—the speed range where drag peaks before the hull gets on top of the water.
Frequently Asked Questions
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.