Hull Speed Calculator
Find a displacement hull's theoretical top speed.
Hull speed is the practical top speed of a displacement boat.
How the Math Works
The Hull Speed Calculator uses the formula hull speed = 1.34 × √(waterline length in feet). This equation derives from empirical observations that displacement hulls (boats that move by displacing water) reach a theoretical maximum speed proportional to the square root of their waterline length. The constant 1.34 accounts for the relationship between speed in knots and length in feet, while the square root means that hull speed increases with longer waterline lengths but at a diminishing rate—doubling the length doesn't double the speed. This provides an idealized upper limit for standard displacement hulls under calm conditions.
Practical Applications
To use this calculator, measure the waterline length of your boat—the distance between the points where the hull meets the water at rest—and input it into the formula. For example, a 30-foot boat would yield a hull speed of 1.34 × √30 ≈ 7.3 knots. This value helps sailors estimate maximum achievable speeds, plan voyages, or compare performance between boats. Note that actual speeds may vary due to hull design, weight distribution, sea conditions, and engine power, but this calculation offers a reliable baseline for displacement hulls.
Day-to-Day Use
Knowing your boat's hull speed helps in practical scenarios like trip planning, safety assessments, and equipment choices. If you're preparing for a sailing trip, you can estimate travel times or determine if your boat can safely navigate certain waters. For recreational boaters, it sets realistic expectations for performance—avoiding disappointment if you can't exceed the theoretical limit. It also aids in comparing different boats when shopping, ensuring you select a vessel suited to your speed and distance needs.
Worked example
25 ft waterline → 6.7 knots.
FAQ
Can boats exceed it?
Planing hulls can; displacement hulls struggle past hull speed.