Boat Speed Calculator
Find hull speed in knots.
Displacement hull speed is roughly 1.34 × √waterline length.
How the Math Works
The Boat Speed Calculator uses a fundamental principle in naval architecture called hull speed. This is the theoretical maximum efficient speed a displacement hull can achieve, determined by the formula: hull speed = 1.34 multiplied by the square root of the Length at Waterline (LWL) in feet. The constant 1.34 comes from the physics of wave-making resistance - as a boat moves through water, it creates a wave at its bow, and at hull speed, the boat is essentially trapped between two waves, making further acceleration increasingly difficult. Taking the square root of LWL reflects how longer boats can achieve higher speeds because their waterline length allows them to ride over their own waves more effectively.
Practical Applications
To use this calculator, simply enter your boat's Length at Waterline (LWL) in feet - this is the distance between the waterline at the bow and stern measured at rest. The calculator instantly applies the formula and returns the hull speed in knots. For example, a boat with a 25-foot waterline length would have a hull speed of approximately 6.7 knots (1.34 × √25). This measurement helps you set realistic performance expectations and plan voyages - if you know your boat's hull speed, you can estimate travel time by dividing distance by this speed, and you'll understand when your boat is approaching its efficient maximum velocity.
Day-to-Day Use
Knowing your boat's hull speed helps you make better decisions whether you're planning a weekend trip or choosing between different boats. Before heading out, you can estimate arrival times and fuel consumption based on this speed. When shopping for a boat, hull speed gives you a quick way to compare performance characteristics - longer boats at the same draft will generally be faster. It also helps you understand why your boat feels sluggish when trying to exceed hull speed, and why small increases in speed require disproportionately more engine power. This knowledge makes you a more informed boater who can plan safer, more efficient trips and choose the right boat for your intended use.
Worked example
25 ft → 6.7 kn.
FAQ
Planing hulls?
They can exceed hull speed by rising onto the plane.