Hull Speed

Hull speed is an estimate of the fastest speed a displacement hull can efficiently travel before it starts climbing its own bow wave and requires much more power to go faster. The key idea is that a longer waterline makes a longer wave pattern, so the boat can move a bit faster before wave-making resistance rises sharply; that is why the formula scales with the square root of waterline length. It is a rule of thumb, not a hard limit, and works best for conventional displacement boats in knots using waterline length in feet.

Use this when estimating cruising performance, comparing similar displacement boats, or checking whether a boat is approaching the speed where wave drag rises quickly. It is especially useful in naval architecture, sailing, and basic boat design calculations.

• Using overall boat length instead of the waterline length
• Forgetting that the result is in knots, not mph
• Treating hull speed as an absolute maximum for every boat type