Hull Speed Calculator

About the Hull Speed Calculator

Calculates the theoretical hull speed of a displacement boat from its waterline length, the speed at which its own bow wave limits further easy gains.

How to use

Enter the waterline length in feet, then click Calculate.

Worked example

A 30 ft waterline → 1.34 × √30 ≈ 7.3 knots.

How to use this calculator

1. Measure the boat's length at the waterline in feet.
2. Enter that value into the calculator.
3. Use the result as the boat's theoretical hull speed in knots.

The formula explained

• \(\text{hull speed}\) = the theoretical maximum speed of a displacement boat in knots
• \(1.34\) = a constant used in the common hull speed approximation
• \(\text{LWL}\) = length at waterline, usually measured in feet

Step by step method

1. Find the boat's waterline length, not the overall length.
2. Apply the hull speed formula using the waterline length.
3. Multiply the square root of the waterline length by 1.34.
4. Read the result as an estimate of speed in knots.

Worked example

For a displacement sailboat with a waterline length of 25 feet, you can estimate its hull speed.

1. Use the formula \(\text{hull speed} = 1.34\sqrt{25}\).
2. Calculate the square root of 25, which is 5.
3. Multiply 5 by 1.34 to get 6.7.

Tips and common mistakes

• Use waterline length, not overall length, because that is what the formula is based on.
• The result is theoretical, not a guaranteed cruising speed.
• This estimate is most relevant for displacement hulls, not planing boats.
• Be consistent with units, since this common version assumes feet and knots.

Frequently asked questions