Hull Speed Calculator

Calculate theoretical hull speed from waterline length using the speed-length ratio formula.

Inputs

Waterline Length (LWL)

Speed-Length Ratio

Tab to move between fields

Results

Hull Speed

7.34 knots

Hull Speed (mph)8.45 mph

Hull Speed (km/h)13.59 km/h

Froude Number0.399

How to Use This Calculator

Enter the waterline length (LWL) in feet — measure from bow to stern at the waterline.
Adjust the speed-length ratio if needed (default 1.34 for displacement hulls).
Read hull speed in knots, mph, and km/h.
Check the Froude number — values above 0.4 indicate you are pushing beyond displacement speed.
Use this result to size engines and propellers appropriately for your hull type.

How the result changes with Speed-Length Ratio

Speed-Length Ratio	Hull Speed
0.9	4.93 knots
1.15	6.3 knots
1.45	7.94 knots
1.7	9.31 knots

What each input means

Waterline Length (LWL): Length of the hull at the waterline in feet. Longer hulls have higher theoretical hull speeds.
Speed-Length Ratio: Typically 1.34 for displacement hulls. Semi-displacement hulls can exceed 1.5.

How this is calculated

Worked example, using the default values

Identify Input Parameters
Waterline Length (LWL) = 30, Speed-Length Ratio = 1.34 = 2 input(s) provided
Calculate Hull Speed
Hull Speed
7.34 = 7.34
Calculate Hull Speed
Hull Speed
8.45 = 8.45
Calculate Hull Speed
Hull Speed
13.59 = 13.59

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