Find the theoretical maximum efficient speed of a displacement hull from its waterline length. Results in knots, mph and km/h, plus a speed-length ratio check for any speed you enter, a chart and a reference table.
Last updated:
Use the length at the waterline, not length overall (LOA). It is usually on the spec sheet as LWL or DWL.
× √LWL
knots
Hull speed by waterline length
Hull speed table
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How to use the Hull Speed Calculator
Enter the waterline length (LWL) and choose feet or meters, or drag the slider.
Keep the 1.34 constant for the classic hull speed, or choose another preset or type your own.
Read the hull speed in knots, mph and km/h, with the formula underneath.
Optionally type a boat speed to see its speed-length ratio, how close it is to hull speed and which regime it falls in.
Use the chart and table to compare boats of other lengths, and download the table as CSV.
How hull speed is calculated
A boat moving through water creates a wave system. In deep water a wave's speed depends on its length: c = √(g × λ ÷ 2π). When the bow wave's length λ equals the waterline length, that relationship gives the classic rule:
Hull speed (knots) = 1.34 × √LWL (feet)
Hull speed (knots) ≈ 2.43 × √LWL (meters)
Waterline needed for a speed (feet) = (speed in knots ÷ 1.34)²
Example: a sailboat with a 26 ft waterline has a hull speed of 1.34 × √26 = 1.34 × 5.10 = 6.83 knots (7.86 mph, 12.65 km/h). To reach 8 knots as a displacement boat it would need about (8 ÷ 1.34)² = 35.6 ft of waterline.
Hull speed reference
Waterline length
Knots
mph
km/h
16 ft (4.9 m)
5.36
6.17
9.93
20 ft (6.1 m)
5.99
6.90
11.10
25 ft (7.6 m)
6.70
7.71
12.41
30 ft (9.1 m)
7.34
8.45
13.59
35 ft (10.7 m)
7.93
9.12
14.68
40 ft (12.2 m)
8.47
9.75
15.70
50 ft (15.2 m)
9.48
10.90
17.55
Speed-length ratio regimes
Speed ÷ √LWL (ft)
Regime
What it means
up to about 1.0
Displacement, efficient
Low wave-making drag; the most economical cruising speeds.
1.0 to 1.34
Approaching hull speed
Drag climbs quickly. Many cruisers motor at about 1.1 to 1.2 to save fuel.
1.34 to about 3
Semi-displacement
Needs a hull designed to partly lift and a lot more power.
above about 3
Planing
The hull rides on top of its bow wave; typical of runabouts and dinghies.
The 1.34 constant is a rule of thumb. Heavy, full-bodied hulls often struggle to reach it, while light, slender or fine-ended hulls may slightly exceed it, which is why you can change the constant.
Frequently asked questions
What is the hull speed formula?
Hull speed in knots = 1.34 × √(waterline length in feet). In metric it is about 2.43 × √(waterline length in meters). A boat with a 30 ft waterline has a hull speed of 1.34 × 5.48 = 7.34 knots.
Why is there a hull speed limit?
A displacement hull makes a bow wave whose length grows with speed. At about 1.34 × √LWL knots the wave is roughly as long as the waterline, so the stern sits in the trough and the boat would have to climb its own bow wave to go faster. Resistance rises steeply, so extra power or sail area gives very little extra speed.
Can a boat go faster than hull speed?
Yes, but not efficiently as a pure displacement hull. Light boats can surf down waves, planing hulls lift and ride on top of the water, and very slender hulls such as catamarans and racing shells make smaller waves. Hull speed is a practical guide, not a physical barrier.
Should I use length overall or waterline length?
Use the waterline length (LWL), measured where the hull meets the water at rest. It is shorter than length overall (LOA), often by 10 to 20 percent on sailboats with overhangs. Using LOA overstates the hull speed.
What is the speed-length ratio?
It is boat speed in knots divided by the square root of the waterline length in feet. Below about 1.0 is efficient cruising, 1.34 is classic hull speed, roughly 1.34 to 3 is the semi-displacement range, and above about 3 a hull is usually planing.
Does heeling increase hull speed?
Heeling can lengthen the effective waterline on boats with long overhangs, which slightly raises their potential speed. That is one reason designers quote sailing waterline lengths. The calculator uses whatever waterline length you enter.
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