Boat Speed Calculator

Understanding Boat Speed and Hull Speed

For displacement hulls—the kind that move through the water by pushing it aside, like sailboats and trawlers—there is a theoretical maximum speed they can achieve. This is known as "hull speed." As a displacement boat moves through the water, it creates a bow wave and a stern wave. The distance between these two waves is equal to the boat's waterline length. As the boat goes faster, the wavelength of the bow wave increases. Hull speed is reached when the wavelength of the bow wave is equal to the waterline length of the boat. At this point, the boat is effectively trapped between its own bow and stern waves, and a tremendous amount of additional power is required to go any faster.

This theoretical maximum is not a physical barrier, but an economic one. Attempting to push a displacement hull beyond its hull speed results in the stern squatting into the trough behind the bow wave, and the boat will climb its own bow wave, wasting enormous amounts of energy for a minuscule gain in speed. Planing hulls, like those on speedboats, are designed to rise up and skim across the water's surface, thus breaking free of this limitation. This calculator focuses on calculating the theoretical hull speed for displacement boats.

How to Use the Boat Speed Calculator

This calculator estimates the theoretical maximum speed of a displacement hull boat based on its waterline length.

Enter Waterline Length (LWL): Input the length of the boat at the waterline. This is the most critical measurement for this calculation.
Select the Unit: Choose whether the length you entered is in feet or meters.
View the Results: The calculator will instantly display the estimated hull speed in knots, miles per hour (mph), and kilometers per hour (km/h).

The Hull Speed Formula

The hull speed is calculated using a simple formula derived from the physics of wave propagation:

Hull Speed (in knots) = 1.34 × √LWL

Where:

Hull Speed is the theoretical maximum speed in knots.
1.34 is a constant derived from the acceleration of gravity and other factors.
√LWL is the square root of the boat's waterline length in feet.

The calculator then converts the result from knots to mph and km/h for your convenience.

Practical Example

Let's say you own a sailboat with a waterline length of 30 feet and you want to know its theoretical top speed under power.

Waterline Length (LWL): 30 feet

Using the hull speed formula:

Hull Speed = 1.34 × √30

Hull Speed = 1.34 × 5.477

Hull Speed ≈ 7.34 knots

The calculator would show that the boat's hull speed is approximately 7.34 knots. This translates to about 8.45 mph or 13.6 km/h. This tells the owner that pushing the engine to go faster than this speed will burn a lot of extra fuel for very little gain. It helps in planning fuel-efficient cruising speeds, which are typically well below the maximum hull speed. For another type of vehicle performance calculation, see our Boost Horsepower Calculator.