Boat Speed Calculator
Estimate the theoretical maximum hull speed of a displacement boat based on its waterline length.
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Understanding Boat Speed and Hull Speed
A Guide to the Physics of Displacement Hulls
How fast can a boat go? The answer is complex, but for a large category of boats—those with displacement hulls—there is a theoretical maximum speed they can efficiently achieve. This is known as "hull speed." This calculator estimates that theoretical speed based on a simple but powerful measurement: the boat's waterline length.
Displacement Hulls vs. Planing Hulls
To understand hull speed, you first need to know about the two primary types of boat hulls:
- Displacement Hulls: These boats move through the water by pushing it aside (displacing it). They are designed to be efficient at low speeds and are typically found on sailboats, trawlers, and large ships. They are limited by their hull speed.
- Planing Hulls: These boats are designed to rise up and ride on top of the water at high speeds, much like a skipping stone. This greatly reduces drag and allows them to go much faster than their theoretical hull speed. Most speedboats, bass boats, and personal watercraft have planing hulls.
This calculator is specifically for displacement hulls, as planing hulls are not bound by the same physical limitations.
What is Hull Speed?
As a displacement boat moves through the water, it creates a wave system. There is a wave crest at the bow (front) and another crest at the stern (back). As the boat's speed increases, the length of this wave system also increases.
"Hull speed" is the point at which the wavelength of the boat's own bow wave is equal to the length of its waterline. At this speed, the boat is effectively trapped between its own bow and stern waves, sitting in the trough. To go any faster, the boat would have to climb its own bow wave, which requires a massive, often impractical, amount of power.
Pushing a displacement hull past its hull speed is extremely inefficient. Doubling the power might only result in a very small, incremental increase in speed, while creating a much larger wake and burning significantly more fuel.
The Hull Speed Formula
The theoretical hull speed is calculated using a simple formula that relates the speed to the square root of the waterline length.
Hull Speed (in knots) = 1.34 × √LWL
Where:
- Hull Speed is the theoretical maximum speed of the boat in knots (nautical miles per hour).
- LWL is the length of the boat's waterline in feet.
- 1.34 is a constant derived from the physics of wave propagation in water.
Example: A sailboat with a waterline length (LWL) of 36 feet would have a theoretical hull speed of:
1.34 × √36 = 1.34 × 6 = 8.04 knots.
Is it a hard limit?
No, but it's a point of diminishing returns. A boat can be forced past its hull speed with enough power (this is called "semi-displacement"), but it's very inefficient. Long, narrow hulls (like those on catamarans or racing shells) can exceed this theoretical speed more easily than short, wide hulls.
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