Hull speed is the speed where a displacement ship begins to run into a natural wave limit set by its own length. As the ship moves, it pushes water aside and creates a bow wave in front and a stern wave behind. When the distance between these wave crests matches the ship’s waterline length, the ship is riding in the trough of its own wave system.
This matters because it helps explain why long ships can travel faster efficiently than short boats of the same hull type.
A displacement hull moves through the water rather than planing on top of it, so its speed is strongly linked to the waves it makes. Near hull speed, extra engine power mostly makes larger waves instead of much more forward motion. The common estimate is V = 1.34 sqrt(LWL), where V is in knots and LWL is in feet.
Submarines avoid much of this surface wave limit when fully submerged because they no longer create large bow and stern waves at the surface.
Key Facts
- Hull speed estimate: V = 1.34 sqrt(LWL), with V in knots and LWL in feet.
- LWL means waterline length, the length of the hull where it meets the water.
- At hull speed, the main bow wave crest and stern wave crest are about one waterline length apart.
- A displacement hull pushes through water and supports its weight by buoyancy, not by planing lift.
- Wave-making resistance rises rapidly as a displacement hull approaches hull speed.
- A fully submerged submarine is not limited by surface hull speed in the same way because it produces far smaller surface waves.
Vocabulary
- Hull speed
- Hull speed is the approximate speed at which a displacement hull’s waterline length matches the wavelength of its main wave system.
- Displacement hull
- A displacement hull is a hull that moves by pushing water aside while being supported mainly by buoyant force.
- Waterline length
- Waterline length, or LWL, is the length of a vessel measured along the surface where the hull meets the water.
- Bow wave
- A bow wave is the wave formed at the front of a moving vessel as it pushes water outward and upward.
- Wave-making resistance
- Wave-making resistance is the drag caused by the energy a vessel spends creating waves as it moves through water.
Common Mistakes to Avoid
- Using the ship’s overall length instead of waterline length is wrong because the hull speed formula depends on the length actually in contact with the water surface.
- Thinking hull speed is an absolute maximum is wrong because a displacement hull can exceed it, but doing so usually requires much more power and becomes inefficient.
- Forgetting the units in V = 1.34 sqrt(LWL) is wrong because the constant 1.34 works when speed is in knots and length is in feet.
- Applying surface hull speed directly to a fully submerged submarine is wrong because submerged motion greatly reduces surface wave-making resistance.
Practice Questions
- 1 A displacement boat has a waterline length of 25 ft. Estimate its hull speed in knots using V = 1.34 sqrt(LWL).
- 2 A ship has an estimated hull speed of 12 knots. Using V = 1.34 sqrt(LWL), estimate its waterline length in feet.
- 3 Explain why adding a much larger engine to a displacement hull near hull speed may produce only a small increase in speed.