Ships and submarines move through water in different ways depending on hull shape, speed, and purpose. A displacement hull pushes water aside and supports its weight mainly by buoyancy. A planing hull rises upward at higher speed and skims over the surface, reducing the amount of hull in the water.
Understanding these hull types explains why cargo ships, submarines, speedboats, and racing boats look and behave so differently.
A displacement hull is limited by the wave system it creates, because the vessel must keep pushing through water and climbing over its own bow wave. A planing hull uses forward speed and hydrodynamic lift to reduce drag after it gets on plane. Submarines are displacement vessels even when fully submerged, because they travel through water rather than riding on top of it.
Designers choose between stability, efficiency, payload, speed, and power needs when selecting a hull form.
Key Facts
- Buoyant force equals the weight of displaced water: F_b = rho g V
- A floating vessel is in vertical equilibrium when F_b = W
- Displacement hulls support weight mainly by buoyancy and move by pushing water aside.
- Planing hulls support part of their weight by hydrodynamic lift at high speed.
- Approximate displacement hull speed in knots: v = 1.34 sqrt(L), where L is waterline length in feet
- Drag increases rapidly with speed, so planing usually requires much more engine power than slow displacement travel.
Vocabulary
- Displacement hull
- A hull that moves by pushing water aside and is supported mainly by the buoyant force of the displaced water.
- Planing hull
- A hull shaped to rise partly out of the water at high speed and skim along the surface.
- Buoyancy
- The upward force exerted by a fluid on an object immersed in it.
- Hydrodynamic lift
- An upward force produced when moving water flows around a hull surface at speed.
- Drag
- The resistive force that opposes a vessel's motion through water.
Common Mistakes to Avoid
- Thinking all fast boats are displacement hulls is wrong because high-speed boats often use planing to reduce wetted surface area and drag.
- Confusing buoyancy with hydrodynamic lift is wrong because buoyancy comes from displaced water volume, while hydrodynamic lift depends on motion through the water.
- Assuming submarines plane underwater is wrong because planing requires interaction with the water surface, while submarines operate as submerged displacement vessels.
- Ignoring waterline length when estimating displacement hull speed is wrong because longer displacement hulls can usually travel faster before wave-making drag becomes severe.
Practice Questions
- 1 A small boat displaces 2.5 m^3 of seawater with density 1025 kg/m^3. What buoyant force acts on it if g = 9.8 m/s^2?
- 2 Estimate the hull speed of a displacement vessel with a waterline length of 36 ft using v = 1.34 sqrt(L). Give your answer in knots.
- 3 A patrol boat and a submarine have similar engines, but the patrol boat is designed to plane while the submarine remains submerged. Explain why the patrol boat can reduce drag by rising onto the surface, but the submarine cannot use planing underwater.