When a ship moves through water, its hull pushes water aside and creates waves at the bow and stern. Those waves are not just a visual effect, they carry energy away from the vessel. The engine must supply that lost energy, so wave-making resistance acts like an extra drag force.
This is why faster ships often need much more power than slower ones of similar size.
Wave-making resistance grows strongly when a vessel moves fast enough that its wave pattern becomes large compared with the hull length. A surface ship is affected most because it travels at the air-water boundary where waves can form easily. A deeply submerged submarine produces far less wave-making resistance because it is away from the free surface.
Naval architects reduce this resistance by shaping hulls, controlling speed, and choosing lengths that manage the bow and stern wave system.
Key Facts
- Wave-making resistance is the drag caused by energy carried away in water waves.
- Total resistance = frictional resistance + pressure resistance + wave-making resistance.
- Power needed to overcome drag is P = Fv, where F is resistance force and v is speed.
- Wave speed in deep water is approximately c = sqrt(gλ / 2π), where λ is wavelength.
- Froude number is Fr = v / sqrt(gL), where L is hull length and g is gravitational acceleration.
- Wave-making resistance becomes more important at higher Fr, especially near Fr = 0.4 to 0.5 for many displacement hulls.
Vocabulary
- Wave-making resistance
- The part of a vessel's drag caused by creating waves that carry energy away from the hull.
- Bow wave
- A wave produced near the front of a moving hull as water is pushed outward and upward.
- Stern wave
- A wave produced near the rear of a moving hull as water flows back together behind the vessel.
- Froude number
- A dimensionless number that compares a vessel's speed with the natural wave speed set by gravity and hull length.
- Displacement hull
- A hull that moves through the water by displacing water rather than mainly riding on top of it.
Common Mistakes to Avoid
- Thinking waves are only a side effect, not an energy loss. Waves carry kinetic and potential energy, and that energy must come from the ship's engine.
- Assuming all drag increases in the same way with speed. Wave-making resistance can rise much more sharply than frictional drag at higher speeds.
- Ignoring hull length when comparing ships. A longer hull can have a different Froude number at the same speed, so it may create a different wave pattern.
- Treating submarines and surface ships the same. A submarine near the surface can make waves, but a deeply submerged submarine has much less wave-making resistance.
Practice Questions
- 1 A ship experiences 80,000 N of total resistance while moving at 6 m/s. What power must the engine deliver just to overcome this resistance, using P = Fv?
- 2 Calculate the Froude number for a 100 m long ship traveling at 12 m/s. Use g = 9.8 m/s^2 and Fr = v / sqrt(gL).
- 3 Explain why a surface ship moving faster may need a much larger increase in engine power even if its hull shape does not change.