A ship moving through calm water already uses engine power to overcome friction and wave-making resistance. In rough seas, the same ship must also push through incoming waves, causing added resistance. This extra load can slow the ship, increase fuel use, and make route planning more difficult.
Understanding added resistance helps mariners predict speed loss and choose safer, more efficient paths.
Key Facts
- Total resistance in waves is Rtotal = Rcalm + Radded.
- Required power is P = Rtotal v, where v is ship speed.
- Added resistance is often largest in head seas, when waves travel toward the ship.
- Speed loss happens when engine power stays constant but total resistance increases.
- Longer, smoother hulls can reduce wave-making resistance but cannot remove added resistance in rough seas.
- A submerged submarine usually feels less wave effect than a surface ship because wave motion decreases with depth.
Vocabulary
- Added resistance
- Added resistance is the extra drag a vessel experiences because waves disturb its motion and force it to do more work.
- Calm-water resistance
- Calm-water resistance is the drag on a vessel moving through still water without waves.
- Head seas
- Head seas are waves that approach a vessel from the front, usually increasing resistance and slowing the vessel.
- Wave-making resistance
- Wave-making resistance is the energy a moving hull loses as it creates waves on the water surface.
- Hull
- The hull is the main body of a ship or submarine that moves through the water and determines much of its drag.
Common Mistakes to Avoid
- Treating rough-water drag as the same as calm-water drag is wrong because waves add extra forces from pitching, heaving, and wave impacts.
- Assuming more engine power always keeps the same speed is wrong because power demand rises with total resistance and may exceed the engine limit.
- Ignoring wave direction is wrong because head seas usually cause much more added resistance than following seas or beam seas.
- Thinking submarines and surface ships are affected equally is wrong because wave motion becomes weaker below the surface, especially at greater depth.
Practice Questions
- 1 A ship travels at 8 m/s in calm water with a resistance of 60,000 N. In waves, added resistance is 20,000 N. What is the total resistance in waves, and what power is required to maintain 8 m/s?
- 2 A vessel has an engine power of 500,000 W. In rough seas its total resistance is 100,000 N. Using P = Rv, what speed can it maintain?
- 3 A surface ship and a deeply submerged submarine travel through the same stormy area. Explain which vessel is more likely to experience large added resistance from waves and why.