aircraft carriers

[scorrpio]

Cavitation is different; that's when a prop (or pump impellor) spins too fast and sucks air (to put it very simplisticly). Suction from a sinking ship is partially a myth. It does exist but, as you see, it won't suck down a lifeboat.

http://en.wikipedia.org/wiki/Cavitation

Basically, cavitation is what happens when propeller spins fast enough to 'tear open' pockets in water. These pockets then collapse with great force, producing sound and a shockwave - potentially damaging the propeller.

As to suction:
Just like propeller's blade drags water along, so does a sinking object. It produces a downward current. Objects of neutral or very low buoyancy (such as people) can be easily sucked down - but more buoyant objects are safe. The danger to lifeboats is not in getting sucked down - but rather the formation of a waterspout which creates a funnel shape and can cause boats to capsize.

[Stick]

Basically, cavitation is what happens when propeller spins fast enough to 'tear open' pockets in water. These pockets then collapse with great force, producing sound and a shockwave - potentially damaging the propeller.

Did you know that cavitation is also a problem for hydro turbines? Same thing happens as to a ship's propeller. After all, they really are just a propeller working in reverse, the water is the prime mover rather than an engine or steam turbine. Only difference is we can't hear it, because the unit turns at a much higher RPM (ours are 112.5 rpm) It actually eats away at the metal and you wind up with spots that look like a sea sponge. Ours at the plant I operate get shut down for 3 weeks every year to gouge the cavitation spots out and weld it back up and grind smooth. Some of the spots are 3" deep or more. It's a lot of welding to fill it in. They go at it 24/7 for 3 weeks on each unit.