Was looking at the drawings for SS-565 Wahoo on navsource.org (which are very good by the way) and noticed two hull details called masker emitter piping and prairie piping. The masker emitter is a tube surrounding the girth of the hull fore and aft. The prairie piping runs down the hull to the prop shaft. I'd never seen them before on subs so I did a search. Apparently they were used to help silence the boat and were referred to as the Prairie-Masker System. Also from the web:
Prairie - injection of LP air through holes in the propeller blade tips. This fills the vacuum left by the rotating blades as the water "boils". Allows cavitation bubbles to contract more slowly as area of underpressure is minimized.
Masker - creates acoustic impedance mismatch between hull and water, by way of the masker belts located around the hull, putting a blanket of air bubbles between the hull's machinery noise and the water.
Ships will routinely operate systems inport and at sea, to avoid marine growth from plugging holes in blade tips and masker belts.
I filed this under "learn something new every day". When I get a chance I'll post some images from the drawings.