[Aktungbby]

I was being humorus U were right the first time! Subs don't give a bilgerats ass about fishing boats...that's just 'collatertal damage'. Although I'd imagine getting tangled in a net causes 'cavitaion' too!http://www.globalsecurity.org/military/systems/ship/systems/shrouded-propulsor.htm

A shrouded propulsor (not a 'ringed propeller') is a directional collar around the submarine's propeller. It lowers the passive sonar signature of the sub and provides improvement over exposed propeller designs in both efficiency. The Improved Los Angeles (SSN-751 onwards) have a shrouded propulsor, as do all subsequent classes of American attack submarines. At least one Russian KILO has had its propeller replaced by a shrouded propulsor, which is now used by new US and British nuclear submarines. A shrouded propulsor is identical to a standard shafted propeller, with a cylindrical ring of metal attached at the the tips of the propeller blades around the full circumference....Improved design in submarine screws (propellers) significantly reduces cavitation. Cavitation is reduced by the shroud, as the duct maintains higher pressure around the blade tips and prevents cavitation bubbles from forming. One method of improving the efficiency of propellers uses a pipe shroud to encase the propeller. This shroud or nozzle forces more of the water directly out from the propeller. And by ducting water flow through the shroud, tip vortices can be harnessed to provide thrust.