[RayChuang88]

Diesel-electric subs have the advantage of running extremely quietly because you don't need the noisy pump systems for cooling a nuclear reactor; however, there is the major disadvantage of limited running time under water. Mind you, the Seawolf and Virginia class nuclear subs have highly-insulated reactor sections, which also drastically reduces the noise signature of the submarine.

[MeanWestTexan]

As above, sound like perfect port-protectors to me.