From The Scotsman, citing an expert:
There is an issue with security. A large part of it is above water, and it is the shape of the submarine that determines its underwater acoustics. It is the bumps and shapes of the hull and the shape of any propellers, if that is seen above water, then someone can work out what it sounds like under water - that is the bit that's really sensitive. You have to be extremely careful.
Link:http://www.scotsman.com/news/Skipper...iry.6595602.jp