Disabling a enemy propeller is a really good thing to do. Great for battleships where a full load of torps doesnt always sink them. Still, its best to launch 2 at the prop if your aiming for that, as for some odd reason 1 torp doesnt always destroy it :/
And if you hit the engine enough, a deckk gun can take down the prop.
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I used to be a huge submarine guy but now I'm a huge submarine girl ¯\_(ツ)_/¯
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