If you don't believe that, lift the hood of your car and listen to the battery.
I too am a retired Diesel/Electric boat sailor, so this is for the non-Navy folks in free republic, not you COB. I am a retired Engineman to be exact.
Battery power with electric motors are very quiet. But when charging batteries with the diesel engine the boat doesn't surface, it snorkels. The snorkel exhaust is below the water and exhaust is dispersed over a large semi-circle area, disapating the exhaust and smoke stays underwater. The diesel engine is run at short intervals. The run time and shutdown intervals are spaced at random to help to avoid detection. Normally, the submarine uses passive sonar. (ie. passive sonar is listen only.) If active sonar is used only one (or short) ping is used. With limited pinging the chance of detection is much lower. Screws are much more sophisticated than any of you suggest. I'll not go there.