SSK stealth is partially a myth. WHy do I say that? Because it is usually very temporary.
(1) A state of the art SSK has a maximum endurance of about 400km at about 4 knots on its batteries. You don't get anywhere at 4 knots and you certainly are not going to be very successful chasing your quarry at that speed. You also do not typically run your batteries 95% flat before a recharge. Rather you tend to do it at conventient times when you don't think there is anyone around to find and kill you. When you surface to run your diesels you have very little stealthy on your side. You are noisy and at periscope depth. In fact, every other thing aside, running fast and near the surface is doubly bad acoustically because your screw cavitate like hell near the surface whereas at depth the water pressures migates the formation of vaccum pockets on the trailing edged of your screw reducing or eliminating cavitation. Radars can find your snorkel, SSNs and ASW ships can hear your from a long way off and aircrafts can literally see you at that depth. You are basically exposing yourself!
(2) There is always the option of AIPs. The problem is that firstly AIPs, probably with exception of the Fuel Cell, is not as silent as motors on batteries. The sterling is a reciprocating piston engine running of separately heated working gas. The Close cycle diesel is exactly that a diesel engine running on diesel fuel, oxygen and part of its recycled exhaust. The MESMA is a steam turbine running on the products of alcohol-oxygen combustion. They all make more noise than a battery does and they all have exhausts to get rid of. The worst thing howeveris that power density is in usually horrible enough that cruise speed on AIP is no better than 5-6 knots and there is every little power left over to recharge the batteries in a timely manner. The Fuel Cell which is the quietest AIP setup also happens to have the worst energy density by a long shot... large PEM stacks, large LOX tanks and huge LH2 tanks, all for less energy yield than the combustion type AIPs. In the end what it means is that AIP boats usually transit or maneuver tactically by running their diesels and running on the surface or at snorkel depth to get close to their quary. In a real war with a massive navy like the USN, a lot of them will be picked off while doing this by ASW aircraft and a forward screen of SSNs.
(3) The other fallacy is that batteries and electric motor equals total silence. This is nonsense. In fact, it is frequently not flow noise and propeller noise which shows up most prominently on a sonar system when an SSK is picked up. It is frequently the inverter buzz from the switching inverters which the SSK uses to convert its DC battery power to AC current to run its motors with. Just about all high power motors are AC induction motors.
(4) The last thing when cosidering using diesels against a major surface action group is that all the silencing advantage is useless against active sonar which is routinely employed on ASW helos and once they catch a glimpse of you, an SSK has neither the speed on the endurance to slip away. Once found you are usually dead meat.
