[Kaptlt.Endrass]

Sadly, the thermal layer is not modeled in vanilla SHIII or any mods that I know of. If it was, then the only thing any of us would have to do is get deep (very, very deep) and slowly creep away at one or two knots.

However, everyone here is giving you ways to become undetected and prevent being depth-charged. I'll give you some tips that I've learned for how to avoid being critically damaged when a destroyer does make an attack run. (This will be from an internal view; no external or free camera)

Go to the command room view and listen...you should be at depth already, and moving anywhere from 1-4 knots. (For these purposes, let's say you're at 75 metres.) The sonar pings will be very intermittent, I'm sure you know this is the destroyer, trying to get a lock on you.

Once the sonar becomes more consistent, be ready to go to flank speed. There will be a point where the sonar cuts out, and you will hear the destroyer's screws as he starts to go above you. At the point where the sonar pings stop, go to ahead flank, and go either hard to starboard or port while diving...the depth charges are already set for a certain depth, which will be the one where the destroyer last had you contacted.

With practice and a certain amount of luck, you should be able to get off scot-free, or at least with minimal damage.

Late-war, however, the odds get heavily stacked against you, with the introduction of Hedgehogs and Squids. With these, a destroyer will keep you contacted, set up for a perfect attack run, and fire these ahead of themselves on your position. Good news: they only explode on contact. Bad news: They only need to hit the hull once, maybe twice on some of the larger submarines, to sink you.

Just be glad they didn't add the Allied anti-submarine torpedoes.

As for avoiding being detected...

Run silent, run deep. Rig for silent running unless you have absolutely critical damage (i.e, your submarine is sinking) This will stop all repairs, reloading, anything that is non-critical. Manually set your speed to 1 or 2 knots, and stay on a course to break contact with the convoy/task force you were engaging; in most instances, the escorts will only drive you off, not hound you until the end.

Remember, if you survive, you can surface to repair, rearm, and recharge in order to go back into the fray. If sunk, well...you can't.

Also keep in mind that any hostile ship that you contact/engage will report your last known position and heading, and depending on your location, that means you'll get some friends in the form of aircraft and hunter-killer groups.