Destroyers have three methods of detecting you
1. visually with the watch crew
2. passively with hydrophones - they listen for the faint sounds of your engines, props, crew activities, various pumps and motors aboard the boat etc their ability to hear these things is affected by how far away from them you are, and/or how deeply you have dived.
3. actively with ASDIC - they send out a sound pulse or ping which reflects off the hull of the submarine and tells the destroyer where and how deep you are.
avoiding passive hydrophone detection is a matter of diving deep and maintaining your distance from the destroyers and by rigging for silent running.
avoiding ASDIC detection is a matter of reducing your aspect ratio relative to the destroyer transmitting the ASDIC ping. meaning that you should keep your stern or bow pointed as directly toward the attacker as possible because...
when you are pointed AT or directly FROM the destroyer his ASDIC "sees" this target (front or back view of u-boat)
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the above is easily dismissed as a school of fish or bubbles or some disturbance in the water and thus, not something to waste depth charges on just yet
When you are at a 90 Degree angle to the attacking destroyer his ASDIC sees this target (side view of the U-boat)
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that... is unmistakably a U-boat and obviously what the destroyer should be depth charging the crap out of
as for diving deep... there are times when i will use a higher RPM like ahead standard or even Flank to gain depth as quickly as possible, but i wont do this unless at least one of my torpedoes has already hit... otherwise the convoy will start zig zagging before i have scored a hit. dont use crash dive while submerged because this may result in a bug which causes your u-boat to slowly descend when after reaching ordered depth
another thing you can do is dive as soon as your last shot leaves the tube. make sure you are rigged for silent running - otherwise your crew will assume it is ok for them to move forward with the VERY noisy activity of reloading the tubes. Destroyers will hear reloading activities from a mile away
Use BOLD decoy canisters if available. Once the destroyer passes overhead and gives you the first run of depth charges, deploy a bold canister as he passes then turn hard left or right at flank speed. as long as you are behind the destroyer you can make as much noise as possible to put distance between you and your attacker however he will turn around to acquire you and attack you again - this means you can only hold that flank speed for maybe 10 seconds. the hope here is that when he turns about, instead of detecting and attacking your boat, he will detect and attack your decoy. this buys you time to sneak away.
finally, 80 and 120 meters is not going to do it. especially in late war, i have had destroyers pound me down to 270 meters before. If you use SH3 Commander - the crush depth is randomized every time you launch the game to a more realistic number. A Type VIIC uboat for example - depending on the historical source of information used - had a theoretical crush depth of 250-275 meters, others estimate as high as 300 meters. this differs from "Test depth" or (red line) of 180 meters because Test Depth is just the depth they took the boat to while testing the prototype.
I've had destroyers push me down to 280 meters before in my VIIC
on important note to remember... Even early in the war, when destroyers were only just trying to figure out how to beat the U-boats, the destroyers still experienced some pretty stunning successes against the kriegsmarine.
as the war progresses, and you have one destroyer sitting still listening, while others ping you - thats when it gets really tough.
GWX is designed to be as historically accurate a mod for SH3 as possible... so just be aware, you have about a 75-80% chance of being killed at some point in your commander's career. especially after 1943
refer to GWX manual pages B-17 through B-20 for more information
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