[Catfish]

Hello treblesum81,
the numbers and situations are from the book i mentioned, and i believe them to be exact. Mr. Topp, who had sunk quite some tonnage, and survived the war, also mentioned that strategy. In fact he said he usually never dived to more than 30 meters for exactly that reason.

A boat hearing a destroyer start its attack run could go to full speed undetected, the destroyer is virtually blind during the attack run. If you are at PD the destroyer has to be REAL fast not to be sunk by its own depth charges, being adjusted for exploding at 15 meters.

Bystanding destroyers would as well not hear you because of the attacking destroyers cavitation noises, and no one wanted to ruin his ears hearing exploding charges at the hydrophone. As well in the first years of the war depth charges often exploded at the wrong depth, having similar problems with depth settings as the torpedoes of all nations.

If the enemy exactly knew where you were (spotted your periscope) you would probaly dive deeper to evade the depth charges: One reason is the deeper you are the more the enemy has to calculate sinking speed of the charges against the movements of the sub. And as well a depth charge in say 200 meters does not have the kill radius it would have in shallow waters - water pressure is your friend here.

Even today e.g. a diesel-electric russian "Kilo" sub can only be found by means of exhaust Diesel detection when running at PD charging the batteries, and the Diesel-generated noise.
As soon as it switches to electric propulsion the only possibility for detection is to use submerged sonar buoys, that direct the impulses upward, and are in that case able to detect the sub. Certainly there are other detection strategies today, but not in WW2 anyway.
One of the advisors for SH3, also a real ex-U-boat commander from WW2, said the SH3 vanilla game was already much too hard compared to reality.

Greetings,
Catfish