Slitherine

The last problem is that german subs make me crazy they appear from nowhere and destroy all convoys. You may ask what’s new ? The biggest fail to me is that Britain has 22 destroyers. Nazi has only 15 subs but they undestructible in GS. Is it possible to turn on system that work in vanilla game that destroyers may attack after move not in next turn when sub swim away???

I would suggest to build more SAC units and make sure your ASW is the priority.
Then you can attack the U-Boats from around the edges. 2 SAC units at ASW 6 puts a pretty good dent into those

U-Boats. Once they take damage they have to return to repair. Sometimes you have to ignore the torpedoes and run the gauntlet.

That is how you set a trap for the U-Boats.