Can the Corvette and Black Swan really be that tough to sink

[Krieg-Schwein]

First I,m using the GW mod, it has provided me with a new lease on wanting to play the game again, just a couple of ? though,
I started a new career starting around 1940 with the IXB and I have run into a convoy with mostly corvettes and Black Swan Frigates and a couple J class destroyers and a Fuji class, any way I decided to go around and ahead of them so I could meet them head on, I started heading toward them at around 1200 am at periscope depth and met up with a black Swan Frigate. and made My save point, I fired 1 torpedo and hit and since he was way ahead of the group I decided to finish him off with a deck gun,..I hit him at least 20 times on the hull, the deck etc and he never ever sank, he finally did stop and sank to about 3 feet from the deck line but that was it, he still had full function of all deck guns and also flares, so by this time I had a corvette coming at me and went to periscope depth, headed toward him and got him with 1 torpedo he turned and I got him in the side with a second and he never went down, if it weren't for the fire I wouldn't have noticed that he was even hit, all guns were functioning as well as depth charges.. I have replayed this save spot over and over and still the same results, I emptied my whole supply at of torpedo's at one point and only had a small merchant to show for it with more than several damaged ships running around like nothing ever happened to them,,
I had one other issue happen also, it seems my hydrophone won't pick up any thing unless it dam good and ready to, The Black swan I had coming at me could not be picked up at 1500 M, but it did recognize a merchant at 6500 M some how, I don't want to be a complainer I really like the mod as a matter of fact I practically file to file had all of the graphics .TGA files already loaded in my last game, any way and thoughts or suggestions would be appreciated

Danke