I just had to go in. Just as I passed, I spotted a convoy of 15 ships (mostly tankers and C2 and C3 cargo's), only protected by an armed trawler and a V&W destroyer. It was too sweet to let pass. But I didn't know the chanel was crawling with destroyers.
By the way, I managed to shoot a torpedo right under the bow of the destroyer (who has shallow depth) to sink the tanker that was sailing after it. Repeat that!
Is there a special tactic to sink ships with one torpedo? I keep needing 2 and already have so little.