The moon is not an issue, found that out, the weather is good, maybe even too good. Radar is also not yet a problem, it's only 1940 and I'm working under the assumption they don't have any...
If they do, I'm going to find out how big a hole a 16" gun make sin a u-boat
What I did was approach carefully decks awash until I was about 2,5km from the outer row of merchies. fired 2 eels at Nelson, both aimed for aft of the bridge, then fired one eel at a medium tanker and one at a large merchant.
Then I did a quick 180 turn and made my egress.
This is where I messed up, I fired the stern tube at the flower corvette. Even though I hadn;t been detected yet.
Only I didn't time it right so the flower got sunk about 1 minute before the rest of the eels were to hit.
As a result they all started zigzagging, the eel for the tanker missed, the large merchant did get hit but not sunk right away. and only one eel managed to hit Nelson, but nowhere critical, right in the bow, which won't sink it.
They didn't detect me though so I got out safely. the large merchant I hit did get slowed down a bit and started lagging about 2km behind the convoy. I did shadow them for about a day to see if the straggler would drop back any further but it didn't, so having no torpedoes left I went on back to base.....
better luck next time, and I must remember not to mess up a well aimed salvo by getting over-eager.
saving that close to a convoy is a sure way to get your save corrupted, so I don't get to retry... oh well, I'll find a similar convoy on my next patrol.