Going in with 'decks awash', that is set depth for 7 metres will help you get a lot closer than being fully surfaced. With luck it is possible to get past the first lane of the convoy. As soon as your torpedos hit, the convoy ships arround you will light up with searchlights, so dive as soon as you have fired. Always check the depth under keel before going into an attack, and have your repair crews selected. Depending on which fatigue model you might be using, it's another element to keep in mind. Keeping fatigue levels as low as possible throughout the crew, thus having enough fresh crew to make repairs when they are needed.
The BE and BF squares are good places to find lots of the larger lone merchants and convoys.
As for which crew members to promote/reward, I select the same names for the same jobseach time. That is engine rooms, watch, control room/radio & hydrophone crews. So I know who was doing what, or at least whom it might have been if I can't recall
