What I would do in your circumstances, and have done many times with success, is to forego this attack. Plot the convoy's
mean course, wait for it to get on the horizon and then surface and do an end-around for a night attack. If you have been detected and 'spooked' the convoy, they will keep zig-zagging for some hours before calming down and returning to course. Don't act in haste, it's normally a waste! (old naval saying or did I just make that up?).
