Just to had some spice, I sunk the three destroyers and then picked off the rest of the convoy with Torp's and cannon fire.
I used the stern tubes with 1/2 meter under the depth listed in the book. The trick is to get them "charging up your rear end" while at low speed then put it in to flank speed while they get within 600m and fire at 400m to 350m and quickly crash dive hard to a turn. Works most of the time. The convoy goes every which way, however returns to course after a brief time.
Sure gets the pulse rate up.
SNAKE1937