Sounds great Teddy - in the current campaign I just finished a patrol (Sept 41) where I contacted a convoy that varied between 5 and 3 knots.
On the subject of convoys I came across some rather interesting info on convoys from Norway to Britain during 39/40 - this is something I have never seen in the game, so possibly an idea for the future?
www.warsailors.com
Also some rather nice pictures of Norwegian ships of the period, along with potted histories.