My convoy had different speed and course. It was confirmed by my radio transmission reporting the convoy as I left with no hits.
What was really cool was a DD sitting back listening as I fired a hail-mary salvo. It heard the fish on their way and fired flares into the night sky, initiating an evasive maneuver by the convoy...that was sweet, but foiled my last shots since it was hard to see the course change at night through the periscope.