Hi C.N. & Derek,
Let me elaborate. Once the wolfpack has located a convoy the aim was to track it during the daytime at the edge of the visible range. Wait to dark and break through the escort screen, go inside at max speed on the surface, pick your targets, shoot all ready eels and withdraw for reloading and hopefully repeat the proceedure without getting clobbered too much by the escorts.
There are more targets in the convoy, thats why the Uboat Cdr. tried to be there instead of looking for stragglers in the night. Stragglers can be looked for during the day, when trying to maintain a loose cover on the convoy. That is why the wiley tanker captain posthaste closed ranks with the convoy at the crack of dawn spending the day in relative safety there.
Also, escort screens around the convoy usually deployed a sweeper a couple miles astern for the purpose of catching boats driven off by the primary screen. Thus the tanker had its own private escort around for the night.
I repeat, that tanker skipper was no dummy. Rgds, RB