[bigboywooly]

Its your wake that is visible at that speed

Small merc or warship
Eyes are eyes

The U-boat surfaced later on and repositioned for another attack. Just as Walker was about to lead his ships back to the convoy and prepare for a bad night, the surfaced U-131 was spotted by Stanley The U-boat was seven miles away, at the ships' guns extreme range and a very small target so the patrolling Martlet was ordered in. The Martlet attacked the surfaced submarine with machine guns, was hit and crashed into the sea. It was twenty minutes later that a number of hits from the four ships penetrated the hull eight times. U-131 sunk and 55 men survived and were picked up by Blankney and Exmoor.

The convoy was steaming in calm weather with excellent visibility. These usual conditions led to an error in judgement by U-434, which had shadowed the convoy during the night. At dawn he was on the surface ten miles out, and could observe the tops of the masts of the convoy without danger of being spotted himself. He decided to close the convoy on the surface then submerge, to conserve his batteries and get into a commanding attacking position. At this moment, Stanley was prowling the deep field on the port side of the convoy. Her Asdic was defective and she only had her lookouts to rely upon. One of the lookouts spied the submarine's periscope and Stanley turned to attack.

http://www.mikekemble.com/ww2/walker.html

Note the distamces there

I have crept past merchants at 3km at slow speeds without being spotted