Simply changed the visual detection to allow subs a chance to sneak up from distance without having scopes or boat spotted from too far out, making the convoys/single freighters zigzag and speed up. I find this numbers works out nicely.
Submerge when at 7km from your contact.
This is highly debatable, "How far should a warship/freighter spot a submarine at?".....
My opinion is only that an opinion, but I dont think that freighters and merchants would spot conning towers 11.5 km away, and if they were, certainly they were not perfect eagle eyes each time.
Here is a bit about Otto Kretschmer and Patroclus, around the time he torpedoed the Laurentic. Note the fact that even at 300m he was unseen:
"A third torpedo was fired at the Laurentic at 23.30 hours from a distance of 250 metres into the hole opened by the first torpedo, at this time the lookouts spotted the U-boat on the surface and Kretschmer had a hard time in evading the gunfire. In the meantime, HMS Patroclus began picking up survivors instead of participating in the fight against the U-boat and her lookouts did not see U-99 only 300 metres away. A first torpedo struck the ship at 00.22 hours, a second at 00.44 hours and a third at 01.18 hours, but then the lookouts spotted the U-boat and Kretschmer had again to evade the gunfire." -taken from uboat.net
So again, highly debatable, I think 7km is just about right.
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