And these subs usually screened for the convoy... running submerged many miles out ahead of the convoy listening for submerged contacts.
as the convoy would catch up they could surface and reposition the sub before submerging again for some time.
As for the red green and blue convoy markers... mine are all dark grey, the only way to ensure the neutral status or enemy/friendly status of a contact is to sail up and check her flag just as in reality
__________________
|