[Crannogman]

IIRC, there is no gain in efficiency below 10-11kts. So you won't gain any distance by going slower than that, but you will remain on station for more time. I guess the moral may be that efficiently patrolling a barrier longer than ~35nm will allow an increasingly large fraction of shipping to escape detection. However, a barrier shorter than 35nm allows you to spot anything transitting while moving slower and staying on station longer.