@Vance:
Well, yes in that case if you need that transport then it’s desperation time so roll and hope for the 1. But he was asking about 1 sub versus 1 sub; not 1 sub versus 1 sub plus a transport, or 1 sub plus 10 transports or whatever.
Transports have no combat value, so having a transport in the defending zone is still basically a sub v. sub scenario.
Have it your way - even without transports, there may be (somewhat rare) circumstances in which a player would wish to fight with a defending sub against an attacking sub.
Point is, subs can be sunk without destroyers, when the defender wishes to fight. And you have to admit it’s a good point I made that one such circumstance in sub vs. sub would be to defend a transport (or 10 transports).