this means that kate has erred in 13a, she refers specifically to ships and vessels (plural) so the answer must be indiamen. but if roach is the answer to 7d, then the answer to 13a is indiaman. which is WRONG according to the clue.
More pedantically, a sail does not have a "leech side". It has a leech, which is the trailing edge of the sail into which battens are sometimes inserted. It does have a leeward side, which is the side furthest from the direction from which the wind is blowing. The roach is that part of the sail which sits between a line drawn straight from head to clew, and the leech.