Well, if you weren't talking about the ships, but about the plural of Drayman itself, then it would be Draymen. However, we are talking about the ships, so in this case it would be Draymans. Doing it the other way around could be argued that it modifies the class designation of the ship. Also, when being briefed about escorting 4 Drayman ships, it would be less confusing to say Draymans than Draymen. Someone not keeping a sharp ear open might think of escorting only one because of the similar sounding words, so I believe Draymans would be correct.