[quote=KabenSaal]
Well, Mary Rose was a Carrack class, and also one of the largest ships in the British navy.
[/quote]

Restricting us to the classes sort of makes more sense for stats reasons, though. In real life, ships can vary even inside the same class, but it'll be a bit off for every ship to be running round with different stats, and a nightmare for the GM's. Having pre-existing classes with pre-existing stats is just easier for everyone.