Upgraded my class3 to a class4, added sailor quarters and ran some speed
tests. Results were pretty much as expected (lvl 6 nav):

LUC - FUR 1:54 Class3, 15 lvl 7 sailors
LUC - FUR 3:21 Class4, 15 lvl 7 sailors
LUC - FUR 2:08 Class4, 20 lvl 7 sailors

FUR - ROT 1:31 Class3, 15 lvl 7 sailors
FUR - ROT 1:42 Class4, 20 lvl 7 sailors

Slight hit in speed, but nothing major if you're going with max sailors. I'm
quite happy with this, now to do the lift/speed quests and max those too and
see how well this thing can fly (class3 was a bit limited in that, even with
lvl 6 lift mages).