First tier
5 lift able objects
15 inventory spaces.
Single masted with small sail
Real example Knarr http://en.m.wikipedia.org/wiki/Knarr
It's more like longer boat with a mast
Second tier
10 lifts lie objects
30 inventory spaces
Single masted but bigger mast and sail (or 2 smaller mast)
Real example caravel http://en.m.wikipedia.org/wiki/Caravel
Third tier (biggest one)
20 liftable objects
60 inventory
Double masted big ship with big sails
Real examples no for now
All numbers can be still changed and so are examples. Resources needed will be added later if I here good responses

For people who hate this idea because of different time period in which game is made.
Old Norway long ship
http://en.m.wikipedia.org/wiki/Longship
This ship could be added it's more game time period ship.
Please respond and tell me what u think