The Keel, the Sheer and the Rudder

I have been doing far too much reading on pre-cannon era ships...

http://en.wikipedia.org/wiki/Square_rig
http://en.wikipedia.org/wiki/Points_of_sail
http://en.wikipedia.org/wiki/Windward
http://en.wikipedia.org/wiki/Broach_sailing
http://en.wikipedia.org/wiki/Clew#Clew
http://en.wikipedia.org/wiki/Brig
http://en.wikipedia.org/wiki/Brigantine
http://en.wikipedia.org/wiki/Hermaphrodite_brig
http://en.wikipedia.org/wiki/Caravel
http://en.wikipedia.org/wiki/Mary_Celeste
http://en.wikipedia.org/wiki/Boat_building

Dungeons & Dragons 3.5 is great at confusing me. A two-masted, square sailed ship, from 100 to 150 tons traveling at 2mph. What?

First of all, strickly square sailed ships can't sail agains the wind, ever, unless they have oars. Secondly, that speed is abyssmal. But, as my Dungeon Master pointed out "While the metric might not make sense, we can rest assured it's balanced with the rest of the movement rates in the system."

What have I learned so far? I've learned that there is a lot of terminology I need to learn to play my next character.