It looks like there's still a LOT of work to be done with encounter chances in the Build Mod, so this topic will keep track of exactly that. 
I did some work to try and balance things out a few patches ago, but I didn't cover every ship.
This was split off from a discussion started here, and Black Bart's reply continues below:
EDIT: You can use the following links to view online spreadsheets containing various information about all ships in the game.
List of ships and their stats
Encounter chances for all periods and nations

I did some work to try and balance things out a few patches ago, but I didn't cover every ship.
This was split off from a discussion started here, and Black Bart's reply continues below:
Nope
I get lots of different pirate ships, but i think them Xebecs are still too common in general - this ain't the Mediterranean. They are good pirate ships though, and look good also.
EDIT: You can use the following links to view online spreadsheets containing various information about all ships in the game.
List of ships and their stats
Encounter chances for all periods and nations