Free Model Plans Site
A top site you need to know is the Free Model Plans Site.
They bring out regularly free boat plans, which are either vintage plans and free for the public domain or plans by owners which give them out for free

Free RC Sailboat Plan “Santa Maria”
I guess most of us have heard of the Santa Maria, as the legendary flagship sailed by Christopher Columbus on his discovery of America. Originally a small carrack (or called nao), roughly 70 feet long carrying 40 men.
The sailboat model plan is actually a whole pack with 10 different pictures / parts of the plan with measurements etc. Here the Santa Maria Sailboat Plan

Wikipedia recommends this book for more details:
The Ships of Christopher Columbus by Xavier Pastor

Free Ships Plans Blog
Another tip is this free model ship plans blog by Emrak. 
A rough plan of this really nice looking racing rc yacht

I will keep this topic as a regular topic and post RC Sailboat plans as I go. If you got any free model sailboat plans or site with plans please contact me.

If you seriously look for more plans read my review on this model ship plans dvd.