I designed the boat
: myself and am trying to figure out how to place the cockpit. I know where
: the center of bouyancy is. How far behind the CB should my seat be? It's a
: 20 foot overall boat.

Good moring Dave,

Perhaps a lesson from the SOF community is useful for your question. Morris describes the process of setting the back brace so that the center of gravity of the boat is just aft of the center of buoyancy. This is so the boat will ride very slightly up in the bow.

He says that your back should rest on a structure 1 foot (that is the anatomic foot of the kayak user) aft of the centerline (center of buoyancy in most cases of Greenland boats) Then the back brace should be put back far enough from that spot to allow padding, etc. In your case, this should put the center of gravity of the kayaker/kayak about 6-8 inches aft of the center of buoyancy.

Hope this helps.