Ok a bit of progress made.
My mistake was lofting the planks in turbocad and then importing them into rhino, even though the drawing was saved as a .3dm rhino file.
I decided to import just the frames and then loft inside of rhino. Aligning tool was then used to make sure the plank sat within the correct constraints (sometimes it would almost look like left was crossing to right from each frame to the next, sort of like a double helix. That was fairly easy to solve.

The plank could then be unrolled to a 2d shape shown below (on the right).

This served as a test and seems to work well. I have just re-drawn the stem using splines, infact 7 separate splines, 1 for each plank. These provide the correct curve which the loft can attach to and ensure the plank will take on the same shape as the stem.
Next issue is a fairly big one. The transom, or lack of one. The transom on the coble is roughly 45 degrees, and the notches must be drawn in order for the loft to attach to. Once that is done, I would say the planks could be trusted to fit the jig in the real world. However from my attempts, drawing on a 45 degree angle doesnt seem easy. I have tried to adjust the workplane but lines seem to be going all over the place aaarrrgggghhhhh!
More soon hopefully
Dan