to update this;
The rib needed to be towed before I got my act together,so three bits of 16mm mooring line 6 bowlines about 20m between the boats and it worked a treat. The only problem with this set up was the rib's center bowline working it's way over to the left,this made the rib tow to one side but still not a problem.
As usual after the plonk chat etc I forgot it was there,decided to press on turned round and there it was, up on the plane at 25knts....
I eased of the power and it took at bit of a fit as it climbed over the wake coming back into line.
this was a calm day, things I've no doubt will be different in the rough stuff .