Sorry should have been a bit clearer, it is the weight of the engine bouncing an unsupported transom that's my issue. The trailer tows sweet, its just that bouncy load.
Thanks Mr G, I didn't realise that the bunks shouldn't be under the tubes. That's a good start.
I think Jeff what you're doing is going to suit me best, it's sort of what I had in mind with my point no. 1. Didn't think about strapping the boat down at the transom. Makes total sense.
When I used it the first time I used luggage strap to the trailer and a wood wedge to keep the engine up a little but that transom support was missing.
So is there an acknowledged method to make and cover a central bunk? I'll start a search....
Cheers all, thanks for sharing in my cheery popping