If you are going to sail it you need to either have the center bar in or rivet the straps, I would not suggest sailing before you do.
My 5.2 doesn't have a center bar and doesn't have any bolt or rivet either to prevent rotation. The bar and its front attachment was removed by a previous owner and not replaced by anything I could find. The only rotation preventer seems to be some black gunk (some kind of adhesive?) between the hull and the beams, but as I never took the boat apart I'm not even sure what it is...
I never had any problem with anything rotating, so far (touch wood). Should I worry about it?
