Originally Posted by JohnHumbert
A 5x7 metal top on a 18' Whaler will flip that hull in a heartbeat. A 4x6 will probably have the same result. Don't do it.
The boat is too narrow and floats too high (there's not enough hull in the water). All you'd need is a small amount of rocking and the changed COG will cause the hull to flip.
When I had the T-top off of my boat to custom fit the home-built electronics box, I was surprised how light the structure was... Maybe 30 pounds total for the alloy/canvas.
Of course after installing the box, the radios, lights, wiring and whatnot, its probably twice that heavy now. You might ask your top builder the net weight will be. I personally don't think that the change in center of gravity will be a deal killer.
Thru-bolting and back-plates might be. You want to have the top mounted very sturdy, and that means nuts and bolts rather than just screws. I don't know if Whaler foam will allow for that. Your dealer should know.
Regarding the sizing, the extra 11-square feet will really be appreciated in going to the 5x7. Go big. That's room for two more people in the shade.
Regarding folding the structure, I have nothing to add other than that's going to complicate things quite a bit. If you can avoid the break-away mechanism you'll have a more sturdy build at a lower cost.