Radar Arch

[maa. melee]

Thinking of custom building a radar arch...very simple one, to support a canvas top and maybe hold nav. lights. I have an abundance of 1/4, 3ply, ACX, very good stock, no voids, no errors on A side. I'm planning to laminate the plywood in a jig, ultimately getting the lamination thickness to somewhere around 1.5-2". My question is, can I use a good polyurethane glue for glue up and then sheath the entire arch in FG cloth and epoxy resin? Since it won't come in contact with too much water, I think it should hold up.

[pipefitter]

How big? width/height? Sounds kind of heavy at that thickness. Since you are going to be laminating it might be better if you consider a "skinned" arch that is hollow inside so you can hide all the wires.Cold molded,maybe?

[Bruce Hooke]

I have never used polyurethane glue but based on what I've read and heard about it, you should not have problems with it surviving in that setting. The big debates seem to be around whether it is suitable for wood that will regularly get saturated.

What I do want to note is that plywood is damn poor stuff for laminating because half the grain is going the wrong way so half the weight of the finished laminate is pretty much wasted material (well, not half the weight but a fair amount of it -- the laminates nearest the center don't really do that much other than keep the two edges far apart (and resist some sheer)). 1/4" 3-ply plywood is better than thicker plywood or plywood with more plys because more the wood is going in the right direction (assuming you run the face grain lengthwise along the arch), but still...

[JimConlin]

A few thoughts-
I'd expect that the curves of a radar arch would be tighter than the 1/4" ply would tolerate, particularly if oriented the long way.

Doesn't PU glue need fairly tight clamping so that the joints don't expand to spongecake? Clamping could be tedious on such a shape.

A 2" thickness of laminated fir ply, topping out at maybe 8' above WL sounds heavier than needed and a real detractor from stability.

Were a stack or 1/4" DF ply mine, i'd be looking at the designs of Sam Devlin and Ruel Parker. They produce some nice boats of laminates of DF plywood.

[pipefitter]

Seems one would have better luck cutting saw kerfs throughout the radiused bend's ID so the only the outer ply was actually bending.Say about a 9-11" radius.then you could make it in 2 plywood layers, one inside the other, leaving a hollow between the two using an appropriare shaped edge band to hold it's shape? Could use spacers in between the 2 skins to keep from collapsing and as long as the top was crowned you woulnt'd have to worry about it ever sagging in the middle.Wouldnt have to use a jig that way.Be tricky to layout the ply patterns though if was a lean forward or backwards arch.

[maa. melee]

The Poly glues do need good clamping pressure, and I had great success with jigs and custom cauls for this. Hrmm the weight might be an issue but there wouldn't be enough crown to allow me to go completely hollow...Just thinking of some smart-ass hanging from it like a chin-up bar. :mad: The span is about 8' with a height of about half that. Bruce, you're absolutely right about the cross grain lams as deadweight...I could laminate the arch as if it were a full bulkhead but that'd take a sheet of ply per laminate...not a good plan. I chose the other route because now I can basically laminate long strips of ply into the jig. The arch wouldn't have that much crown to begin with figuring its just holding up a canvas top and my boat cover for storage, and maybe some nav. lights. I like the hollow idea, wherein I can probably make the back and front from 2 sheets of ply and then space them off with well-placed webbing and epoxy filleting, although to be honest, I doubt the extra weight would even matter.

[pipefitter]

Ok,I pictured a canvas bimini top that folded and was attached to the arch and being most of those are crowned that the arch would have to be as well the same to match the bimini as it was folded against it.What kind of boat? Sail or power? Also I had figured it to support a radar so I wasn't thinking very simple or small.I have had to build skinned arches before but with sheet aluminum over a tube frame.Not an easy task.Probably easier to do an airplane wing or something.Good luck with your project though.Take some pictures,would be nice to see how it works out.

[maa. melee]

It's an arch for a power boat. I might built the bottom foot or so out of wood lam and then the upper part from SS tubing or Alumnum tubing, depending on how much I want to cut down the weight high up. I like having the option of mounting a radar reflector, an antenea, nav lights, and a spot light on the arch. Keep the fordeck and helm clean.