Modulus of elasticity? You gotta be kidding!

I will reveal the morals of this story straightaway: Assumptions are dangerous. Willingness to change course is necessary. Humility is to be desired.

Please recall from the last post, that elegant set of rafters I built on the bench. Recall how nicely the new top curved over the rafters, when screwed down with the special fasteners I designed myself. And, remember the "item of faith" I cited: That the new top would retain its curvature after it was fiberglassed and removed from the strongback rafters.

Get all that out of your mind. It's gone. All gone. Poof! And that is good.

After that last post, Dan left a comment that changed everything. Thank goodness! I could have messed up big time. He wrote:

The modulus of elasticity (elastic modulus) for fiberglass is much different than for wood. In other words, the fiberglass-epoxy matrix will bend much easier than the plywood. In fiberglass boats it is the stringers and frames that give the hull it's stiffness. In the case of your top, the fiberglass-epoxy matrix provides protection against the elements of weather and provides very little to the structural integrity. 

 What does that mean for you? It means that what ever the plywood wants to do when it is "unhooked" from the strong-back it will do. The epoxy-fiberglass coating will do very little to make the roof stay in a curved condition.  

My suggestion: try a small test piece. Take a strip of plywood, hold it in a curved position, cover with epoxy and fiberglass, and release it. Let me know what happens. 

A follow-up exchange with Dan also established another likelihood: Not only would the top not hold its curve, but forces locked inside the structure by fiberglassing it while bent would be working in destructive directions when the top went flat. The fiberglass sheath and the wood would want to de-laminate.

I am convinced. But I used to work at a university, so I set up the suggested experiment. I took a five-foot strip of scrap top plywood and bent it between two chocks on the bench. I applied fiberglass fabric and epoxy. It looked like this:

I let it cure overnight, then trimmed off the excess fiberglass cloth.


Next, I applied another coat of epoxy; just enough to fill the weave of the cloth. Just as we are planning to do when we glass the whole top.

The photo at left shows what happened after tension was released this afternoon.

The test piece did not go completely flat immediately, but I am willing to bet that it will be near-horizontal by morning. There is no evidence of de-lamination, but that would probably take more time than has passed so far.


There's another piece of information you might pick up from the above photo. The new top, part of which is shown in the upper portion of this photo, is now flat. We removed the special fasteners and took the top off the strongback. The rafters are gone. The top has gone flat. I am patching the holes left by the fasteners now.

My friends Major and Paul are coming over in the morning and we are going to start fiberglassing. This project should begin to move pretty fast now. And, thanks to Dan, I think we will be moving in the right direction.

Lessons learned: Don't assume. Listen to friends. Experiment. Learn from mistakes.

And, please, please keep the comments coming.