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Is fiberglass flexible?


John Stevens

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I have finished glassing the floor of my fir plywood pram. It looks nice, first coat of paint is on. But... man was it a lot of work.

My question is: Can I glass the plywood before I bend it? I'm going to be building a Minuet and thought it might be easier to glass it before it goes on, just the inside. Will epoxy/glass bend?

TIA,

John

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It depends upon which cloth you are refering to, but in general, no it won't bend 'that' much. Glassing even one side creates stiffness in the panel, think of the strength created in a glulam, same concept.

Sorry, I would definitely put off the glasing until the panel are bent in place.

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Agree, however- you most likely won't have to glass the inside of the Minuet- the checking comes from exposure to the sun more often than not, Normally interiors don't have the problems.

Other than the cockpit of course.

If the bend you are making is a very soft one then you can maybe get away with it. BUT- if it doesn't work you are out the ply, the glass and epoxy AND your labor.

'Tain't worth the risk usually.

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Here's an interesting experiment. Take a piece of wood about 2 or 3 feet long by 3/4" thick and fix one end to a workbench top. Hang a weight on the end and measure the deflection. Now lay up one layer of glass cloth on the top side. After it sets up remeasure the deflection the same way as before.

Now cut a new piece of the same wood and dimensions except make the thickness the same as the total thicknesof the other including the glass cloth. Measure its deflection and report all the results. I expect you are going to be surprised.

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Here's an interesting experiment. Take a piece of wood about 2 or 3 feet long by 3/4" thick and fix one end to a workbench top. Hang a weight on the end and measure the deflection. Now lay up one layer of glass cloth on the top side. After it sets up remeasure the deflection the same way as before.

Now cut a new piece of the same wood and dimensions except make the thickness the same as the total thicknesof the other including the glass cloth. Measure its deflection and report all the results. I expect you are going to be surprised.

You have to cheat and tell us how this one turns out ... I suspect deflection is the same on both laminated pieces, right?

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I did this experiment 30+ years ago with a daggerboard. The deflection was measured with the bare wood daggerboard with about 50# weight suspended from the end. Then one layer of 10oz cloth was laminated on each side. With the same 50# suspended, the change in deflection was not measurable.

Does this prove anything?

If the experiment were repeated with unidirectional or biaxial glass instead of woven cloth, I think the result would have been different.

Because woven cloth threads wander about in the weave, very little tensile stress is placed on the longitudinal fibers in the experiment. No tensile stress, no bending resistance. I have found laminated biaxial glass to have a lot of rigidity.

Whatever, go figure.

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I believe that Phil Gowens encapsulated his bottom panel in epoxy and then glassed the exterior surface prior to assembly. Since he was building the Weekender as a stitch and glue project so that it wouldn't have any metal to corrode in The Great Salt Lake.

I've seen a couple of others try to glass both sides of a panel and then bend it and it resulted in a total failure of the panels. (These were not Weekenders or Stevenson's Project boats.) Once you attach the glass with resin it becomes a pretty rigid and unforgiving composite. Slight bends maybe, side panels, cabin tops, bottom panels with a lot of rocker to them could all have issues.

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I posted a message on this topic a few days ago on UseNet. Other than the usual block of people who didn't read anything other than the fact that I was using polyester I got one "probably it will work" and one "maybe".

For me I've pre-coated the parts that don't bend. For the lazrette and transom, I used really large stringers that I pre-cut to the right curve, attached the plywood and then glassed that. The reasons I have for doing this was for convenience - the parts can be be laid flat and are easily accessible, and to do a better job - no drips, sags or runs. I also pre-coated and painted the underside of my deck for the same reason - I just masked off where the joints were.

For my sides though I'm now planning on attaching one side, coating and painting that and then doing the other. That way I'll only have one side that is really awkward to get to. I'm not going to fiberglass the inside of the sides though - just a fairly thin coat of resin and then a couple of coats of paint. I "did" fiberglass all the parts of the sole though as I fully expect to see water laying in the boat at some time and I also want it to be able to take some abuse.

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I actually glassed almost everything plywood - Inside and out. Then assembled it pre-glassed. I was worried about the cabin trunk taking the bend, but I did glass the exterior of it, just not the interior. Expansion is eaiser to take than compression. It still looks fine. I do find some surfaces have a bit of an imprint from the glass after 7 years, but haven't noticed it on the trunk.

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