PDRacer Sailboat - epoxy and fiberglass stress experiments

Epoxy and Fiberglass Stress Experiments
By Gougeon Brothers / West System Epoxy

I had done an experiment about bending sticks of wood with different coats of epoxy and fiberglass. [click here] Gougeon Brothers have done extensive testing with their epoxy and fiberglass, I contacted them, asking if they had experimental data they would like to share & publish here, below is their response with some interesting data.

From: J.R. Watson
Thanks for the opportunity. Your test with the bucket and water that progressively increases cantilevered stress and allows measurement of deflection is great. So simply yet so effective.

We did some tests to plywood where we exerted pressure to a 2 x 2 flat panels and measured deflection under a certain force. Then coated it with epoxy and measured deflection, then added 6 ounce glass to one side, then both sides, then with carbon fiber; all with the goal to measure the benefit in stiffness for the weight increase penalty.

To summerize: All panels were the same thickness (7 ply 3/4 inch thick material). Basic stiffness of Luaan was 9% more than Okoume.

6 oz woven fiberglass cloth to one side (tension side) showed 20% increase in stiffness with a 7% weight increase.

With both sides reinforced, stiffness increased by 32% with a weight gain of 11%.

Reinforced with 5.7 3k woven carbon fiber to one side stiffness increased 30% with a 5% weight increase.

Both sides reinforced with carbon fiber resulted in a 66% increase with a 11% weight increase.

Similar results would occur with lumber vs plywood only flaws in lumber may affect stiffness and ultimate strength whereas flaws are randomly spread with plywood and similar laminations. The results would be more dramatic with thinner material (say 1/4" thick).

Stiffness and strength are separate issues. Usually, structures are strong enough but not stiff enough. Also, usually lack of strength is due to flaws. If under built, it simply is not going to be strong enough.

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