Boatworks Today: Testing Resin Strength for the Transom

Andy Miller knows a thing or two about resins and boat repair. But when it’s time to reconstruct the transom of the Bertram powerboat he’s restoring,  he wants to be certain that he’s using the right resin for the job – and it’s a big, heavy, important job! Andy thought it might be wise to actually see which resins are going to give the best results for both bonding, as well as laminating to the Coosa Board that will be the core of his new transom.  This video gives great insight into the differences between Polyester Resins and both strengthened and straight-up epoxy resin.   Watch and learn from the fiberglass repair pro, Andy Miller and Boatworks Today.

16 responses to “Boatworks Today: Testing Resin Strength for the Transom

  1. Something that has been pondered around the fire many times but not something I was willing to do. Great test thanks so much,

  2. Good comparative analysis. To take it to next level there are standardized tests for peel and shear. See https://duckduckgo.com/?q=astm+peel+testing&t=hl&ia=web

  3. Noticed Andy wearing a respirator when cutting the Coosa board. Is the stuff that dangerous when tooling?
    Wait, are you really putting 2 outboards on a beautiful Bertram inboard?
    Thanks for sharing the test.

  4. If you glue to perfectly flat surfaces together, you basically push all the “glue” out, a common problem with aluminum sheets. The solution is to put thin spacers in the “glue” such as little pieces of welding wire, about 30 thou diameter, this makes a big difference. Because of this I was surprised that the additional fiberglass layer in between did not make more difference

  5. A safer and more quantifiable test, without going off the tech deep-end with instrumentation, math, etc. would be laminating both sides of Coosa “planks” (same cloth, different resins) and stacking weights in the center while bridged across a measured gap.

    By the same token, glass on the outsides of all the Coosa layers will add much more strength and stiffness to the transom than in between.

  6. Polyesters are inherently/generally more brittle than epoxies. Obviously, this generalization is formulation dependent. But your testing method (stress riser/notch sensitivity) puts emphasis on these basic material characteristics versus more standard two point loading bending shear strength tests. Maybe more importantly, in any marine application the joint strength after repeated environmental cycles is the practical concern rather than the post fabrication ultimate prying strength.

  7. Yeah, vacuum the Coosa dust as you cut it! It’s worse than fiberglass dust. I’ve cut lots of Coosa. Great stuff but nasty to work with.
    Regarding the test, you didn’t try what I always recommend. The surface of the Coosa is so rough you need to resin coat it and let that cure, then scuff up and bond the two pieces together. Yes, that’s more steps, but the failure point (as you found out) is the weak surface of Coosa. If you fill all the tiny divots in the Coosa surface (actually air bubbles in the foam cut in half) with straight epoxy, then bond with epoxy/Cab-o-sil, you’ll get the best bond yet.

  8. Andy – Do yourself a favor and wear safety glasses and use the right cartridge on your respirator for these dusty projects. I recommend a P100.
    I love the Bertram project. I brought a 1977 26’ Bertram back to life. It is an amazing boat.

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