I have been meaning to post this information for some time now. I just re-discovered the pictures on my computer and decided to share this information. I might be of interest to some of you builders.
Back when I was contemplating building a DB I decided I didn’t like the look of lap joints common in drift boat construction. I wanted a cleaner looking frame for the boat. So I decided I’d like to use half lap joints between the bottom and side frame pieces. Also I decided there were to be “no screws” in the boat’s hull. Only epoxy would be used to secure the boats frames, sides, bottom, etc.
The problem then is will the half lap joins be as stiff and handle the loads as the screwed lap joints. I called Gougeon Brothers (West System Epoxy developers) and discussed the issue with one of their experienced engineers. He suggested I make a sample of the different designs and they would test them for me (free of charge too). So I made three frames. The first was a standard lap joint with glue and screws, the second a butt joint with a plywood gusset also with glue and screws and the third an epoxied half lap. I made them all to frame No. Five specs and delivered them.
They then prepped them to run the test in their tensile tester and took each to yield. The pictures below show the set-up in the machine.
The graph shown in the last photo shows the three curves, force over distance. The first is the lap joint the second the butt joint and the third the half lap. The steeper curve represents the stiffness and the height of the curve the load to yield (fracture).
I went to Gougeon Brothers to pick up the results and the test samples and after reviewing the results with their test engineer decided the half lap joints would be sufficient.
After four years, they are strong enough.