Monday, June 12, 2006


So here i was thinking "Great! I've finished all my scarfing, time to get into the lofting!" So I get one of my 10 inch wide planks and manhandle it onto the table. I'm almost there when, SNAP! My carefully made and smoothed scarf joint snaps! AUUUUUGH! As Charlie Brown would say. At this point I had a choice to make. Would I test my other scarfs? maybe this is a one off. So I tested another one, it snapped too. I remember my Dad suggesting I do the scarfs this way instead of using the bevel. "Why?" I asked, "because its stronger" came the authoritative reply. Stronger = Better as far as i'm concerned. So off I went.
Some background on the lap-scarfs.
My Dad and I built a 37 ft fiberglass yacht together. We built it from scratch. Starting with a rectangle pegged out in the back yard we built a shed, a strongback (for setting the frames on) and then proceeded to construct a male mould and laminate it with fiberglass. So far so good. When Dad came to the bulkheads, he had to scarf three sheets of ply to make something wide enough. He used a lap-scarf rather than a bevel. I can see now, why he did. A lap scarf is easy to make with a router, it's self aligning and it's neat. But wait, there are no bending loads on the bulkhead, mainly torsional. Plus, the whole thing was laminated with fiberglass, making the ply a core rather than a sole load bearing structure.
The ply in the LT 17 is a core too but it is subject to bending stresses during construction and in service. Bevel scarfs make much more sense in this situation. Look at my laps. As soon as the resin bond fractures on the butt end of the scarf, the material becomes half as thick at the next point of failiure. At which pint 4mm ply will break easily along the line cut by the router. In other words DISASTER!


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