Wooden Boat People
By McKenzieDriftBoat.com
Lap Joint vs. traditional scarf for joing two big bottom pieces
As I get closer to putting a new bottom on my boat I am leaning towards doing a lap joint to combine the two 4' x 8' marine grade plywood sheets I will need for my new bottom. I've been reading up on scarfing them together but with this being my first restoration and also lack of space/tools I feel more comfortable with a lap joint. Any thoughts or feedback on this? I'll be using 1/2'' hydrotek marine plywood for the bottom. Or, is there anyone in Utah that would want to help me scarf the bottom for some free beer/food? :)
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Replies to This Discussion
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Permalink Reply by Randy Dersham on
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A scarf joint is much easier than expected once you get one done. A single lap will want to pop at the seam like a crease in paper. One way to prevent this is to not make the lap line straight. I do this with a CnC and do many small laps.
The scarf is easier than the non-straight line. There is a lot of instruction on how here on the site if you do a quick search of the forum for scarf.
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Permalink Reply by Lawrence E. Long on
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Jon: Get some scraps of PW- thickness about what your bottom will be and lay out an 8/1 scarf. Use a jack/fore plane(block plane will take a little longer) and practice cutting a scarf. It might take a few trials to get the tecnique down pat. This is not rocket science! Go for it.
Good Luck
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Permalink Reply by Kevin C. on
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Don't fear the lap joint....the interior layers will show you the way...
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Oh sure just when they convince me to go back to the scarf you make me want the lap! :) I've got some spare plywood in the garage so I'm going to try both and see how they go. Kevin, do you usually us a router to do your lap joints on the 4' wide pieces?
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Nice, then I guess I'll just practice the scarfs with the spare wood! Whats your favorite epoxies for the joints. I've got some west system six10 left over I'll probably use for the practice but want the good stuff for the bottom.
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any of the epoxies work. I like System 3 since its a 2 to 1 ratio rather than the 5 to1 West system. Coat both sides , let soak in- end graion plys will soak it up and look "dull". Recoat with unthickened then lay on a coat with thickening agent- about like runny peanut butter. Not too much pressure on joint- weigh down with concrete blocks.
Good luck
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I built a 37 foot sailboat out of plywood. The hull was two layers of 1/2 inch plywood. It took a lot of sheets to do the job and I joined every sheet with scarf joints. I got real good at doing them. I put the two sheets together, the front edge of the top sheet at the rear edge of the bottom scarf. If it happened to be a four inch scarf, then I had an 8 inch wide surface to plane down. I took off the bulk of the material with a power planer and then used a combo of a beautiful old jack plane and a block plane to finish the joints off. Lots of work and very good joints. Two coats of epoxy with time for it to soak in before putting the pieces together.
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Permalink Reply by Greg Jensen on
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The scarf joint will be MUCH stronger than the lap joint. Expecially on the bottom wher there will me much more pressure and torque forces in play. I used a router for all of my scarfs, and once you get the initial cut done, you will never worry about scarfs again.
good luck.
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