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Scarfing

Posted by DonTyson 
Scarfing
July 21, 2019 02:55PM
Are Butt Scarves acceptable when scarfing Ply sheets together?
Re: Scarfing
July 22, 2019 05:37PM
Not in my book. Long scarfs are worth it.
Re: Scarfing
July 25, 2019 04:17AM
Depends.

I wouldn't use a butt joint on a dory side panel because it has a nice smooth arc. A butt joint will most likely have a hard spot when bent to the degree a side panel arcs.

Dave Carnel (hope I spelled that close enough) and Dynamite Paysen published building instructions for epoxy butt joints covered in fiberglass, and their testing suggested that the plywood fractured before the joint. They both argued for using glassed butt joints. If you google you should find links to descriptions.

The boat I am currently working on is 54 inches wide. The extra 3 inches on each side of a 48 inch plywood panel was butt jointed, not scarfed, and covered in glass on both sides. Most of the length of the extra width is also either covered in, or close to, epoxy chine fillets. The extra piece was 8 feet long, constructed from two 4 foot sections butt jointed together. In this case I elected to use a butt joint to get a full 8 feet from two 4 foot sections. They were cut off the end of the two sheets of plywood that made up the primary bottom panel. By using a butt joint to join the 3 inch piece it didn't need to be very wide, and I didn't need to attempt to make an 8 foot scarf.

I think the curve in the bottom of a river dory (the rocker) is gentle enough that a butt joint would be ok. Certainly strong enough if there are a couple layers of glass inside and out. The bottoms of the boats I have built have scarfed joints. I wasn't worried about the extra wood used.
Re: Scarfing
July 27, 2019 12:09AM
I agree thAt it is the sharpness of the curve that would dictate wether or not to use a butt scarf. I would agree that either of your bottom scarfs ( the lengthwise and/or the crosswise scarf) would be fine regarding strength and stability. But I would be worried about butt scarfs in side panels potentially showing hard spots. JEFF Spira recommends butt joints on most of his long and narrow power dories.
Re: Scarfing
July 27, 2019 12:20PM
I have to put two side panels down flat on my shop floor in order to glass them. I've been putting it off for half a year because of so many resin putty bumps on the floor. And I'm retired now. I can afford to be lazy. I don't have no deadlines a no kind. Other than the undertaker?

Yesterday I rolled everything out of the shop. Put on a mask and used a big Milwaukee right angle grinder to smooth all the bumps. Vacuumed. Blew with air compressor. Moved everything back. Now I can proceed. It wasn't that bad once I did it. But I had a mental block and just kept putting it off.

Some mostly beginning boat builders get put off by scarfing in a similar way. Build a scarfing jig for a saw. It will take a whole day to get it built and mounted on the saw. But so what? Hey. You are making a boat. That's a big deal.

Then make a few scarfs. It isn't that hard. Calculus and differential equations are hard. Scarfing is just work. And it is so so so much stronger than butt joints of any kind.

https://montana-riverboats.com/?robopage=Drift-boat-info/Homemade-Boats/Scarfing-Jig
Re: Scarfing
July 28, 2019 11:08AM
I remember having to do that. Friendly old Gumps of Goo, grinding, sanding and vacuuming the floors.
Thanks for sharing the Scarf jig. Looks straightforward. I've always cut scarfs with a razor sharp block plane.......no wonder I hated cutting them.
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