advise on scarff joints please

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advise on scarff joints please

Post by gssmith » Sun Mar 04, 2018 12:32 pm

I am Gregg Smith, New member to the forum from Corvallis Oregon.

I am building the scrambler, a 15 1/2 ft sled with traditional plywood on frame construction. The plan call for 1/4 inch plywood on the sides and 1/2 on the bottom using two layers of 1/4 plywood. I want to use scarf joints to join the plywood panels. Please give me input on scarf joint methods please. Pro's and pitfalls.
ALS, Being 16 ft in length and not having a local source of already made 16 ft plywood panels, the two 8 foot panels join within 6 inches of the center of the boat. if Start at the transom with the first layer, the joint is just forward of the center of the boat. If I start at the bow with the second layer with an 8 ft panel, the joint will be about 6 inches aft of the center. My question is will the joints and the resulting layered bottom be strong enough givin that the joints are so close to each other.
ALSO, Is a scarf joint better, ( Stronger and better looking) than a but block joint with 1/4 inch plywood on the sides?

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Re: advise on scarff joints please

Post by joshuab » Sun Mar 04, 2018 2:31 pm

Welcome Greg, I'm about 2 1/2 hrs. south west of you, i used a butt joint with backer on my Zip. I have seen several different methods for scarfing plywood, setting up a jig for router, building a guide for a skil saw, even a belt sander alone, there are several videos on youtube demonstrating each type of setup, i think having the two layers with staggered joints even 6" to 8" apart will be plenty strong, in my opinion a scarf is stronger than a backed butt joint.

FYI: I bought all my marine ply from crosscut hardwoods in eugene (just down the road from you)
I cut twice and it's still too short :mrgreen:

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Re: advise on scarff joints please

Post by JimmY » Sun Mar 04, 2018 5:33 pm

Hi Gregg,

I scarfed the 1/4" plywood on my Squirt. I used a router and a jig to do the joints because I have one and felt comfortable working with it. This worked pretty well, but you need to have some way to clamp the plywood flat across the width of the joint. I tried double sided tape which did okay. I did a 12:1 ratio, so my joints were 3" long on the 1/4" plywood. When gluing the joints, I used System 3 Silver Tip (thin glassing epoxy) and painted this on each half of the joint, and then thickened the epoxy a bit with microballoons and slathered this on before clamping the joint together.

The joints are very strong, and I don't worry about them in the least. I don't think you will have any problems if the center joint on the bottom is staggered by 6". The Scrambler also looks pretty flat in the middle, so there will be very little initial stress on the joints.
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Re: advise on scarff joints please

Post by PeterG » Mon Mar 05, 2018 9:47 am

Hi Gregg, I scarfed the plywood for my Malahini (1/4" and 3/8" okoume) by hand using hand planes. I also used a 12:1 taper for these to get maximum surface for the epoxy and glued it the exact same way JimmY did. I had tried the router method and was not happy with the results and I wanted a neat glue line because it was going to be visible inside the boat. I had excellent control with very sharp hand planes for removing the bulk of material and then finished them with a 4" x 12" sanding block with a piece of 60 grit sanding belt attached. I match marked the joints to line them up properly for gluing later.
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Re: advise on scarff joints please

Post by Hercdrvr » Mon Mar 05, 2018 10:06 am

Your botttom planking is unique compared to the responses so far because you are laminating to layers of plywood to achieve 1/2” thick bottom. I would scarf both joints and put butt blocks on the inside of the boat between the battens.
You’ll be fine with your scarf joints within 6” of each other . These boats are tougher than you think. My 2 cents.
Matt B

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Re: advise on scarff joints please

Post by Bill Edmundson » Mon Mar 05, 2018 10:52 am

The setup that I liked for plywood was by Falcon (Porter Harvey). He made a "table" sloped at 8:1. He put a fence on the end to guide his circular saw. He could clamp whole sheets of ply to the table, 1 up & 1 down. He cut whole sheets with one pass.

I personally think 8:1 is plenty.

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Re: advise on scarff joints please

Post by chugalug » Wed Mar 07, 2018 9:18 am

:D Found this in old "Small boat journal"
wish I knew about this earlier :roll:I think the "7 inches should be 7 degrees.
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Re: advise on scarff joints please

Post by hoodman » Wed Mar 07, 2018 12:01 pm

That's pretty much exactly how I made my table saw sled to do the scarfs for longitudinals.

Building a Geronimo......!

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