scarf joint

Hi Everyone,

I'm about to start a kaholo 12-6 from plans. I've only built kits in the past so I'll have lots of questions to be sure.

Cutting the scarf for the plywood is my biggest concern. I have a couple specific questions that I haven't found an answer to (yet) in the forum or the shop tips.

1) the scarf for 3mm ply is one inch. The manual shows the pieces stacked in a stair step fashion in order to cut multiple sheets at once. Is the stepback in this stair step approach the one inch or is is something more or less? I hope that made sense.

2) When stacking the sheets to cut the scarf am I correct in assuming they should alternate "good side" up then "bad side" up in order to have the sheets glue together with the entire length then being a "good" side for the exterior of the board and the other being the "bad side" for the interior of the board. I did a mock up using post-it notes and this seemed to work but I want to make sure before committing to the real thing.

I'll expirament with some scrap and be using a sharp blade. Any help with these questions is greatly appreciated!

Patrick-17lt, duckling, kaholo 12-6

8 replies:

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RE: scarf joint


You are correct, for a 1 inch scarf stair step the pieces one inch.  You are also correct about the good and bad side stacking proceedure.  Here are a few tip I'v developed over the years scarfing plywood.

1.  Stair step the scarfs the length of the scarf.

2. Alternate the sides of the plywood  so that you end up with two good sides up.

3. Place a dummy/waste piece of plywood on top of the top piece of plywood because when you sand or plane the top piece of plywood you have a tendency to round over the top edge. 

4.  Place a firm piece of wood or a flat steel plate or the sharp edge of you table saw under the bottom piece of plywood in the stack.  The  bottom piece of plywood will be cut to a feather edge and you want it well supported in the cutting process.  This piece of material is even with the edge of the bottom piece of plywood.  You will now beable to run the heel of you sander or plane below this firm edge.

5.  When cutting the scarf use the laminates as a guide.  That is the glue joints of the laminates will be straight edges when the scarf is correctly cut.

6.  For 3/4 inch plywood I use a power plane but this is not necessary for 3 mm material.  For my Kaholo I used 60 grit on a 5 inch disk sander.  You can also use a hand block plane that is razor sharp and finish up with sand paper on a block.

When glueing scafts I do the following:

1.  I tape off the edges of the scarfed  joint the full width of the scarf.  Where ever you don't want glue put  down tape.  It is a heck of a lot easier to pull off tape than sand off glue Also,  tape is cheaper than sand paper.  During the entire build I always taped off the boundries of the glued joints 

2.  After I have taped off the scarfed joint about 4 inch back from the joint I put down waxed paper, spread thicked epoxy, and place the piece of plywood to be glued.  The tape on the edges of the scarf will assist in placing the pieces.  Now you can place a piece of tape the full width of the scarf right on top of the area that the glue is being squished out of.  Then put down a second piece of waxed paper.  On top of this place a 1 x 4 strip of wood and weights.

Note, when scarfing the two bottom pieces to the aft bottom piece be sure to overlap the bow sections in the area of the tip of the bow and frame 2.  This detail may not be obvious but you will see it if you extend the center line with a straight edge to the bow.

When cutting out the 3 mm ply I used a sabersaw with a metal cutting blade.  I cut next to the pencil line and then block planed to the edge.


RE: scarf joint

Catalina Charlie,

Thank you very much for your response. It's really informative. The tips you gave I would never have known about. I especially like the idea of using a sander. Good to know I have that option.

I'm hoping to get some pieces cut out this weekend. Now if I can just find some 3mm okoume in Colorado...


RE: scarf joint

I'm wondering if anyone has used a butt joint instead of a scarf joint for the Kaholo.  I've built 10 S & G boats and have always used butt joints.  I'm much more comfortable doing that.

Any thoughts out there on the feasability of using this method?  I usually use 6 oz. biaxial tape with a 4 oz/ directional FG finish veil on the exposed side(s).  Plenty strong I think.

Thanks!  Marc B.

RE: scarf joint

Yes, a butt joint with fiberglass tape (AKA Payson joint) should work just fine. In fact, CLC recommends fiberglass tape on the inside of their "finger joint" used in their kits. Or you could use a small piece of 3mm on the inside. I've done that as well, and zero on outside save for the fiberglass sheath cloth layer.

RE: scarf joint

You will be much happier with a scarf joint.  Take a little time practicing cutting a scarf.  Epoxy glue with filler is very forgiving when gluing scarfs.

RE: scarf joint

Charlie C ... great piece of advice on the sacrificial piece of plywood on top.  I will use it on my Mill Creek build.

RE: scarf joint

   I just watned to echo some of the advice above. I have ruined, or come colse to ruining joints when not using an extra piece of plywood on the top. I also use a sander primarily when making the joints. The handblock plane caused a lot more cursing and frustration for me than actual progress. I realize this is likely my own failing, but I just had/have trouble with making scarfs using the block plane.

The time you save in stacking cannot be overstated either.

Just don't make two left sides of your Kaoholo like I did. Its really depressing when you start to wire up your board and realize you need to go back multiple steps.

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