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Hand plane for boat scarf joint?

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Forum topic by cootcraig posted 06-09-2016 06:25 PM 496 views 0 times favorited 5 replies Add to Favorites Watch
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cootcraig

58 posts in 673 days


06-09-2016 06:25 PM

I think this will be my entry to stitch and glue boat building.

storer quick canoe

I just know I want to do a scarf joint instead of butt joint. Would you talk me out of a scarf joint or advise me on a hand plane and other tools and technique.


5 replies so far

View runswithscissors's profile

runswithscissors

2183 posts in 1487 days


#1 posted 06-10-2016 12:05 AM

Scarfs are often cut with hand planes. Can also be cut with a router, which would require some sort of tilted sled to work. But for one boat, a hand plane should be fine. Check out Chesapeake Lightl Craft online to see examples of cutting scarfs with a plane.

Another option is to make butt joints with fiberglass. This is how Pygmy boats (kayaks, a wherry, and a canoe) are being built. The joint is made by laminating a layer of glass on both sides of the joint. As I recall, Pygmy supplies a finer weave of glass (called S glass, I think) to make an almost invisible joint. Of course, that joint is reinforced when the whole boat is covered in glass and epoxy. The but joint (2 of them in a 17 1/2’ Coho, for example) goes right around the hull. It doesn’t look strong, but in practice has proved to be totally adequate. I built my boat in ‘98, and have since paddled it on the open coast of Vancouver Island, in rock gardens, and have crashed in surf more than once. Also have done numerous rescues where you drag the capsized kayak over your cockpit to empty the water out. Never a problem with that joint. But one caveat: Pygmy kayaks are a more complex shape than the canoe you are looking at, which stiffens the hull a lot.

Pygmy’s first designs/kits called for a joint made with a butt block. Ugly, hard to glass over, and tends to create a stiff spot in the plank that throws off the desired fair curvature. That’s how I built my first one.

Scarf joints are as good as you make them. A sloppy job of planing or gluing would create a weak joint. As for using duck tape to assemble before gluing, that should work. Of course you’d do the gluing from the inside. Once that glue is cured, the joint will be surprisingly strong on its own. CLC wants you to tie joints with copper wire, which is then left in the hull permanently. I see no reason in the world to do that, as it contributes zero to hull strength, and makes the interior taped seam lumpy, ugly, and not as strong (because of small voids under the wire). Soft iron wire is the thing to use. After gluing, the wires are snipped and pulled out. Easy peasy. The whole thing probably takes an hour. In difficult areas, such as tight narrow crevices at bow and stern, heating the wire for a few moments with a soldering gun frees it from the epoxy, and out it comes.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View Planeman40's profile

Planeman40

805 posts in 2222 days


#2 posted 06-10-2016 02:15 PM

I would suggest hand planing both ends of the joint at the same time by clamping them together, checking constantly to make sure the planed surface is square to the sides. But I think you would find it much faster. more precise, and easier doing it with a machine and a jig.

Years ago I prepared the main spar of a home built sailplane by joining the multiple layers of the laminated spar with scarf joints, staggering the joints along the spar. I cut the scarf joints with a table saw using a special jig. Slow and tedious, but very strong! Its been a while, but as I recall the angle requited by the FAA for scarf joints used in wooden aircraft was 16:1.

Planeman

-- Always remember: It is a mathematical certainty that half the people in this country are below average in intelligence!

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runswithscissors

2183 posts in 1487 days


#3 posted 06-10-2016 08:42 PM

For boats, scarf bevels are usually recommended to be at least 8:1 up to 12:1

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View sras's profile

sras

4391 posts in 2591 days


#4 posted 06-11-2016 01:16 AM

I used scarf joints on my Stitch & Glue kayak. Like Planeman says, clamp both halves of the joint together.

Let’s use 8:1 as an example. If your plywood is 1/8” thick, set the top board 1 inch back. Draw a line 1 inch back on the top board. As you plane down, Use the reference line on top, the interface between the top/bottom boards and the bottom of the bottom board to guide your work.

When you’re done, you should have a bevel that is planar, starts at your reference line, ends at the end of the bottom board and passes through the interface between the boards right at the end of the top board. Hope that makes sense.

-- Steve - Impatience is Expensive

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runswithscissors

2183 posts in 1487 days


#5 posted 06-12-2016 04:22 AM

For what it’s worth, be aware that a scarf joint will shorten the finished panel a bit. Of course, that may not matter.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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