Joining plywood end to end

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CarvedTones

Board of Directors, Vice President
Andy
Thinking ahead. It will be early July when I do this. Making a 13' stitch and glue kayak. The scarf joint has fallen out of favor. Finger joints or other interlocking joins seem to do better in thin (4mm is what I will be using) ply. I will epoxy and glass tape it. How do you make a box join for boards that big?

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smallboat

smallboat
Corporate Member
I've used the scarf joint and a butt joint with a butt block with thin plywoods ( 3/16 or 1/4") for stitch and glue boats.
Both work fine and are easy to do.

I've seen the "puzzle piece" approach on kit boats and I suspect it is chosen because its easy and quick on a CNC (vs a scarf) and provides alignment of the panels (vs a butt block)

Structurally and esthetically I'd use the scarf over the other two.
It is easy to accomplish with a router sled and a tapered jig ( if you want I can send you photos of mine)
I've seen it done with a hand plane or a power hand plane as well.

This link from Gougeon Bros is a great resource for anyone contemplating a build like this.
http://www.westsystem.com/ss/use-guides/
You can even download their 406 page encyclopedic The Gougeon Brothers on Boat Construction — for free.
I bought the original hardcover back in the early 80's and have since down loaded the current version.
Much good info there on all aspects of boat building and engineering composite wood structures.

If you were going to be doing this full time you could purchase their "Scarfer" which is a circ saw attachment that guides the cut at the shallow angle required for the scarf joint

For occasional use I'd stick with a router and sled or a plane.
 

CarvedTones

Board of Directors, Vice President
Andy
I did the scarf joint on a previous boat that used 6mm and it came out okay, but it definitely bent a little different. But I was told that for the thinner 4mm, it is more difficult to get a good scarf joint and even if done perfectly, for half the length of the join one side or the other will have a thickness of 1mm or less. Supposedly the puzzle piece planks bend more evenly along their full length because there is no single point at which it's all join.

Butt blocks are the method recommended by the designer and I probably should just go with that, but I read a few posts by people who went with overlap/interlock and like the results better.
 
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smallboat

smallboat
Corporate Member
Actually on 4mm ply, 1/4 of the length will be less than 1mm but that's immaterial.
once the layers are laminated it is acting as one piece.

Any scarf joint I've ever done went to a feather edge never mind 1mm

To make that consistent, I drilled some holes in the base of my jig and use my shop vac to provide a vacuum assist to hold the work in place while the router trims it clean.
No issues with the thin edge and it bends like the joint it isn't there.

Another useful technique is to stack several layers of your thin material and cut all the scarf joints at once.
that way each one backs up the next and they match
Of course a vacuum assist will only help on the bottom layer.

Not sure how the puzzle piece joint (or finger joint) works in flex since its all edge glued.
Guess if there's enough surface it will bend just fine.

Good luck with your build which ever way you decide to go!
 

CarvedTones

Board of Directors, Vice President
Andy
1/4 on one side at each end; 1/4 + 1/4 = 1/2. The point is that half the length has very little strength.

I didn't have a complicated jig last time. I used a plane with a miter ripped 2x4 clamped as a guide. I think it worked pretty well but I could definitely tell where it was. I only have second hand info on the interlock/overlap.
 

smallboat

smallboat
Corporate Member
Andy-
Don't mean to beat a dead horse, there are many boats out there built using any of these methods.

As for the scarf joint, I'm not sure we're talking about the same thing so maybe a pic can help.
Here's what I'm talking about when I say scarf joint.
The top drawing shows the two pieces stacked as if you were going to create the scarf with a hand plane.
Next shows them reoriented for gluing
Last- the finished joint.

scarf-joint.jpg

A couple things to note-
regardless of the thickness of the wood, you'll always be tapering to a feather edge. Doesn't matter if you start with 4mm or 3/4"
The finished joint is the same thickness end to end and the same thickness as the original material.
This is what makes it bend in a nice fair curve with the rest of the panel.
It also creates a smooth transition for distributing loads, no stress concentrations.- this is important in lightly built boats.
The 8:1 slope is the recommendation I've used from Gougeon Bros. and it has always worked for me.

Finally, when you bend the scarfed panel part of the glue joint is in compression and part in tension- none of it is in shear.
Compared to the "finger joint" where it would all be in shear.
Epoxy is great stuff, but my understanding is that shear is the weakest axis for the glued joint.

Here are a couple pics of the jig I built for using a router to scarf panels.
If your panels are very wide you'll need to adjust.
P1080319.JPG


P1080320.JPG



I use a piece of plywood as the sled base for the router to slide back and forth.
Just make sure its long enough to support both ends at full reach.
 

CarvedTones

Board of Directors, Vice President
Andy
No dead horse yet; you may have revived the old nag... :D

I did nowhere near 8:1 (~7 deg); I did more like 3:1 (~20 deg). I can see how that would work better. That would be a 32mm glue surface and even if I ignore the 8mm on each end where one side has just a flap of veneer, that's still 16mm (~ 3/4") in the middle. I am still a little confused about the big jig, but I guess with thicker panels you have a long taper. If I am doing my math right, for 4mm an 8:1 taper would only be about 1.5". I could probably use the plane method again with a lower angle. I have a jointer length hand plane (forget the # and too lazy to go dig it out right this second, but it is plenty long) so as long as I can get a 7 degree wedge cut on a board I should be able to use that. So you may have talked me back into scarfing.

Any tips on alignment for gluing? The other concern I had before was the fear of having the glue line too thick (panels too far apart) or having almost no glue and a tiny bump (panels pushed too close).

And finally, unless that shop vac hose is huge, I am thinking that sled joins small panels. The instructions I used before were for whole sheets and the plan I have now gives panel offsets to be marked off the whole sheet so I was thinking of joining whole sheets again.
 

smallboat

smallboat
Corporate Member
Yep - the jig is for narrow panels, think multichine boats.

I'm going to check my WEST System manual - I think there's some detail on scarfing whole panels there.
If so I'll PM you the pertinent stuff.
In the mean time here's my method.

To glue up a whole sheet I would be most concerned with clamping evenly across that wide distance
First arrange some scrap pieces to support the panels flat on the floor making sure there's support under the glue joint.
I would use more scrap to create a "fence" on one or both sides so the panels align lengthwise.
Then the issue is end to end
Here I would do a dry fit and make reference marks on the fence piece(s) for each panel part (top and bottom).
When its time to glue put down some wax paper or heavy poly film on the support under the glue line, even packing tape will do.

put your "bottom" panel between the fence rails with the bevel up and align it with your marks
mix up your epoxy and wet out the end grain on both pieces. (were talking penetrating epoxy here like WEST or System-3)
Keep adding more as it gets soaked into the endgrain, this will prevent a glue starved joint.
when both are thoroughly wet, mix some thickener with your remaining epoxy and spread it on one side of the glue joint. the one that's facing up is easiest
set the top panel over the bottom and align with your marks.
put another layer of poly over the top glue line and clamp.

I'm partial to drywall screws through a batten for clamping in a situation like this but if you don't like the holes left when you remove them you can use staples through a piece of scrap. These will go straight through both panels and into the support underneath
just wait for the epoxy to cure and remove the screws or staples. you'll fill whatever holes are left when you do the finish fairing on the boat.

Also, take a second to squeegee out the excess glue that comes out when you clamp. smoothing it under the poly film will leave a nice smooth surface and you'll be left with very little cleanup.

FWIW I much prefer a scraper for finishing hardened epoxy vs sanding.
 

CarvedTones

Board of Directors, Vice President
Andy
Last time I lined the boards up on the garage floor, duct taped the edge with 3 overlapped pieces using a wall paper roller, enlisted a helper to flip it over, slid a small batten under it to open the gap, put epoxy in the gap, slid the batten back out, wax paper (yeah have since heard plastic wrap is better) underneath and on top with a board on top of that with all manner of heavy things sitting on it. It was a pain to sand/clean after; little wrinkles in the duct tape left epoxy "veins" and cleaning duct tape residue off of wood is a chore.

If you do a nice write up, please consider posting in How To instead of just PM'ing me, but either way I appreciate it. I am not too old to be corrected... :embaresse
 

smallboat

smallboat
Corporate Member
Andy-
I checked and there's a 12 page chapter on scarfing in the book.
One section is specifically devoted to scarfing plywood panels.
I made a PDF of just that chapter and if I could figure out how to attach a pdf to this post or to a PM I would.
If you would like it just PM me with an email address and I'll shoot it to you directly.

Or you can download the whole book at the link in my first post.
Its well worth it and its free.
 

CarvedTones

Board of Directors, Vice President
Andy
Andy-
I checked and there's a 12 page chapter on scarfing in the book.
One section is specifically devoted to scarfing plywood panels.
I made a PDF of just that chapter and if I could figure out how to attach a pdf to this post or to a PM I would.
If you would like it just PM me with an email address and I'll shoot it to you directly.

Or you can download the whole book at the link in my first post.
Its well worth it and its free.
I have seen the Gudgeon book. The method I used last time referred to it also but claimed to be an improvement. It required less expertise, time and equipment but while it worked, I wasn't real happy with it. I ended up adding a piece of glass tape over it because it seemed like a soft spot in the bend. I will PM you for the PDF of just that chapter and share it on Google Drive.
 
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