Butt Joint

I know the common method of joining 2 sheets of plywood is the scarph joint but I am considering using a blocked butt joint.

 

From what I can tell in my research is that a epoxied blocked strapped joint can be as strong or stronger than a scarphed joint.

 

I dont need to bend the joints too much as they will be on the bottom of a flat bottom boat and the blocks on the back of the joint will be "hidden" in the benches of the boat. I could also use the extra length by not having scarf joints.

 

Here is a link for reference: http://www.boatbuilding-links.de/Jim-Michalak/joining-plywood-sheet...

 

Just looking for some input on why this may not be a better idea.

 

Thanks,

 

Andrew

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Permalink Reply by Rick Newman on April 19, 2011 at 3:02pm

Andrew, I have an older drift boat bottom that has a butt joint with a block running across the length of the joint. Other parts of the boat rotted but the joint is still solid. If you can hide the joint, don't place unusual stresses on and use the proper glue it may work as well the the decades old one on my drift boat. Will it be as strong or stronger that a scarfed joint, I don't have an engineering degree to quantify that. As some have suggested previously do two sample joints from similar wood, cure them and try them with your glue, your wood and your techniques then what you build will be based on what you know to be true.

 

Good luck,

 

Rick Newman

Permalink Reply by justin nicola on April 19, 2011 at 6:47pm

the boat im building now is just a glue joint.  i have framing running across it in 3 spots (one down the middle, one on each side for strength and stability.  any type of joint (in woodworking in general) you make is as strong or as weak as the glue you use to seal it.  clean cuts, glue brushes, and high quality waterproof glue will hold a butt joint together just fine.  like rick, i am not an engineer, nor am i a professional boat builder, so i could be talking nonsense.  and scarf joints, by their very nature, are exceptionally strong when done properly.  but i don't have the least bit of worry about the butt joint in my hull, neither should you.

Permalink Reply by Lawrence E. Long on April 20, 2011 at 5:12am

Andrew:  don't sweat the block joint. It will  be strong enough - but won't take as smooth a curve or bend as will the scarf joint. The first skiff I built used them on the bottom and sides-before I learned how to scarf.  It's still in use after 11 or so years.  And as you say it will "save" you 3 inches or so.

Good Luck

Permalink Reply by Andrew on April 20, 2011 at 6:02am

Thanks Guys,

I was pretty confident that the butt strap joint would be fine.

 

The other reason for using the butt joint it I am using a plywood that is faced with aluminum and it would be far easier than trying to incorporate the aluminum face with the scarf.  

 

Andrew

Permalink Reply by Lawrence E. Long on April 20, 2011 at 10:24am

What's with this aluminum backed plywood? Aluminum "glued": on?  Sounds like you will have a painted wooden boat on the inside and aluminum boat on the outside.  Where did you run into this stuff?  Sounds nuts to me- but at one time I thought the electron was the smallest atomic particle possible.

Keep us posted

Permalink Reply by Andrew on April 20, 2011 at 10:49am

I ran into it by accident really.  I work for a large canadian municipality and we have a sign shop that makes all of our traffic signs etc. and found some there.  I was then able to find a source of some used stuff.

 

I am just using it for the bottom, the sides will be 6mm Occume and the chines & gunnels of hemlock, top decks will be exterior grade baltic birch.

 

I have been calling it a neo-traditional boat build.  I will post my build thread when I have a minute.

 

Here is a link to the product http://www.sterlingpromotions.ca/Omega-SignPly%AE.html

Description:

SignPly Single-sided, painted aluminum over furniture grade exterior plywood core

Benefits
Impact resistant, colorfast surface is warranted not to fade, flake or peel.
Average outdoor life expectancy is 5 to 7 years if properly edge sealed.
Construction
SignPly is composed of painted aluminum face on premium furniture-grade, exterior plywood.
Factory-baked polyester paint on the finished side with a mill-finished aluminum backer to provide balance and prevent warping.
Permalink Reply by Dave Z on April 20, 2011 at 11:30am

I actually considered a butt joint on my current builds.  As others said, put it in a flat spot of the boat.

 

I built a little weekender skiff once that used a butt joint and its no big deal.

 

Permalink Reply by Andrew on April 26, 2011 at 1:44pm

I am pretty sure I am going to be using a strap/block butt joint for the bottom panels of the boat.

 

Would there be any problem with using 3M 5200 as the adhesive? I know epoxy is preferred for scarphs, and I will epoxy those, but would 5200 not work here if I coated the butts and the block/strap liberally and screwed it all together?

 

Andrew

Permalink Reply by Dave Z on April 26, 2011 at 5:44pm
please use epoxy on the butt joint.  5200 is great stuff, but not in this application.
Permalink Reply by Andrew on April 26, 2011 at 7:42pm

Since you put it so nicely Dave I will use epoxy. :)

 

Andrew

Permalink Reply by Lawrence E. Long on April 26, 2011 at 7:18pm
forget the 5200.  Epoxy, several coats to soak in and then final coat with cabosil/milled fibers

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