butt block assembly

[Bluegill]

Splain how you assemble a buttblock with only 1/2 inch plywood ??? I am about to attached my 1/2 inch meranti to the bottom of my boat. The plans call for using a buttblock for the bottom. I intend to use a scarf joint on the bottom and perhaps a buttblock. Buttblocks look so clunky. Splain how you assemble a buttblock with only 1/2 inch plywood ??? I do not understand how you can pull the boards tight with only 1/2 thick stock.

[Gib Etheridge]

You could lay it out on the bench, epoxy the surfaces and weight the 1/2" plywood butt block with sandbags. I would still use something of a scarf joint as well as the butt block, maybe 4 to 1? Just wouldn't feel right otherwise.

[Jamie Orr]

I used a 1/2 inch thick buttblock, same as the 1/2 inch planking, with 1" bronze screws from the planking side, (outside) countersunk so I could leave them in to permanently back up the glue. The screws drew up the butt nicely and after the glue set I ground off the points. They came through because of the countersinking and because the half inch ply was only 12mm, but that was okay as the screw was full width nearly all the way through so the threads had a good grip.

After 12 years sailing she's still holding together.

Jamie

[Lewisboater]

You generally use a butt block about the same as you would use a scarf...12:1 with the same thickness so your butt block should be 1/2" ply with a width of 6" and a length suitable to the joint. The grain should run in the same direction as the original plywood. You normally make the joint on a flat surface if you can, that way you can weight the block down to ensure a good solid joint. OTOH I just did it with the bottom panels installed on the boat and though it was 1/4" ply it went together just fine. If the joint is in an area where the panels have a lot of curve... well that just wasn't thought out very well. You will have difficulties and probably a flat spot at that point. Butt blocks are normally installed at the point of least bending or stress.

Here the sides and the bottom are buttblocked...the butt on the bottom is inside and about 3" back or aft of the side blocks.



Here are the side panels with their butt blocks untrimmed yet.



This boat here shows the joint where one panel of the bottom will be butted to the other. There are 2 butts on the bottom of this boat and one on each side.



You can just see the side butts under the frame in the middle.



None of these panels are bent a whole lot so the block works well and there are no noticeable flat spots. The joints are just as strong as if I had scarfed them but I was able to keep the full length of the panels on the sides...16' so the boat runs about 15 1/2' long instead of being 3 or 4" shorter. I was also able to install the blocks on the inside of the bottom after the panels were glued and screwed onto the boat...yesterday afternoon.

[Ian McColgin]

If you do a decent 12:1 (or even 8:1) scarf you do not need or want butt blocks. Butt blocks in plywood construction have their place if there's very little bend and very little stress at the joint. Even so, in plywood it's glued and screwed. It's the glue doing the work with the screws really serving as left-in-place clamps.

G'luck

[TerryLL]

... Butt blocks in plywood construction have their place...

Have to disagree with you on this one. To my way of thinking a 12:1 scarf is the only proper way to do the job.

[David G]

Have to disagree with you on this one. To my way of thinking a 12:1 scarf is the only proper way to do the job.

I have to agree with Ian. There are a couple ways that butt blocks are better.

1. If you're trying to squeak the last bit of length out of your joined panels, and the scarfing loss would require adding another scarfed length.

2. It's much easier to achieve a 'good' grade of butt block joint than it is to accomplish a 'good' scarf.

3. It's easier to align the panels when butt blocking.

4. A butt block is quicker.

My main objections to butt blocks are:

1. They can look a but clunky.

2. People sometimes forget to leave limbers.

3. They can create an unfair curve when incorporated into a hard bend.

[TerryLL]

I have to agree with Ian. There are a couple ways that butt blocks are better.

1. If you're trying to squeak the last bit of length out of your joined panels, and the scarfing loss would require adding another scarfed length.

2. It's much easier to achieve a 'good' grade of butt block joint than it is to accomplish a 'good' scarf.

3. It's easier to align the panels when butt blocking.

4. A butt block is quicker.

Nope. I ain't buying it.

If you're into quick and dirty boat building, or can't take the time to properly lay out your panels, or teach yourself how to scarf, and are content to look at an unsightly patch on the inside and a lump on the outside, then by all means go with that butt block.

But, if you plan to build a proper plywood boat, butt blocks will not be used.

[switters]

You generally use a butt block about the same as you would use a scarf...12:1 with the same thickness so your butt block should be 1/2" ply with a width of 6" and a length suitable to the joint. The grain should run in the same direction as the original plywood. You normally make the joint on a flat surface if you can, that way you can weight the block down to ensure a good solid joint. OTOH I just did it with the bottom panels installed on the boat and though it was 1/4" ply it went together just fine. If the joint is in an area where the panels have a lot of curve... well that just wasn't thought out very well. You will have difficulties and probably a flat spot at that point. Butt blocks are normally installed at the point of least bending or stress.

Here the sides and the bottom are buttblocked...the butt on the bottom is inside and about 3" back or aft of the side blocks.



Here are the side panels with their butt blocks untrimmed yet.



This boat here shows the joint where one panel of the bottom will be butted to the other. There are 2 butts on the bottom of this boat and one on each side.



You can just see the side butts under the frame in the middle.



None of these panels are bent a whole lot so the block works well and there are no noticeable flat spots. The joints are just as strong as if I had scarfed them but I was able to keep the full length of the panels on the sides...16' so the boat runs about 15 1/2' long instead of being 3 or 4" shorter. I was also able to install the blocks on the inside of the bottom after the panels were glued and screwed onto the boat...yesterday afternoon.

Nice transom shape! And having just launched a boat with buttblocks, I didn't notice that the lines were not fair, because as stated above, they were in the right place. One thing I have learned, put a fillet of epoxy around the edge of the block, covers the end grain, easier to clean with a hose and you dont get dirt and water collecting there.

[JimConlin]

For those builders who aren't up for a full-bragging-rights 12.78:1 scarf with gopherwood trunnels, there's also the fiberglass butt strap, as described by Dynamite Payson and Jim Michalak.



The method produces a clean and sound joint with modest effort. You just need to be handy with the materials.

[Lewisboater]

Nice transom shape! And having just launched a boat with buttblocks, I didn't notice that the lines were not fair, because as stated above, they were in the right place. One thing I have learned, put a fillet of epoxy around the edge of the block, covers the end grain, easier to clean with a hose and you dont get dirt and water collecting there.

Thanks... It was really for practical reasons...I wanted to be able to mount a trolling motor for river use...the current is too strong for the sail and my shoulders can only take so much paddling.

In this case it was a fillet of PL Premium Advanced. Basically PL with added fillers to stiffen it up a lot. First tube I used split the plastic cone so I had goo coming out 3 different places. You really need a quality gun to use it and Popeye arms to get it to go. It really helps if you let the tube sit in the sun a bit... I think I'll only use it when I need the thickness... regular PL for most stuff. If you need to stick something and have it stay in place while you grab screws or clamps or whatever...the new stuff works quite well for that. I believe it cures quicker too.

As to the butt blocks... in this instance I gained 1/2" in beam by butting the frame on top of the blocks plus it eases the hard spot where the frame pushes on the ply...kind of added benefit.

[Lewisboater]

For those builders who aren't up for a full-bragging-rights 12.78:1 scarf with gopherwood trunnels, there's also the fiberglass butt strap, as described by Dynamite Payson and Jim Michalak.



The method produces a clean and sound joint with modest effort. You just need to be handy with the materials.

I've used both those methods before also... It takes pretty much as much time as scarfing so no gain there although it allows you to use the full length of the pieces of ply.

[JimConlin]

Right.
It depends on what skills, tools and materials you have.

And gopherwood trunnels are getting hard to find.