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Puzzle joint vs butt or scarf joint

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Puzzle joint vs butt or scarf joint

Postby swede » Thu Feb 24, 2011 11:37 am

I hope I put this question in the correct section. I'm building a 14 ft. skiff and am curious about the strength of puzzle joints vs scarf or butt joints to join 2 sheets of okoume plywood for the hull. It won't be a clear finish, so not worried about the look of them. Seems to me that the puzzle joints are very strong. Anyone here ever used them ??

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Re: Puzzle joint vs butt or scarf joint

Postby Laszlo » Thu Feb 24, 2011 1:15 pm

I've successfully used all three. As long as you go with what the designer recommends, you'll be fine.

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Re: Puzzle joint vs butt or scarf joint

Postby ks8 » Thu Feb 24, 2011 1:46 pm

I respect Laszlo's comment, and the designers and maunfacturers who implement them. I'll try to add some *off the top of my head* here...

When a supplier cuts a CNC kit, a puzzle joint is an easy matter as the programming and the cutter do all the work. For an amateur build, where the builder is cutting out the panels, a puzzle joint is an unnecessary complication, imho, unless the builder is *into it*, and that's fine too. Butt blocks or fiberglass splices will be plenty, *depending* on the program of the boat and the skill of the designer in placing them properly. And they are MUCH easier to implement. When the finish will be paint, you will be fairing anyway, and for some, a bright finished puzzle joint, well, it certainly will look *different*. :wink:

What are the advantages of a puzzle joint? More edge glue surface area, automatic alignment of *precut* parts, and might add some strength in the *plane* of the panel, but you again, may be glassing over that joint anyway. Conclusion, imho, they are a convenience in small lightweight CNC precut kits, for *some* builders who don't want to be bothered making three simple tick marks on the shop floor or shop bench before making a butt block or glass splice, or, for builder's who are *into the novelty of making such a joint successfully*, like an 8:1 or 12:1 scarf (usually no need for it with a well-designed composite glass-core-glass design). I'm an amateur, and I'm sure a professional advocate of puzzle joints could make my words look stupid and ignorant if they desired to do so. Yet, consider, a puzzle joint may make it more difficult to get a good and healthy quantity of epoxy filler blend into that tight joint, and cause it to be a dry or starved bond! The additional edge glue area on a puzzle joint can be a non-issue due to this potential for a starved bond, and as soon as there is a butt block, or glass tape, and a proper structural positioning of either, the starved bond is less likely an issue. Puzzle joints may be wonderful in those lightweight pre-cut kits for aiding in alignment :D but I personally would not make the effort on a scratch built and cut build, nor would I personally want to risk a starved bond as epoxy gets soaked into the edge cuts (precoating would be recommended to prevent that). On the other hand, I am one who might do a puzzle joint for the novelty and challenge of it, and if I did, I would likely document it here, and communicate whatever issues I encountered. If it is already in the panel because one purchased a kit, then I agree with Laszlo... follow the recommended assembly procedures, particularly to make sure that the glue joint does not get starved, and I would likely do this with a serious precoating or two of the edges to be glued. Though a puzzle joint might be strong in the plane of the panel, I wouldn't want to have one right at a point where there is a high twisting load on that panel, without glass skins, especially without good precautions to avoid a starved bond. One reason why we as amateurs must trust the designers. We shouldn't draw up our own skiff and throw in a skinless puzzle joint just anywhere. :wink: From one amateur to another, hope that helps. :)

Got any pictures Laszlo, or additional thoughts on your experience? :)

Swede, are you building a Bateau design? FL14? FS 14?
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Re: Puzzle joint vs butt or scarf joint

Postby shine » Thu Feb 24, 2011 2:16 pm

they (puzzle joints) work very well, but they waste a lot of wood compared to butt blocks or fiberglass splices. I think we had a VG20 cut with puzzle joints a long time ago, maybe 10 years ago.
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Re: Puzzle joint vs butt or scarf joint

Postby swede » Thu Feb 24, 2011 3:24 pm

ks8-

I'm building a Tango-14 skiff, its a Hank Bravo design (out of NC). He calls for scarf joints, but said puzzle joints would be fine. I came across puzzle joints when looking for plywood. The company sells 8'x47"(loss of an inch when cutting them) sheets of Okoume with puzzle joints cut via CNC. My thought is the puzzle joints would be easier (time saving) in joining the sheets of plywood. After the 8' x16' sheets are cured I would measure and cutout the panels for the hull and other parts as well.

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Re: Puzzle joint vs butt or scarf joint

Postby Laszlo » Thu Feb 24, 2011 7:40 pm

swede wrote:...a Hank Bravo design (out of NC). He calls for scarf joints, but said puzzle joints would be fine...


There you go, question answered.

My direct experience with puzzle joints is with a kit. When I built from scratch I used butt joints with blocks. The puzzles had just enough of a gap that there was no problem with starvation. They were CNC cut by the kit seller. I would never go to the trouble of cutting them by hand or the expense of buying the tools to cut them.

The amount of time they save over scarfing, butt joints and glass splices is negligible when compared with the total time it takes to build any boat. But if they tickle your fancy and the designer says OK, then by all means go for it.

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Re: Puzzle joint vs butt or scarf joint

Postby ks8 » Fri Feb 25, 2011 3:11 pm

8)
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Re: Puzzle joint vs butt or scarf joint

Postby jacquesmm » Fri Feb 25, 2011 3:23 pm

The puzzle joints are not stronger than a plain butt joint unless you consider that the strength is increased by the length of the joint line which is minimal.

Designers and suppliers sometimes use a puzzle joint in a CNC cut kit because it makes parts alignment easier. It also waste some plywood.

If your plans show a puzzle joint, use a puzzle joint. If they don't, don't.
Our plans never show a puzzle or a scarf.

Since this forum is reserved to the building of our boats or the use of our materials, this is off topic and I close this thread.
Sorry but we will be stricter about staying on topic, too much time is spent discussing details irrelevant to our plans.
Jacques Mertens - Designer
http://bateau.com
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