Thread: Interesting assembly method

Must admit at first glance I was dubious but I have seen tabbed joins on things from Chesapeake Light craft (see example below). The CLC tabs are cut into the board while the Eco5.5 guys have just put key holes in both ends and floating joining tabs, which may make for a longer craft from the same lengths of plywood for the strength trade off of having joins on both ends of the joining tabs. Interesting they did not show how that worked but I assume you have to fibreglass back them.

Chesapeake Light Craft :: Kaholo Stand-Up Paddleboard: Kit Option Details


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This is a discussion I've avoided on another forum and will mostly do so here, as to most reasonable engineers and naval architects, this system has several obvious flaws. These types of fastening systems are typical of designs where the "inventor" has little practical experience, but lots of CAD experience.

OMG!!!, Ikea building boats now? It should be called a "Skit båt".

The photo of the finished boat is from the designers home page and would most likely have been built using the (dare I say traditional?) stitch and glue method. Follow the links page to have a look.

I shudder to think of the difficulty in filleting all those joints, let alone fairing the hull once the "Ears" have all been trimmed. Or are we expected to believe the hull will be strong enough if we merely tape over the outside edges of the hull?

Well those aren't the major issues I have with this type of "creativity" in assembly, but generally requiring joints only a CNC machine can cut and extra pieces to an already difficult joint, seems counter productive to home building, unless of course you're selling CNC kits. The dog bone joint is a joke, as are puzzle joints in anything other then a CNC setting. The next time I have $20,000 I don't know what to do with, I'll go out and buy a cheap CNC machine, just so I can make much more complicated joints, that are weaker then a traditional scarf or Payson joint.

I will stick with the tried and proven, tested scarf joint.
Those little dog bones, just do not give enough gluing area to secure panels together.
I also think they would create "flat spots because of their short length.
There is too much work in this cutting method to make it worthwhile and I don't see any real strength in the finished product.

I think the chap has too much time on his hands.

Just my Opinion.

Paul.

PAR, are you trying to tell us there may be some
bugs in this system?

Par, excuse my ignorance. But what is a Payson joint?

A Payson joint is that fostered by several people (my self included in a modified format), but generally Harold "Dynamite" Payson has been credited with this style of butt joint.

In a nut shell two pieces of plywood are butted together, then cloth or biax (I prefer biax) is applied over the seam, with thickened epoxy. Both sides of the seam are done this way and the result is a butt joint that is stronger then the surrounding wood. Unfortunately, it also causes a slight bump, which needs to be faired out.

I (as well as others) have taken the joint to the next logical level, which is to still butt the two pieces together, this time plowing out a shallow depression in the plywood, so the goo and fabrics have a place to live, without making a bump that needs fairing later. Of course both sides are do the same way and if you have some practice, you can make these joints so there is not fairing necessary, which can be a time saver.

Generally, you need a specific amount of fabric on the seam to make a structural panel. Additionally, you can use the full length of the plywood, instead of having to reserve some area for a scarf. There's no difficult machining of the edges (like a scarf or other joint) and anyone can make a good one the first time out. The disadvantages are: it takes longer then a skilled scarf maker can make a scarf. If I was racing myself to make both a scarf and a Payson joint, I'd kick my own butt with the scarf, as I've made so many I have nightmares about them. The Payson needs more goo work, which to some folks is objectionable and you can't finish it bright either.

I like the Payson joint because you can get the most out of plywood panels and they're self fairing if done right. Yep, they're more work, but after you've done enough, you can do both sides at the same time and also stack them on top of each other, assembly line fashion. I've used this joint to fool clients into thinking I've managed to find 20' long plywood planking for their boat, wood grain and everything!

On a lapstrake powerboat repair, I joined 3 pieces of plywood end to end for planking that was 18' to 24' long. These joints were made as I described, with a shallow depression on each side and filled with goo and biax. I then placed clear polyethylene sheeting over the goo and used a scrap piece of the same plywood to bridge the joint, which served to level off the thickened goo and biax during the cure (part of the stacked up assembly line thing I mentioned). When I disassembled the setup and peeled the polyethylene off the planks, the wood grain from the leveling bridge transferred through the polyethylene into the epoxy. The client came by in the process of all this, but after I'd already gotten a couple of coats of primer on the planking. All he saw was 20' long planks, with continuous wood grain their full length, which he was wildly impressed with. In his eyes, I was the magician with 20' plywood at my disposal. I just explained there was a good reason he was referred to me in the first place and left it at that.

This is Payson's site describing his way. You'll note the broken joint as a test. This is the not scooped out style of joint and it broke just at the edge of the fabric, meaning a stress riser was likely present. The scooped out joint doesn't do this.

If you do a web search you'll find many discussions about this joint, including some with me in them as well as other pictorials of the process.

See posts 23 and 26 here. How Do You Scarf 12mm Ply Bulkheads? - Page 2 - Boat Design Forums