Zipper Seam Construction

[ancient kayaker]

I have an idea for assembling a small ply hull which is an alternative to stitch and glue. If it works it will eliminate the holes, and with no drilling or wire twisting may even save time.

Ply planks are developed and sawn out and the edges are planed fair. They are placed together flat with their edges in contact at midships. A stapled strap or adhesive tape (I am trying duct tape) is installed across them to keep them together. The planks are then lifted and draped over a pair of forms. Bungy cords are applied over the outside of the hull from gunnel to gunnel, starting from midships and working toward the stems, to pull in the seams. Finally the planks are nipped together at the stems.

Naturally, as the planks are pulled together their edges must be kept in alignment to form a butted seam. I do that by adding small wood tabs on the outside edge of each plank, overlapping the edges, about every 4 inches alternately, so the planks at each seam close in a manner resembling a zipper. Try pressing your hands together with the fingers interleaved and imagine a bit more space between the fingers ...

The tabs don't take long to make, the trick is mass production. I cut a pack of popsicle sticks in half, unwrap it and arrange the halves in a line, then stick them together with a strip of double sided tape across one end. Then I pull off the backing and separate them with a razor. A fairly agressive tape is sold for use in carpeting and holds the tabs well enough to control thin ply. Once the hull is dry-assembled the tension of the bungies and friction between the planks holds it together while gluing.

I have tried this out on a partial hull model and it works. The indications are that it will save time compared with stitch and glue. I welcome opinions and suggestions before I try this out on a full sized, complete hull (a good belly laugh from those who heard about this years ago is also fine but try to be kind).

The main challenge now remaining is how to bond the seams. I have tried injecting epoxy but I get variable results, 55% of the ply strength is the best I have got, sometimes less than 20%. The glue gets absorbed into the end grain leaving a starved joint, or does not penetrate fully and the joint fails at the glue/wood interface, or I get a good fillet of epoxy which cracks during testing. I intend to add tape or a fairing to the inside of the seams to stiffen things up but the main issue is to get adequate tensile strength across the seam; I prefer not to use glass cloth if I can avoid it. Any suggestions?

[marshmat]

Interesting concept. I can see how it could be quicker than stitch/glue for a hull whose panels need some coaxing to take their shape.
Have you seen a Phil Bolger "tack-n-tape" design under construction? He gets around the stitch-with-wire part altogether, by matching the curvature of the knuckle/chine lines in such a way that the seam is actually the natural intersection of two conic surfaces.... in non-math terms, he shapes the panels so that they naturally fit at the seam without needing to be pulled together. It's somewhat trickier to design but the build goes oh so smooth....

[ancient kayaker]

In Phil Bolger's "tack-n-tape" method it sounds like the builder uses adhesive tape to hold and seal the seams prior to gluing. I did an Internet search for "tack-n-tape" but did not find anything describing the process. Could you summarise it for me or provide a link?

One of the interesting things that is not clear in my first description is that the tension in the gunnel-to-gunnel bungy cords across the outside of the hull causes the planks to curve in to fit the forms without needing tension between the gunnels. Much the same thing happens when stitching. A mathematician would probably say the structure seeks a minimum energy configuration. It is essential that all planks meet at angles less than 180 deg everywhere for this to work.

With hulls that have a lot of narrow planks drilling and stitching takes a lot of time and getting all those thin, floopy little planks to come together could be a hassle. The "zipper seam" method (if it works - I've only tried it small scale so far) should most useful here. I think it would be less effective with stiffer, wider planks where the bending forces might pop the tabs away from the double sided tape.

The phrase "nothing new under the sun" comes to mind, however; is this new, or have I just reinvented a wheel here? Anyone out there heard of this dodge?

The day after I wrote there was a fire in my workshop. I always unplug power tools and sweep up dust, no oil soaked rags, definitely not arson, cause is a complete mystery. Nobody was put at risk but I lost much of my tools and materials. All was insured and the workshop is repairable but it's going to be a few weeks before I can get back to work and I will need to rebuild the ready-for-painting canoe I was working on before I can return to this project. I hope I will be able to try out the idea full-size before Winter.

[ancient kayaker]

Update: the fire damaged was repaired and I built a boat over the winter which is now getting wet on a regular basis. Presently I am working on a small canoe but have been developing techniques to supplement the zipper seam idea described in earlier posts.

I will try the concept on a Wee Lassie canoe after my summer break. The original Wee Lassie (by Rushton) had 6 lapstrake planks per side, I ran the offsets through a spreadsheet to eliminate the laps in preparation for butted seams and found the lower two planks merged into one; I am guessing that Rushton used two narrower planks to reduce the assembly forces. The bottom planks are the only ones that are really twisted, nearly 90 deg and the force to do that is significant for such a small boat if one wide plank is used per side as I plan to do.

In a half-hull test I glue half a keelson to the bottom edge of each plank, twisted them into shape, but 90 deg out of alignment so the joint was horizontal instead of vertical. Imagine cutting a hull down the centerline then folding the two halves of the hull outwards 90 deg. The center joint would now be horizontal on both sides: I get that by planing a flat. When the planks flattened out the flats became two perfect rolling bevels. When the planks were brought together I glued them. Instead of epoxy I used Titebond III which is not forgiving of joint gaps, but I got a perfectly fitting, strong joint. Very promising for the real thing! I pass on this idea for others to use, far easier than carving a keel with a rolling rebate on each side to fit twisted garboards. Unfortunately I can't find the photos.

The rest of the assembly calls for butt joints along the seams. I an undecided between two methods; the first would involve adding a 1/2 inch batten across the inside of the butt joint to act as a long butt block, with the face ply on the inside surfaces of the batten and planks stripped off so the joint is between the core veneers with grain running across the joint for max strength. The other method involves a 1/2 inch doubler along the mating edges of each plank, bevelled to create a birdsbeak groove inside the hull that would be filled with epoxy.

After I build and test some pieces using these methods (and finish the currently building boat) I will be ready to start.

thanks for encouragement and ideas, I will post pictures if it works.

[PAR]

This "zipper" concept is currently in use, typically with CNC cut, taped seam kits. The use of bungee cords, ratchet straps, fat friends leaning on twisted panels, etc. are also common methods to hold things together.

The difference I see between what is described above and the machine cut versions is the tabs are part of the cut and not an addition. Panel edges literally look like zippers, with interlocking teeth, which makes alignment quite easy and also simplifies holding the seams together during the taping process.

This is a CS-17 kit and the zipper should be self explanatory.

[ancient kayaker]

Very interesting. That should go together really easily.

That was actually my first approach a couple of years ago when I was looking for a way to eliminate the stitches but preserve the simplicity of S&G, but it would have required CNC machinery so I did not proceed.

The tab idea came later when I was looking for a method for an amateur boatbuilder to build boats designed to be built by the lapstrake method, only without the laps using butted seams a la S&G. I first used it many years ago as a teenager, to join balsa sheets for model aircraft. Due to the strength limitations of the adhesive tape the tabs will only work on narrow, thin planks.

A further limitation seems to be that a plank development with a concave edge results in a seam that is difficult to close. I notice the CS-17 developments are all straight or convex.

Thanks for the information and your interest.

[Manie B]

the zipper concept works VERY WELL
i have experimented with my own version of the concept and then filled the corners same as stich and glue
all good and very strong

if you have access to a CNC router all the better
i have often wondered why the "kit" makers dont do "zipper" it really works easy