Zipper Seam Construction

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?

 

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....

 

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.

 

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.

 

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.

 

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.

 

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

 

Brian, thanks for the interest and welcome to this thread. Although some time has passed since the last dated post I have been quietly working this idea.

Rushton and his workers were boat-building masters of course, and I wouldn't presume to place myself in such exalted company. My approach is to employ modern tools and materials to the maximum advantage. I have no doubt whatsoever that Rushton would have done the same. Given access to marine plywood, epoxy and other water-proof glues, a router or two and some other modern marvels, who knows what he would have accomplished. In my case, I do not have a CNC router - who does? - but like most folk these days I have some power tools, a computer and access to free software like Free!Ship to work up a hull design and do the plank developments.

Translating my own take on zipper-seams to the Wee Lassie, the principal challenges over the last 2 years have been:

1) Converting the offsets for the lapstrake-construction hull to suit butted seams (done).
2) Creating a design in computer software and developing the planks (done).
3) Getting proper developments for the heavily twisted bottom planks (image) which FreeShip does not handle well (in progress).
4) Developing a method to fabricate the Tee-shaped keel of the Wee Lassie accurately, with my limited skills (see post #4).
5) Developing a method to bond the edges of the ply planks along the seams (done, image).

I think the reinforced seams (see 2nd image) will look very attractive especially if I varnish the hull. It involves some fiddley work to remove an outer veneer from the ply planks and reinforcing strips but the end result should be worth it. The idea is an adaptation of the metal batten system used by canoe builders in Peterborough, Ontario, Canada around the same time as Rushton was operating in New York.

It is clear that many of the problems are related to the choice of hull design on which to try out the idea. However, this is good as it has forced me to develop a complete system rather than one suited only for hulls with low-twist planks.

There is a tendency for the carpet tape to cause slight damage to the ply surface because the adhesive is quite aggressive. I think this can be avoided by giving the ply a coat of varnish, but I may have to mask a couple of areas for gluing later.

 

The zipper method of assembling boats has ben around for quite a while although I did not know that name for it. The CS17 that PAR shows goes together amazingly easy and accurately. It's more accurate because of the fact that the alternating CNC tabs locate the panels precisely. The boat forms up almost automatically. After an S&G hull is stitched up, it can still be moved quite abit and must have the alignment checked often to keep it right but these zipper cuts allow much less movement fro the design lines. Those who did this with hand cut tabs had a lot more trouble keeping things lined up.

 

I don't believe it would be all that laborious to hand cut the "zipper" teeth in thin ply with a good template setup. CNC is cool but I don't think you have to be John Henry to get by on a small boat.

 

I was unaware of the existence of the CNC cut "zipper" planks when I came up with this idea. What I was trying to do at the time was eliminate all them tiny little holes in a finished stitch and glue hull, plus the drilling and threading and tightening and adjusting, and the need for thick fairings to cover up the little bits of copper that remained.

This was my experience when I was helping a friend build a canoe, and I know people will disagree with me about the amount of work but it was emough to make the poor guy just give up. I have built about 6 boats since then and his boat is still hung up in his garage waiting, waiting ...

I appreciate the neatness of the stitch and glue idea but just felt there was another way. A couple of years later I am beginning to realize just how many other ways there are.

 

This thread has been a bit moribund lately, due to other boat builds getting the way, then health problems, and more recently the weather has been so hot I just hid in the basement rec room which stays cool. But that doesn’t mean I have forgotten the idea or my plans to build the Wee Lassie.

I would like to change the name of the thread since the name is associated with another method, but I don’t know how to do that; at least I should think up a new name for this method, perhaps Tab Seam. Anyhow, here’s an update on recent activities.

Someone (I didn’t keep a record of who - maybe someone who deleted a post) helpfully pointed out that offsets in the plans I got from the Algonquin Museum are measured to the inside of the laps for a lapstrake build. Therefore to my list of actions in post #8 I added converting the offsets to measure to the outside of the plank seams, so I can get proper plank developments from FreeShip.

As I wrote in post #4 I merged the bottom 2 planks into a single wide (6", 150 mm) garboard. My scheme for building up the keel - also described in post #4 - involves twisting the garboard nearly 90 deg from midships to each stem, which needs a lot of force for such a small boat. In addition, as noted in post #8 item 3, FreeShip seemed to have trouble generating a proper development for this plank and was giving a weird looking shape. I think that was because there was so much strain in the plank; however when I divided the wide garboard into 2 narrower planks as per the original design the development looked perfectly normal and will be much easier to bend.

Having got to the point of a computer design that I felt I could build, I turned my attention to what I really wanted. Several people who have paddled the original Wee Lassie design have noted it tracks well, perhaps a little too well. The original design has virtually no rocker so I added a bit of rocker to provide the agility I will need paddling in the narrow waters near my home. This has resulted in the bottom edge of garboard development becoming flat rather than concave, which saves material.

The plank developments are attached, packed for best use of material: the plank order from the top is 2nd or lower sheer, 1st sheer, 2nd bilge, 3rd bilge, 1st bilge, garboard; the assembly order is of course 1st & 2nd sheer, 1st/2nd/3rd bilge and garboard.

The original Wee Lassie design is symmetrical from the top and side: I have found canoes like that turn into a strong breeze, making cross- or down-wind progress tiresome. I have a small canoe being finished which is based on an earlier boat I built, but I have raised the sheer at the bow and lowered it at the stern to reduce the windsock effect. If that is successful I will modify my Wee Lassie derivative the same way.

By then I should be ready to start building. In the meantime, I hope nobody was holding their breath!

 

Wheeww!!...I was getting a bit light headed...

Looks like an easily assembled set. For future reference...get rid of the waterline in the development...even the color too. Makes it a lot easier to see properly. You are looking for the ship bow button and the can of paint button. The ship bow button removes the waterline and the paint button removes all color...leaving your lines and perhaps the station lines where they land on the developed sheets. These correspond to where the station lines of the model will fall on the developed panels...so they won't be at regular intervals.

 

I left the waterline so I could figure out which plank was which, and I displayed the planks packed as they would be cut on the ply sheet rather than arranged for assembly because I was pleased with the way everything fell out so efficiently.

I am wondering which is the best way to go, transfer the plank measurements onto the ply sheet for cutting, or take the print from the computer to a local print shop that can make photocopies of any length up to about 3' wide, as all the planks for half the boat will fit in 20" width: I could split the ply sheet down the middle, paste the photosopy on the ply and cut both sides at the same time. I will check the photocopy for dimensional errors before I do that, of course.

Here the planks are shown in order for assembly. The gap between the garboard (shown at bottom) and the next bilge plank is in the middle and will likely be the most difficult to close. These 2 planks were previously merged into one. Merging 2 planks into one creates stress that is probably proportional to the size of the gap; the lower 2 bilge planks could be merged into a single plank but I think it would look odd if I did that.

As noted earlier, the garboards and keel will be built as a separate sub-assembly.

I plan to assemble each side consisting of 5 planks - 3 bilge and 2 sheer - as a sub-assembly and offer it up to the garboard/keel sub-assembly, which it should fit with a minimum of pushing and pulling. Because the gaps are at the ends I can use duct tape to hold the planks together midships while I am pulling the ends in to meet the stems. However, it has never been done before like this so I am prepared to make changes to my plans!

I don't think this boat would be a good candidate for stitch and glue, too many holes to drill and align and the narrow planks would be floppy and hard to control. Once stitched, making the adjustments to get the seams butted neatly ready for gluing would be frustrating as one would pop out when another was pushed into place.

 

Duct tape or packaging tape...and to be honest...you would really only need a stitch per foot or so and the really small 4 inch wire ties only require a 3/32" hole. If you were to hybrid the build...a stitch for every 2-2.5 ft to start and then use the tape to finagle any gaps closed. Then use a few dabs of 5 minute epoxy to get a quick lock down of the shape and fillet at your leisure. Krazy glue might work too...I have heard of people using it on wood for a quick tack