Zipper Seam Construction

I see that this is an older thread and you may have moved on to other projects by now. *

How thin of a panel are you talking about? *I did a kayak with 1/8 inch ply and I can relate to the difficulties in aligning a very thin ply. *A subsequent kayak, I constructed using 3/16 inch ply with significantly reduced alignment problems. And a reduced requirement for ties. *I understand your problems with trying to maintain the alignment of an entire boat prior to any application of goo. *My solution was to only concern myself with a pair of seams ( p & s ) while loosely stitching the others. *This could be all of them or just a limited quantity that would still allow hull shape definition.

I would also incorporate temporary and permanent bulkhead to further assist in hull definition/allignment/symmetry. *I was a bit too casual in the first build thinking there was only a single mathematical solution to panel equation. *Ha! *It looks OK as long as you don't look too close. *

But to continue with your stitch problem, you can apply your fillet goo between stitches only slightly thinner or smaller than the depth of the final fillet and let it kick off. *I thought this was a personal epiphany in technique, but was rereading an s and g book of mine and what do you know. *It was right there in print. *So much for personal epiphanies. *Anyways, this allows you to pull your stitches before they get get glued in or covered.*

Working this way adds a step, but if you only work a pair of seams at a time, it becomes a one time event. *My progression would be to stitch up as much hull as is reasonable, align the first seam pair and "tack weld" it with your favorite goo. *Once cured, remove the stitches from the first seams, align the next seam pair, fillet and tape the first seam pair and tack weld the second seam pair.*

Thinking about it further, I might try to get the entire hull tacked up, in several sessions most likely, and then fillet and tape to my hearts desire afterwards. *This way you are not having to deal with a fillet thickness that has to incorporate your stitches. *I also try to tape my seams as I apply the fillet. Maybe this is already standard practice. *I do like the results though as the fillets are not so temperamental with the fabric covering over them. **

 

I haven't given up on this idea or the Wee Lassie build, but I got involved in another project which I am determined to finish before I get back to this one. There is already one unfinished boat lurking in the workshop which was put aside while I built a sailboat and I will finish both boats together. I hope.

I was originally planning to use 3 mm marine ply but I think the slight extra weight of 4 mm (3/16) will be worth it, and I have a stack of that already. Saves me buying more ...

I agree that a center station mold will be advisable although theoretically not necessary - but I don’t trust entirely theory and the mold will assist forming the hull shape.

Just a note here - this is not a stitch and glue project, in fact the whole idea is to eliminate the stitches, and the complex building mold associated with alternative construction techniques. I will cut the planks to the developments as for S&G but then I will use wood tabs such as popsicle sticks held to the outside of the planks using double-sided tape, so they overlap the edges like teeth. These will prevent the butted edges sliding over each other as I pull the hull from a 2D into a 3D object. Hence the term zipper-seam, although I can’t use that as someone else has claimed it!

Pulling the hull into shape will be done using bungy cords around the outside, from gunnel to gunnel. It works in miniature, I haven’t yet tried it full-size.

What has delayed me is figuring out a nice way to secure the seams. I don’t want to use the usual glass tape and epoxy fillet method. I have been experimenting but am not satisfied with either the strength or the appearance of what I have done so far. One approach is to use a narrow epoxy fillet and cover it with a ply batten planed to half-thickness, but keeping the batten aligned over the seam is difficult. When/if I get it to work I might use 1 mm aircraft ply for the batten.

I have been thinking about another method which involves going back to lapstrake. With this approach the plank developments would have an extra 3/8" / 9 mm added to their lower edges, a row of staples would be inserted along the original plank edge - the seam line - which would hold the planks in alignment while the hull is pulled into shape. Then I hope to be able to flip the hull and fill the laps with epoxy. Glued-lap is a well-established technique used by Iain Oughtred, Tom Hill and others. It is related to the lap-stitch concept developed by Chesapeake Light Craft. If the butted seam method does not work this is my backup!

By now it should be clear that I have a severe case of grasshopper mind ...

 

Despite appearances this thread is still alive! It got stopped by ill-health, but I hope to get back to it over the next couple of months. It has been over 4 years since I started on this project, far longer than my normal attention span, so I must really want to do it.

I think I have a way to achieve seam strength with butt joints, avoiding the need for laps, battens, or glass taped seams. Earlier test samples with butt joints between ply planks had joint strengths of at least 58% of the ply strength, despite using “5 min epoxy” which is not a true epoxy. The joint strength can be increased considerably with a combination of a structural epoxy and 30 deg bevels on the ply edges, since joint strength is proportional to joint width cubed.

The strength and toughness of the fillet is key. System Three’s Gelmagic is intended for structural fillets and conveniently comes in a self-mixing tube for use with a caulking gun. I have used these tubes before and they are great, but expensive. Fortunately the high cost is offset by the small cross-section of the fillet. For this design I estimate the amount at 1/3 L - plus and waste allowance. I will do a series of tests to validate the theory and calculations before starting, once I find time to drive to my S-3 supplier.

I have an alternative to bungy cords for pulling the plank seams together. I plan to use 0.5 mm monofilament nylon fishing line - 25 lb test - stretched across several planks using something similar to a violin tuning peg, so I can get as much tension as I need. The epoxy should not stick to the nylon - another advantage over bungies. There may be a few places where I will have to resort to a stitch or two to pull things together.

Probably build sequence will be:

1) bottom assembly consisting of keel, stems and garboards
2) Port and starboard plank assemblies, bilge through sheer planks and (probably) inwales
3) Assembly port/starboard/bottom
4) Add outwales, thwart etc.

- this needs to be verified on a smaller scale half-model.

The problem - addressed in an earlier post - of how to transfer full-sized plank development shapes to the ply sheets for cutting has been solved. The full-sized blow-ups I had done by a printer a year ago appear to have remained dimensionally stable, stored carefully in a reasonably constant environment. If I can get a similar environment in my workshop they should remain accurate for long enough to trace the cutting lines on the ply. I also have 1/4 and 3/4 scale copies which I can use for scale experiments.

 

Currently I am making a 1/12 scale model of the stretched Wee Lassie to ensure that FreeShip's plank developments will go together. I also picked up some West Systems epoxy today so I can test the strength of the porposed seam joint design. If those go well I will put a 1/6 scale model together using aircraft ply to check out the assembly sequence before I commit to a full-sized build.

West System now have their epoxy available in a self-mixing cartridge pack similar to System Three's but my supplier doesn't stock it yet, so I will have to make do with the blend-it-yourself stuff.

Normally I'd just go for broke and build the real thing but the workshop is full of boats being stored, built, finished or just waiting for maintenance. Whereas the kitchen table is doing nothing much . . .

Every boat building addict should have a wife as tolerant as mine! Of course, she just came back from a shopping trip to the US a couple of weeks ago, so this is as good a time as any to get my claim in . . .

 

I agree and I believe I do also. I just bought her a copy of Dynamite Payson's Instant Boats for her birthday and she is reading it. We have plans to build her a kayak together so this is a primer for her. Most likely, she'll be telling me what I'm doing wrong by the time we are done with the project.

 

The 1/12 scale Wee Lassie model went together reasonably well for such a small cardboard model which confirms FreeShip's plank developments, at least within 0.1" at full scale. Cutting errors appear as seam gaps, which is not surprising.

Lessons learned include need for keeping the bottom assembly (keel plus garboards) on the building mold to prevent it being pulled out of shape when adding the side planking, and also internal stems should be glued to bottom assembly before the side planking.

Building the the keel and garboard as a separate bottom assembly worked; the stems can be added at the same time. I think adding the planking for both sides at the same time - instead of assembling the two sides separately - to the keel/garboard/stems assembly will work best, then I can pass the bindings around the bottom of the hull from gunnel to gunnel to hold the planks together for gluing. I can confirm that on the larger model.

The 1/6 model will be more accurate as well as larger since I can sand the ply exactly to the lines and should confirm FreeShip's plank developments within 1/32" or so. This is the first time I am relying on FreeShip to create accurate developments for a boat design so I am being extra cautious.

 

@PAR... in your pic, it looks like there are either dots to mark location or there are some stitch holes drilled. Am I seeing it wrong?

 

I can't see it. The planks are held in fore-and-aft alignment by the notches in their edges.

The curved stringer attached to the sheer plank is interesting; initially I thought it was just laying there but it is glued. It would cause the sheer plank to become convex when it was bent inwards to fit around the bottom plank. That's something I wanted to experiment with after I noticed the same but opposite effect on a sheer plank that I had attached an inwale and chine log to before bending it. LP has been experimenting with it at http://www.boatdesign.net/forums/wooden-boat-building-restoration/15-lapstrake-kayak-40486.html

 

If you click on the picture and click it again so zooms in, you can see some dots or holes about 1/4" inward from each tooth. They show up clearer toward the lower part of the picture.

 

Yep, them there be holes, about 1/8" diameter. This type of assembly permits the use of a dowel and some wire inserted (the holes thing). The dowel aligns with the seam and forces the tabs to remain in vertical alignment with the notch before and aft it.

Again, as my posts might have suggested, I think these types of seam assemblies are more marketing gimmick than actually assembly easing methods. The moment you introduce more pieces to the mix, you're on the wrong path, in engineering terms. Admittedly the dowel trick works on this type of seam, but only is useful on CNC cut zippered steams, where perfect alignment can guarantee a sweet seam. Don't even start me up on the dog bone or other similar seam alterations I've seen.

 

- ah ha! Now I see it, then I didn’t . . . and for over 3 years here I was thinking it was all done without stitches. My apologies Jem, you have better eyesight than I have.

- I must say I agree with that assessment; I would have thought a few gunnel-gunnel bungies would have held it together with the "zipper" teeth there to keep the edges aligned.

- I must try out my idea soon, maybe I’m on to something new after all.

 

Zipper teeth can become miss-aligned vertically, which is what the dowel solves. I've never had a need for special seam shapes or contrivances and very rarely use stitches.

 

I found the “Core Sound 17" kit on the B&B Yacht Designs website. They don't use “Zipper Seam” to describe their concept, they call it "step scarves", so I can still use it for this idea of mine, which saves me renaming it! I like the name: it's edgy!

 

Actually, B&B's "stepped scarves" are exactly as you'd expect them to be, notched, steps, instead of continuous slopped scarf joints. I'm not Sure what Graham calls the CNC seam. Tom, are you watching this thread?

 

Interesting. What's the thinnest plywood you think this could be achieved with? My guess, and that's with zero experimentation on a CNC, would be 9mm. Maybe 6mm. Anything thinner and I'd think you'd run a good chance of ripping out the thinner "lower" step.