Skin-on-frame dinghy hull

Hello boatdesign.netters,
I've been playing around for some time with a concept for a small, collapsible skin-on-frame sailing dinghy that would be light enough to be transported in a moderately sized duffel bag, and I'm at the point where I have what should be a workable design. The only remaining uncertainty I have is about the hull skin material; from my research it seems that professional builders' consensus choice is polyurethane-coated ballistic nylon (e.g. Spirit Line kayaks or Aerolite) but I've never worked with the stuff and have no idea if polyurethane is flexible enough to be folded or stuffed into a bag of some kind without damage, or if it's considered "flexible" only because it absorbs stresses during normal use that epoxy cannot. That's my 1st question; the 2nd is whether there are any drawbacks to possibly using 18oz vinyl tarp instead (aside from carcinogenicity), which would additionally save me the work of sewing grommets and reinforcements on the edge of the skin (since it'll be temporarily lashed to the frame when assembled instead of stapled or glued). Is PVC relatively saltwater resistant or will it turn brittle quickly? Rough calculations tell me its strength and durability should be sufficient (ultralightweight boat, ~50lbs, and lightweight usage) but I imagine that there's some accumulated practical knowledge out there about how PVC works for SoF boats. Thanks!

 

There is a skin on frame sailing kayak design on the market. It might give you the answers you seek.
Kayaksailor Sail System https://www.klepperamerica.com/kayaksailor-sail-system/

 

Kleppers are very cool but unfortunately I don't think Hypalon is possible for me, it's not cheap (if you can even get it!). I think it's a similar enough material mechanical properties-wise to PVC but I can't find much information about using the latter as a skin

 

Hi Greenflower, you might try various free materials from industry for your prototype, to start cutting patterns. Try trucking companys for old discarded tarpaulins, they come in different synthetic materials, and thicknesses. Or even old canvas tarps; sewn patches can be waxed or oiled to waterproof again (mostly). Plain old canvas isn't greatly expensive new, and easy to work with. Some car covers, boat and caravan covers may be big enough material, suitably waterproof, and available locally. I made a canvas skin on frame kayak, with unimproved medium weight waterproof canvas, which was adequate for testing, and I can still use it 30 years later. My skin was held on with long velcro patches, contact adhesive and stitched through, so no eyelets to tear out, or position restrictions. I just put more velcro patches wherever needed to create and hold the best shape. An uncle made a pretty canvas on wood frame row boat, with painted canvas, but I think the old enamel paint he used would crack up damaging the canvas; I never saw it on the water. Post some pics of your frame, and you may get some more useful suggestions. Do you have a centreboard case, or lee boards ? Sealing around a centreboard case is probably your greatest waterproofing challenge. You may need 2 duffel bags in the end, for rudder, C board, mast, rigging, and sails. Then buoyancy vest, water, and other personal items. You won't be back packing it. Good luck with your project. Show photos.

 

The velcro's an excellent idea, and I wasn't convinced of its long-term waterproofness but the canvas is too, also less poisonous to marine life I'd imagine.
I can't really go into detail yet about the design (and I don't have any photos because it's not built, I've only got a Delftship model) but I'm almost certain it'll be legitimately backpackable, the frame isn't a conventional keel and rib kinda thing. And yes I'm going with leeboards, most likely made from 1/2 or 3/4 inch epoxy-sealed baltic birch. 50lbs as an estimate includes mast/rigging, sails, leeboards, rudder, etc, and the frame is so compact that it's probably a minority of the total space the whole thing'll take up. Still haven't quite figured out how I'm gonna haul the yuloh around however, maybe I can have some sort of bindle set-up going on so as to reflect the boat's essentially cheapskate nature. The design requirement is for it to be a fully functional 9ft sailing dinghy that I can transport on foot and that's small enough to pack onto a train. I'm confident in its structural integrity, less so in its actual sailing qualities, but I've done as much theoretical groundwork as I can so I'm moving onto a prototype. I'll for sure post some photos if it ever gets built, thanks for the sage advice

 

Hi,

PVC comes in different material. Some are UV resistant, some not. Their mechanical properties can also vary. PVC is also beginning to be forbidden in many applications due to its environnemental impact. You should perhaps consider PP, as this company Folding Kayaks That Go Anywhere | Oru Kayak Europe https://eu.orukayak.com/ has developped a folding kayak, made out of double-layered, custom-extruded polypropylene.

Cheers,

 

Yes the toxicity of PVC definitely gives me pause. Insane that the stuff's been used for so long in everyday objects when the danger's been objectively known for decades, it has its uses (boats possibly being one of them) but it's probably best to avoid it. Do you happen to know if a polyurethane-nylon skin is flexible enough to be stowed in a bag? At the thicknesses required for a hull skin I believe polypropylene isn't. Also quite a cool boat though

 

We had this discussion a few times. To spare you the search, here an overview:
There are two basic possibilities, precoated fabrics and post (user) coated fabrics. The underlying fabric is made out if either nylon or polyester yarns, the coating can be on one or both sides.
Precoated fabrics:
1. Synthetic rubber on both sides of polyester fabric, aka. Hypalon. EPDM is another synthetic rubber but usually not available with enough reinforcement for boat use. Can only be glued, heavy, difficult to procure, best UV resistance, very good abrasion resistance. Plenty of folding kayaks and inflatable boats use it.

2. PVC coated fabric, usually on polyester backings, can be glued or hot air welded. Medium UV and abrasion resistance, easy to procure, cheap. The amount of fabric reinforcement and PVC coating varies greatly, and that influences its properties. Normal uses include traps, printable billboards, pond liners, truck coverings, inflatable boats and folding kayaks.

3. PU coated fabric, polyester or nylon. We distinguish between PU and TPU coatings, with TPU beeing preferred as it has better UV and abrasion resistance then PU. TPU can also be welded by hot ironing. There is a wide array of fabric weights available, typical uses beeing technical clothing, tents, fokding kayaks, inflatable boats, etc.

4. Silicone coated nylon, aka. Silnylon, good UV resistance, medium to poor abrasion, very lightweight. Typical use is outdoor gear.

Post coated fabrics:
Raw polyester and nylon fabrics are coated by the user. None of the usual coatings used on rigid SOF boats is flexible enough for folding, so don't try them. Success has been reported with liquid hypalon and recently PU construction adhesive.

Now to your question about 18oz PVC tarp. PVC coated fabrics are reinforced in many ways, from tightly woven fabrics to very open weaves held together by the PVC. What you get depends on the intended use, something intended as an actual tarp has less reinforcement then something intended for more structural applications trailer enclosure or inflatable boat. 18oz/sqyd is a sufficient weight is properly reinforced, but on the lightweight end of the scale. Typical weights for folding kayaks and inflatable boats range from 24-36oz/sqyd, but people have used even 12oz/sqyd fabric intended for billboards.
Instead of buying tarps buy the PVC off the roll and choose something with a higher reinforcement. Applications include pond liner, biogas reactor covers and actual inflatable boat fabric.

 

back packable Canoe https://www.boatdesign.net/threads/back-packable-canoe.60237/#post-829145
I built the back-packable canoe in the above thread using PVC coated nylon for the skin, with brass grommets to attach tensioning cords etc. About 16kg and 3.3m long packs into 90cm x 30cm back pack.

I don't understand your fear of PVC, you have to burn it and breath the fumes to be in any danger as far as I know. Nearly every home in the UK has PVC window frames and guttering; it's a true commodity plastic.

From the British Plastics Federation:
The essential raw materials for PVC are derived from salt and oil. The electrolysis of salt water produces chlorine, which is combined with ethylene (obtained from oil) to form vinyl chloride monomer (VCM). Molecules of VCM are polymerised to form PVC resin, to which appropriate additives are incorporated to make a customised PVC compound .
.... which also says that about 57% of the weight is from common salt, the rest from oil.

 

Agent Orange's precursors were just lye and methanol, doesn't have much to do with its chemical properties ultimately. The additives you mention are typically carcinogenic phthalates that offgas and leach out of the material over time, and there are various dioxin synthesis byproducts that can contaminate both the plastic and the environment (and which end up bioaccumulating in the food chain basically for eternity, which is why as Alain noted there's a real policy effort to curb its use, at least in the EU) that are teratogenic and acutely toxic. Nasty stuff, probably OK if produced and used carefully but that's not the case currently. Your canoe has almost exactly the tensioning system I had in mind, but it does seem like the PVC doesn't have quite the same form-fitting stretchiness as nylon or polyester skins. Was there any noticeable extra drag from the slack?

 

I'd previously come across your thread about a similar concept, lots of interesting information but I couldn't find out whether or not PU/TPU can be folded repeatedly without damaging the coating. You're saying that it's used for inflatables and folding boats already, so I imagine it can?

 

TPU coated fabric was used by Feathercraft, but please understand this is not a coating you can apply by yourself over some random fabric with a brush, it's done by the factory and to you it looks just like any other vinyl tarp material. You then have to cut and sew, glue or weld it to build a skin. It's available from specialized fabric stores in different weights and coating thickness.
You probably own TPU and PU coated fabrics since they are a mainstay of outdoor apparel and gear (clothes, bags, tents, etc.).

When people talk about PU coatings for SOF they mean rigid boats coated with some form of PU varnish or paint, and those are not flexible enough for folding. It has been said that PU construction adhesive can make a flexible waterproof coating but I have no experience with it and can not confirm. You want to try it, buy some PL Premium and smear it on fabric, then see how it holds up.

 

Greenflower, the PVC coated nylon fabric in that canoe of mine was quite difficult to stretch and to shape around the stems. I had a PU coated polyester fabric skin made (at very great expense) and that was practically unstretchable and couldn't be used. The PVC/nylon skin was completely smooth below the waterline so didn't cause drag but the boat was a bit too unstable for me.
A few years ago I built a skin on frame rowing boat (skin-on-teak boat somewhere on this forum) with both a PVC on nylon layer and an uncoated polyester layer, the whole thing then painted. I moored it over a sharp rock which went through the polyester but didn't damage the nylon based fabric - but as Rumars said I don't think you can fit and remove a painted fabric from a folding boat.

 

Sorry Rumars, I read right over the answer in your first post, apologies for making you repeat yourself. Thanks for the detailed technical info, TPU precoated nylon seems like the best bet for my needs, and it's available very inexpensively from Chinese suppliers (far cheaper in fact than buying ballistic nylon and 2-part PU separately, somehow). Plus the ease of heat-welding seams makes questions of stretchiness moot, I can just cut out a pattern for the hull shape specifically (it's hard chined), which'll also add abrasion reinforcement on all the edges. Link if anyone's interested: Danlu Textile Both Sides Laminated Tpu Fabric/ Nylon Oxofrd Laminated Tpu Coated For Inflatable Boat/relief Tents - Buy Waterproof Tent Fabric,Waterproof Coated Nylon Oxofrd Fabric,Tpu Fabric Product on Alibaba.com https://www.alibaba.com/product-detail/Danlu-Textile-Both-sides-laminated-TPU_1600179550002.html?spm=a2700.wholesale.0.0.7d9e10a0XM7Ze3. this stuff is 700g/m2, or 25oz, should be plenty sturdy

Alan, I came across your teak boat while searching the forums earlier, she's a beaut. This thing isn't gonna be an aesthetic masterpiece but the structural design as far as I know is completely novel and should be of interest to people here, I'll be sure to post updates as it progresses.

 

rough comparison of different fabrics' tensile strengths, nylon is significantly stronger than polyester or canvas at a given weight so it makes sense that the nylon held up where the polyester didn't. I imagine its greater elasticity helps prevent punctures as well
https://www.orientbag.net/post/2016/05/31/the-fabric-strength-challenge-1