Inflatable kayak construction, part 1: materials and fabrication

I’m not an expert on any of these matters; it’s just what I’ve learned over the years. I update these pages as I learn more and welcome your corrections.
This original article got a bit rambley so was split into two. Part 2 covers hull forms and rigidity.
And you can read about glues and repairs here
Updated Summer 2017

Broadly speaking, quality inflatable kayaks are made from two types of material: a woven fabric core coated in rubber or plastic. And IKs are fabricated or assembled in two ways: either with ‘inner tubes’ which inflate a fabric shell to give it a rigid form – like a bicycle tyre – or the panels of hull fabric are glued together make a fully airtight load-carrying monocoque. I call this ‘tubeless’ and the pros and cons of both are outlined below.

Synthetic rubber hull materials
nylonpolyThese use a nylon or polyester (PES) woven fabric (see right; broadly speaking nylon is more stretchy which is good against punctures but  less good for rigidity). It is coated with tough natural and/or hypalon fabricsynthetic rubber and sometimes a softer neoprene on the inside surfaces. Examples include the original hypalon, invented many decades ago by DuPont but actually referring to the coating, not the finished material.
Hypalon fabric is composed of three plies all bonded together:
• an inner coating of neoprene makes it airtight and easy to glue
• a fabric core of polyester or nylon weave gives tensile strength and resists tearing
• the hypalon outer layer resists UV rays and chemicals

A near identical product is now supplied, among others, by Pennel in France under the ‘Orca‘ brand (graphic, above left) as used on US-made NRS IKs.
gumotexfabrics1Gumotex’s various Nitrilons (left) are similar, as is EPDM aka: ‘Nordel’ used by Grabner as well as some folding kayak makers and raft manufacturers. In 2012 Gumotex brought out a material called Hevealon based on their single-side coated Lite-Pack (now renamed Nitrilon Light). It had EDPM on the outside of the hull and a teflon treatment on the inside of the boat to repel water and so speed up drying. This is a problem with the original Lite-Pack material as used in their original Twists, compared to the classic, full coat Nitrilon which dries fast on both surfaces. Other problems with earlier Lite-Pack boats may explain the renaming as Nitrilon Light. The Swing models use Hevealon and the Twists use Nitrilon Light (NL), but are much cheaper than the Nitrilon models. As you’ll see in the table, NL has only a third of the tensile strength of Nitrilon, which can only be down to a less robust and thinner polyester core but is 30% lighter.
Gumotex in Czech Rep. produce Nitrilon fabric in their factory as well as various other rubber-based sheeting and products besides inflatables. The tougher boats in their inexpensive recreational gumoglubiIKs are made from regular Nitrilon (see graphic above) and are glued together by hand. The gluing of some of  these boats limits the operating pressure to 0.2 bar (2.9); still-impressive by the standards of many well known American-branded IKs which run as little as 0.1 bar. But notably, a couple of Gumotex IKs including the long-established and expensive K whitewater series and the Seawave sea kayak run 0.25 bar (3.6 psi). It is possible that these boats are not hand-glued, but heat vulcanised, a join that can contain higher pressures.
heatrweldAustrian fab-frabGrabners say their EPDM boats are heat vulcanised which can be compared to heat- or radio frequency (RF) welding of plastics like PVC. Heat welding PVC is so easy and effective you can do it yourself with roller and a heat gun fitted with a flat nozzle (left), though trying to make an IK like this would be quite a challenge (as would be gluing up an IK from Nitrilon sheet).
doudoufeatherfApparently of heat welding PVC was invented in 1948 in Vitry, France by SEVY, the company that went on to become Sevylor (‘Sevy of gold’). They produced an inflatable PVC bath that became a hit in post-war austerity France. A velour coated air mattress followed and since that day Sevylor has never looked back.

Vulcanising is a chemical reaction which either molecularly bonds rubber to itself or makes a rubber product, such as a tyre more durable. In a bonding sense vulcanisation is what happens when your punctured tubeless car tyre is professionally repaired on the inside surface, whereas gluing is like sticking a patch on an inner tube (stop me if I’m going too fast, here). Assuming I have the right end of the sticky stick, that means Grabners have a superior construction process to most Gumotexs, and may also explain how, despite running 50% higher pressures, Grabner boats don’t feature pressure release valves (PRVs); their boats are so well bonded there’s little risk of them bursting if left in the hot sun. But that’s still not recommended.
Don’t be put off by the picture of delaminated Grabner on the left – it’s from a 12-year-old Grabner Holiday 2 and occurred at a point where a metal eyelet on the demi-deck didn’t get on with the EPDM fabric (that boat has since been renovated). It’s there to illustrate the densely woven and stretch resistant underlying fabric. On full inflation a good IK doesn’t want to stretch like a balloon (or a PVC Slackraft come to that), it wants to be taught like a basketball. As folding boat makers, Pakboat put it on their website: ‘The abrasion resistance [and so, waterproofing] is in the coating, and the tear strength and tensile strength are in the woven fabric.’

PVC
Otherwise, IKs are made from a cheaper but stiffer PVC-coated fabrics or more expensive [poly]urethane, the latter like Incept, Alpacka packrafts and a few more listed below. Remember, this is not pure PVC plastic film with the pool toy and Slackraft connotations of poor durability, but PVC coated onto a nylon or polyester fabric base, just like hypalon. The difference is like a tent made out of a plastic bag; light and waterproof yes, durable no. Or a tent made of normal woven fabric with a PU waterproofing coating: much more durable. contubesIn environmental terms, PVC has become a bit of a dirty word (Greenpeace report) which is why Innova in the US make such a song and dance about their Swing IKs not being made from PVC (like all Gumotex IKs). It’s notable that when an IK maker like Sea Eagle describes their hull material as ‘1100 Decitex Reinforced’ or ‘1000 denier fabric with a high ripstop capability’, the missing letters are ‘PVC‘ because mentioning it has become bad for business.

Rubber vs plastic
Synthetic rubber Nitrilons and EPDM are both tough, durable and more expensive than PVC, metre for metre. On a same-sized boat they’re also more pliant and will roll up or fold down much more compactly than PVC (below right, a 4-metre Nitrilon Sunny). Synthetic rubbers also have very good resistance to UV and solvents, are heavier than comparable PVC/PU and come is fewer colour options. Rubber used to be the classic material for river running rafts (as pictured left at Lees Ferry near the Grand Canyon) that put in years and years of reliable service. But being a form of rubber not plastic, it cannot be heat welded in a machine like PVC/PU; it must be labouriously glued which, when done right, increases costs. In Grabner’s case, it can then be heat vulcanised which might be regarded as a form of heat welding.
This is why Grabners, NRS MaverIKs and proper river running rafts all cost so much. They’ll probably outlive your pet and might be considered over-the-top for a recreational IK’ing. Inexpensive recreational IKs are made from PVC-coated fabrics. Polyurethane (PU) approaches the better characteristics of rubber and it’s likely that not all PVC is nasty, toxic crap, just as not all metal bicycle frames are the same quality. In the hands of a careful owner – as opposed to the hard use from a rental- or river-running outfit – with 303proper care and maintenance involving anti-UV 303 protectorant (right), thoughtful handling, drying and storage, a PVC or PU IK will still last for years provided the initial material and quality of manufacture are high. And while suitably chunky PVC or PU IKs like my old Incept K40 are hard to fold, that stiffness translates to a more rigid and therefore faster boat on the water. This was apparent when I first tried a K40 after running a Nitrilon rubber Sunny for years.

vanguard

Table lifted from: http://www.vanguardinflatables.com/service-support/inflatable-boat-materials-construction
Bear in mind in may be a little ‘optimised’ to validate their choices of material.

Fabrication ‘air tube’ or ‘tubeless’ airecell
Sometimes an IK’s hull form is a shaped, casing or ‘envelope’ made of PVC, Cordura or any similar hard-wearing fabric into which slip light, removable sponsons or air bladders, made either from stiffer, ‘brittle’ vinyl, more durable and flexible urethane, or PU-coated nylon. Examples include Aire (an Aire ‘cell’ or bladder pictured above left), bladdioAdvanced Elements and the BP Trinity II (see OtherIKs). I call this the ‘American’ method. Pictured on the right, vinyl and urethane may just all be plastics, but might be compared to Platypus water bladders (vinyl, stiff, slippery) and the blue Camelbak (more soft and rubbery). That is why in bladder boats like Aires PVC is good for the shell and urethane makes an ideal, very slightly stretchy bladder.
pointerdiagramCompare that to the Gumotex, Grabner, NRS and Incept style of fabrication which is like a tubeless car tyre: the perfect gluing of the tough hull sections keeps the air in. Aire-style is like a tyre with an inner tube that pumps up to give rigidity and form to the outer cover or hull casing. Both are repairable with patches in broadly speaking the same way. I notice my ‘tubeless’ analogy has now been adopted by Innova, the US Gumotex importers, except that they try to make out that ‘tubeless’ is superior. It certainly is for automotive tyre use, but with IKs it’s more down to manufacturing ease and therefore, costs. PVC (welded is best,
like Incept or some Aires, not glued like Advanced Elements or sewn like Tributary) is stiffer once pumped up, less durable, doesn’t abrade so well on grit (out of the water), but is less expensive than synthetic rubbers, quicker to weld together, and is slipperier in the water, so giving better response when combined with the superior stiffness. The difference between the ‘tubeless’ or ‘tubed’ construction style is merely down to the cost of manufacture and materials.
yakkistanBladder boats can use PVC shells and so save costs (or spend it on design) because fully sealed welding of the hull sleeves isn’t critical; they can just be easily heat welded, sewn, glued, or even zipped together and the sponson ‘inner tubes’ can be slipped in and pumped up. But durability of the outer hull is a factor in how the panels fit together to make the sleeved hull, and here quality heat welding is best, certainly compared to vulnerable stitching. I also read that on cheaper IKs sponsons can get twisted in the sleeves during unrolling and inflation which can get to be a faff. That and that much quicker and easier drying is another reason I prefer tubeless IKs. If your boat is in and out of your car boot or motorhome hatch, then tubeless won’t give those problems. If you’re more into multi-day trips, a bladder boat has no disadvantages dsfabriconce pumped up correctly.

Drop-stitch fabric (right) – a type of construction from flat tubeless panels – is discussed in the next article.

Conclusion
sempauto - 1
Nitrilon/EPDM ‘tubeless’ construction seems to be the traditional or ‘European’ method, and if well made will last for decades as rafters know well (left, a hypalon Semperit probably from the 70s or 80s). Our Nitrilon Gumotex Solar looked as good as new when  I sold it some nine years on, and if my former Incept K40 had been made from the same material I’d have probably kept it. Bladder boats seem to be a cost saving way of doing it, and are popular in North American. All cheap IKs use bladders, but not all bladder IKs are cheapies. A top of the range, 18-foot Aire Sea Tiger costs $2650.
bladderboyThe problem with bladder boats is that although the best made ones may perform better, it’s normal for some water to get inside the hull sleeves which contain the pumped-up bladders. Result: the boat takes ages to dry. This may not matter in sunny Californi-yay, but it sure does in Scotland or Scandinavia. Packing a wet boat is as undesirable as packing a wet anything, not least if it’s sea water. Mildew may develop, grit may get in and who knows, something may rot and shorten the life of a boat (although Aire say that a little water in the chambers does no harm, even long-term). When I come back from a sea paddle I always hose my IK down.
gumk44I’ll admit I like the simplicity of the tubeless style of IK fabrication – a tough outer shell that is well sealed; there is no cheap way of doing it and so any tubeless IK ought to be a well made IK. In my experience in the US, IK rental outfitters tend to use tubeless hypalon IKs even if most recreational IKs sold are bladdered. Tubeless will cost much more but they’ll last much longer, espwciually if made from synthetic rubber.

Part 2 covers hull forms, rigidity and drop stitch IKs

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One Response to Inflatable kayak construction, part 1: materials and fabrication

  1. Col Yeo says:

    I bought a Gumotex twist Nitrilon Light. On it second outing because it is so light it was flipped over by a light gust on to a small stick which punctured it. Our Gumo Solar which we have had extensive use for over 4 years would never have puncture but it is made of the standard Nitrilon.
    Gumotex said the stick would have puncture any inflatable which is obviously not true.
    The retailer has offered to send it back to them but with that attitude I don’t expect them to accept that the Nitrilon Light is flawed or up to the job.

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