Hull forms and rigidity
Glues and repairs
A ‘PVC vs rubber’ article from 2011 by The Boat People
PVC vs PU vs synthetic rubber: slightly biased. Skip to: 2. ‘Characteristics of the Material’.
IKs are fabricated or assembled in two ways: either with airtight ‘inner tubes’ which inflate inside the floor and side compartments of a (sometimes porous) protective fabric shell to make a rigid form, like a bicycle tyre inner tube.
Or sections of hull fabric are assembled to make fully airtight chambers and a load-carrying monocoque. I call this ‘tubeless‘ (which is also found on the very cheapest heat-welded vinyl IKs) and the pros and cons of both are explained below.
An IK’s hull can be an envelope or shell made of three chambers (two sides and the floor) usually made of PVC, woven nylon or both. Inside the casing fit light, removable sponsons or air bladders, made either from more PVC, more durable and flexible urethane, or just PU-coated nylon. Examples include Aire, some Aquaglide, Sevylor, Intex, Decathlon Itiwit and Advanced Elements.
Vinyl and urethane (PU) may all be just ‘plastics’, but might be compared to a Platypus water bladder (vinyl, stiff, slippery) and the blue Camelbak PU (softer and rubbery).
That is why in bladder boats like Aire, tough PVC is good for the outer shell and urethane makes an ideal, slightly stretchy bladder. Cheaper Aire Tributary IKs use vinyl bladders.
Compare that to the Gumotex, Grabner (both rubber), NRS (hypalon and Star PVC models), Sea Eagle, Incept and AquaGlide Chelan (all PVC) style of fabrication which involves the perfect gluing or heat-welding of the hull sections into a tubeless airtight monocoque.
I noticed my ‘tubeless’ analogy was adopted by Innova, the former US-based Gumotex importers, except that they tried to make out that ‘tubeless’ is superior. It certainly is for automotive tyre use, but with IKs it’s down to manufacturing ease and costs. PVC (welded is best, like Incept or some Aires, not glued like some 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, easy to weld or sew, and is slipperier in the water.
The difference between the ‘tubeless’ or ‘tubed’ IKs is down to manufacturing costs and materials. It costs more make a tubeless IK because the seal must be perfect. All bladder IKs are made of PVC; no one makes rubber IKs with bladders
On boats with air bladders the shell doesn’t have to be airtight which is why it can be sewn like the boat above. The shell takes form as the bladder is inflated. Shells can be heat welded, sewn or glued, and need sleeve openings or zips to access and repair or replace bladders, if necessary.
The robustness and durability of the shell is a factor, as it takes all the knocks and abrasion. Reliable heat welding of quality PVC is best, certainly compared to vulnerable stitching or laborious gluing. The green shell of the boat above was ripped on its second outing (possibly left in the hot sun). I also read that on cheaper IKs, sponsons can get twisted in the sleeves during unrolling and inflation.
The big drawback with bladders is that it’s normal for some water to seep inside the hull shell in the spaces between the bladder and shell. Result: the boat takes ages to dry properly. This may not matter in your villa in Acapulco, but it sure does in Scotland or Scandinavia or Seattle. Coming back tired from a long paddle and then packing a wet boat is as undesirable as packing a wet anything, not least if it’s seawater. Mildew may develop, grit may get in and who knows, something may rot and shorten the life of your boat.
If your boat is in and out of your car boot or motorhome, tubeless is the way to go. If you’re more into multi-day trips and have time and space to dry it later, a bladder boat has no disadvantages.
Tubeless construction is the original or ‘European’ method when all boats were made from rubber like Hypalon. If well made, as most were back then, a boat can last decades as rafters know well (left, a hypalon Semperit from the 80s).
Our Nitrilon Gumotex Solar looked as good as new when I sold it some nine years on, and had my Incept K40 been made from the same material and not PVC, I’d have probably kept it. No matter what they tell you (‘easy repair’), bladdered IKs are about ease of assembly and cost-saving.
I like the simplicity of tubeless IKs: a tough one piece shell with less bulk. There is no cheap way of doing it and so any tubeless IK like a Gumotex, Sea Eagle (heat-welded PVC), Chelan or Grabner ought to be a well-made IK.
In my experience in the US, IK rental outfitters tend to use tubeless Hypalon IKs like NRS or Hyside, even if most recreational IKs sold are bladdered. Tubeless will cost more but they’ll last much longer, especially if made from synthetic rubber.