Tag Archives: ik hull design

IK autopsy

Performing my cutting-a-kayak-in-half trick gave me a long overdue chance to see exactly how they’re put together, as well as other stuff, like why it was failing and how well certain glues stuck.

The neoprene inside
I used to assume it was the same coloured coating inside the boat as out; it’s just simpler. But of course the diagram right is clear: what’s hypalon fabricoutside and what’s inside an IK hull is not the same stuff. There’s no need to waste UV-resistant hypalon sempauto - 16coating (or colouring or that matter) inside the boat’s benighted chambers. All it needs to be is the same durable and airtight coating, and neoprene – the brown rubber-like coating left – does that fine.
I bet I’m not the only one to mistake ‘neoprene‘ as simply that closed-cell sponge used in wet-suits or laptop sleeves. In its solid form it’s a durable synthetic rubber, but I presume lacks the full-on UV resistance of hypalon which DuPont invented shortly after.

I-beam floor
As mentioned here, an inflated vessel will seek equilibrium by attaining a rotund form, be it tube or sphere. A flat inflated plane such as an airbed or an IK floor needs to be a series of parallel tubes – or just a non-inflated sheet, like packraft and white-water raft floors. It also works the other way with bed mattresses. The springs and foam must be constrained by straps or whatever to keep the spring mattress flat.
sempauto - 17So this is an IK I-beam floor (left): probably the same tough core of nylon or polyester scrim, but without the impermeable hypalon and neoprene coatings of the exterior panels.
Note the pre-folds or creases to help the Semperit pack flat. I imagine modern IKs do the same, but it all explains the necessary attention to detail which makes ‘tubeless’ IKs like this so labour intensive, compared to ‘bladder’ designs like Aire.

sempauto - 2Twin side-tube IKs like this Forelle, the Incept and Grabner Holidays, have two smaller tubes one on top of the other, rather than one fat side tube like my Seawave (left, red) or Amigo. It gives the same buoyancy, more freeboard (above water height), a slimmer profile (more speed) or make more volume inside (easier packing). The red Seawave on the left is 82cm wide; the Semperit is 72. It makes the boat look a whole lot better too and overall because it’s also no less stable, I’d say it’s the best design for an IK, but it also needs I-beam sections to constrain the two side tubes.
I can’t say I could suck air through the scrim easily, but I’m pretty sure it’s porous – I didn’t find any transfer holes to allow air to flow between adjacent tubes – they might be a weak point.
sempauto - 14When an IK like this is over-inflated (or left in the sun) and is unable to purge through PRVs (none on the Semperit), you imagine it’s this scrim which either tears apart, most probably at the T-join where it’s glued to the neoprene (left). I tried tearing sections of scrim by hand;  impossible where it was uncut, but as soon as you nick it with a knife it would tear quite easily. This fabric was at least 40-years-old and had one or two patches of mildew, but was still tough and the whole assembly of the boat has held together amazingly well over the years.

sempauto - 4Where mine failed
sempauto - 10Inspecting the fatal second leak alongside the earlier repair, it seemed air was pushing through where two sections of I-beam scrim butted against each other. Perhaps the old coatings stretched differentially here or were just worn out.  It did look like the hypalon was simply flaking away – as you’d expect after four decades.
I could have fixed that leak but, as mentioned, another would probably pop up somewhere else, quite possible while at sea in either my- or a new owner’s hands.

Glue test
sempauto - 3semp- - 6I repaired the big original ‘L’ tear with a 5″ round patch of hypalon and two-part glue (left). I then patched a down-to-the-scrim scratch under the hull with one-part Bostik 1782 (right). I used the same glue to repair the first new leak inside (bubbling above left).

sempauto - 11Although I’m pretty sure they’d have lasted, I could easily pull off the Bostik patches by hand. Pulling off the big round Polymarined patch was another matter. It just so happened I’d sawn through the round patch but, only once I got some pliers under a lip (left) was I able to separate it from the hull. As you can see in the big image below, either the ancient orange hypalon coating of the IK, or the newer red hypalon of the patch separated from their respective nylon cores – the glue’s bond was stronger than the actual hypalon coatings.
sempauto - 13I get a bit lazy about having to faff about with two-part glue, and I also wonder if I ever guestimating the 25:1 ratio (or whatever it is) correctly. But as you can see, this stuff sticks. If you absolutely, positively want it to stay stuck, use two-part ami-polyadhesives. I still don’t know if the second part curing agent merely speeds up the drying process, or is actually chemically integral to creating the very strong bond. I’d think it’s the latter, otherwise why bother.

There’s more about glues and repairs here.


semp-bowwowOther stuff
sempauto - 15Well, the distinctive marine plywood bow has lasted fine – no warping at all and the rivets still intact.
It may have been an early design solution to easily joining the three sections of the hull in a nice sharp point, though they managed that join easily enough at the back. Maybe it was as much for protection and a frontal tracking aid.
I now have enough hypalon patches and D-rings to see me out. Other images from the autopsy below.




Inflatable kayak construction part 2: hull forms and rigidity

Part 1: Materials and Fabrication is here
See also drop stitch IKs (or more below)
Updated January 2020


An inflatable kayak differs from a hardshell in many ways. An IK is wider and sits higher in the water because the hull of the boat is effectively several inches thick, not a few millimetres as on a SinK (‘sit-in hardshell kayak’).
While length for length an IK is usually lighter than a plastic hardshell, it’s less rigid. On a short, rock-bashing, creek boat this can be either be unnoticeable or even a slight benefit when it comes to impacts. But here at IK&P we’re into long IKs of 3.6m (12 feet) or more. Years of experimentation have proved that this can add up to a do-it-all boat that’s manoeuvrable on rivers up to WW 3 and fine for coast hopping up to Force 4. Such a boat can either carry a second tacomapaddler or a solo camping payload for a few days paddle touring. grabananaProblem is when an IK gets beyond a certain length it can sag in the middle with a single heavy paddler (‘taco chipping’). As on the Java above (even with rocks piled in each end) or the ‘uphill‘ Grabner, left. There are various ways of making a long IK rigid, but first…
Hull profiles cons-2botsA hardshell sea kayak can have various hull profiles that, combined with other design elements defines the boat’s stability in various sea conditions (good article here). The picture on the right may be merely down to timing but illustrates how a very long hardshell runs more level than an IK bobbing on rough water, partly because it’s heavier and less buoyant, but also because – like a 29-inch MTB wheel – a long, slim kayak ‘flattens out the bumps’ better than a 26-incher. Some sea kayaks with V-shaped hull profiles become more stable the rougher seas get but at the cost of stability in flat waters.
sai13The flat and slidercutwide hull profile of a traditional  IK is not so sophisticated, neither is the box profile of a DS IK (right). For better or worse most IKs are as stable as a raft. On rivers or at sea an IK hull is what it is once they glued all the sections together: in most cases a very stable kayak until things get exceedingly rough. I’d guess that of the two hulls pictured left (a red Grabner Amigo and an Incept K40), the less boxy red boat would take more leaning over (or steeper waves) before it suddenly tipped. javahullHowever, it’s presumably obvious why the grey Incept is some 20% quicker through the water (as was the slender FC Java on the right). The Incept is 3.5 inches narrower than the Grabner and has a more pointy bow (its waegfabric and being 15% longer are also factors). And yet, this 27-inch wide Incept could never be described as ‘tippy’ even in seas up to F5-6 (other problems did occur). It proves that an IK doesn’t have to be wide (look left, would you paddle that?). Many, many otherwise functional-looking IKs are up to a foot wider than the Incept. Even if you are extremely nervous about padding, such excessive width is unnecessary and makes paddling inefficient.

cons2padsStability and centre of gravity (CoG) Having said that, an IK needs to be wide enough to compensate for the fact, that compared to a SinK or a packraft, you’re perched higher over the water on an air floor and probably on an inflatable seat too (graphic, left). This adds up to a higher centre of gravity which compromises stability, just as a 4WD is top-heavy in turns or on slopes compared to a regular car. Your butt is the axis on which you pivot when wobbling/capsizing and on a hardshell, a folder or a packraft you sit just an inch or two above the hull bottom and just below the water level: lower CoG, better stability. AESEThen, when you factor in self-bailing IKs, they require an even thicker floor to be above the general water level so again the boat becomes less stable unless it becomes wider. You can see how high the floor appears on the self-bailing AW StraightEdge on the right and the Feathercraft Java, below left. javabailerAnd although until recently it wasn’t something I saw mentioned, your physique/size can also produce an impression of instability in otherwise well-liked boats that most paddlers find fine. At my size, I found my old Sunny’s 30″/76cm beam was more than enough and at 27″ my Incept was also fine. But the 28-inch Java (right) and even more so the as wide Mk1 Safari (a self-bailer) were a bit tippy. The graphic below shows a regular IK in calm water and then swamped in rough water (centre). Right is a self-bailer like the Java or StraightEdge which drains in the same rough conditions but requires a thicker floor and/or higher seat to keep you dry. Result: IKs like Java or Safari Mk1 get tippy (for some), or you sit in water (not ideal) or the IK becomes over-wide.


comeonibeamIK floors: I-beam and drop stitch Most IK hulls are made of three chambers: two round sides and a flatter, wider floor made of many tubes which are interlinked (image left). And in case you’re wondering, an inflated floor is an important element of an IK’s buoyancy. Obviously, round sided tubes are easy to make and have take on the required form on inflation. Over inflating here is no great drama as the round profile distributes pressure equally. Steel-IBeamThe flatter, lilo-like floor is another matter. To make this section uniformly thick, before it’s all sealed the floor is joined top to bottom with I-beam dividers like the steel beam on the right. It’s said this is the most labour-sempauto - 8intensive and expensive part of traditional IK fabrication and explains why easy, slip-in bladders are preferred by most manufacturers. It saves time, effort and cost. Without I-beams or other constraints, the floor would balloon into a useless rounded form. But with I-beams too much pressure can pull the floor apart. Result: the floor balloons, the hull becomes deformed and a repair is very complicated or expensive. This is why some IKs including better Aires, some Gumotexes and Incept Ks feature a pressure relief valve (PRV) in the I-beam floor. Even though this part of the boat is in the cooling water more than the exposed, rounded side tubes, it is vulnerable to damage from excessive pressures which can occur when an IK gets hot out of the water. javasectionI-beam is the best design they’ve come up with, but an over-inflated I-beam floor without a PRV can mean a ruined boat if left in the hot sun for too long. Although it had four separate bladders (left) as opposed to an I-beam floor, I learned this lesson the hard way when my one-day-old Feathercraft Java went Krakatoa on me. Following that disaster, one thing I liked about my Incept K40 was it had PRVs on all three chambers. I no longer needed to be paranoid about exploding my £1500 boat by allowing it to overheat accidentally out of the water. I could leave it on a car roof or a beach all day and it would just purge harmlessly via the PRVs at the price of being a bit limp once it all cooled down back on the water. a quick blast with the K-Pump was all that was needed.

Other ways of making an IK hull stiff twinpointerWhether your IK is bladder (as left) or tubeless, one way to achieve a rigid hull is to use twin side beams; two thinner tubes attached one over the other as pictured right. There may not seem much in it, but two tubular sections resist longitudinal bending better than one big tube, and all for a negligible gain in weight and complexity. This design also has the advantage of making a slimmer boat compared to a big fat, single-sidetube design. Higher sides without width also keep out waves but do offer more windage – the bane of many IKs. Examples of twin side tube IKs include the Grabner Holidays, Incept Ks and the Sevylor above. One assumes that Gumotex weren’t able to make a long sea kayak out of Nitrilon that was suitably rigid. Instead their discontinued, permanently decked, 5-metre Seaker used a Korean PVC-coated fabric called Mirasol, but the Seaker ended up weighing 34kg, double the Incept.
backboneAnother method includes fitting metal frames or spars. Advanced Elements offer a Backbone (right) for some of their kayaks, though I’m still unsure whether this is as much to impart more of a ‘V’ into the otherwise flat and loose hull and so improve tracking and ner - 13speed. A picture here, and a forum full of discussions here. As the picture right at the top of the page shows, the metal frames in the FC Java (see also green graphic above right) didn’t keep that boat rigid, at least with my weight.
The Neris Smart IKs (left) use more substantial metalwork based on their folding kayaks. Using metal frames is a perfectly valid way of gaining stuffiness, but Incept and Grabner and maybe others too have proved that you can design an IK without resorting to such measures. Just as I’ve found with the Java and Grabner’s alloy backrests, incorporating bend-prone metal bars with inflatables isn’t such a good idea. If nothing else it makes damage-free transportation more of a gamble.
mod-bendloadsThis sagging or ‘taco chipping’ was always a problem on my 13-foot Sunny (left). While paddling in France one time I tried putting a 1-metre plank under the seat to reduce the sag. It did seem to give me an extra inch or two of draught in the shallow river. Later I tried a couple of straight branches jammed into the cavity between the floor and the side tubes.
My unscientific impression was that by levelling the boat out in the water the Sunny was indeed faster and more responsive. The fact that later on the river sticks popped out of their slots through some rapids suggested how much the Sunny flexed in rough water. A couple of years later I tried to do a better job, fitting some chopped-down cheapo paddles that ended up 1.55-metre long (left). Another 20cm would have been better as this where the Sunny hull starts to taper inwards to bow and stern. Some form of attachment for the poles needed to be glued to the 3-inch wide flat section where the floor meets the side wall (left). I got as far as this but then gave the Sunny away. Had I finished the job I’d have expected a little more response to the paddling stroke with a less taco chipped hull in the waves. If I owned the longer Gumotex Solar 3 I’d seriously think about enacting this mod or something like it.

slidercutifloorIn the last couple of years drop stitch (DS) panels have appeared on a few IKs. Lord knows how they make it, but it’s a way of joining two sheets of PVC with countless loose nylon filaments, dsfabricall of the same length. When the two sheets are sealed to make a chamber and then pumped up, the filaments act as multiple ‘I-beams’, distributing the tension over the entire surface area, as right (more on DS IKs here).

With this evenly distributed load constraining the two sheets of the panel, the pressure can be considerably higher and so the finished form much more rigid. It’s an ingenious solution to making slab-like inflatable chambers and Selytech-DShas proved a particularly effective way of producing iSUP boards (right) which are very light and nearly as stiff as a rigid foam and fibreglass surfboard. A drop stitch floor can take up to take 10 psi or nearly 0.7 bar’, 60% more than the highest pressure IK tube hulls. inflatoe
Incredibly, in the 1950s Goodyear used a similar technology to design an experimental inflatable aircraft, the Inflatoplane. This was a very light and portable plane, not an inexpensive inflatable decoy as armies have used previously. It did actually fly as a project it was abandoned when a valid military use for ‘an aircraft that could be brought down by a well-aimed bow and arrow‘ couldn’t be found. More Inflatoplane pics here.
Seagleea Eagle’s Fast Track 385 (right) uses a drop-stitch floor, so does the suspiciously similar but half-priced ZPro Flash (all sold via the same Korean factory). And now there are IKs which are fully made from DS panels.

The thing is, they haven’t yet found a way of making a drop-stitch panel that’s anything other than as flat as an ironing board. While full DS ought to make a kayak faster, the smooth, flat underside and basic, box profile may make handling in rougher water a bit sai13tricky. One side benefit of the I-beam floors on ‘tubeless’ IKs as pictured left, is that the channels formed by the many razorskegtubes create a keel or long skeg effect. You’d imagine completely flat floor would skate about in the slightest wind, were it not for a huge skeg, with additional skegs on the front or similar fixed bow forms. On the Sea Eagle 385 they’ve incorporated a drop-stitch inflatable front keel (right) that looks rather exposed and prone to damage. sea385skegI’ve not seen a 385 but I suspect a skeg anywhere other than at the back is a sign of a compromised design. You’d also assume a front skeg makes turning difficult, although with enough paddle cranking and some edging any IK can be turned easily enough. 

airkayaksAs well as Bic Kayaks, IK makers like Sevylor and Advanced Elements who feature separate floor panels on some of their boats, now offer a drop-stitch replacement for their regular, low-psi ‘air mattress’ floors. There’s a good discussion about doing that on AE kayaks here and more details at airkayaks here. Their image left dramatically illustrates the difference between the two floors. The consensus seems to be on AEs that the Backbone helps the flat-bottomed kayaks go straight and the DS floors are certainly lighter and way more rigid than the low-psi I-beam floors.
Not surprisingly, a Chinese company has come up with a crude IK based entirely on three Sportekies0.5-bar DS panels: the rather boxy Sportek KK1 and 2 on the left. You’d imagine some IK manufacturer is working on making something better. And sure enough, here they are.
The day they can manufacture DS panels which are more sophisticated than the slab-like platform shown below, mimicking the complex forms and curves of a hardshell kayak, will mark a big step forward in IK design, and that could be the way it’s going. For the moment Sea Eagle and Kxone/AirKajak have upped the game, but Incept and Grabner and now Gumotex with the Seawave have proved that a long, touring IK can be made stiff enough, even out of Hypalon, by using twin-tube design and/or a higher pressure hull.