Inflatable kayak construction part 2: hull forms and rigidity

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


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 side tube 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 theor 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 back rests, 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 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 that 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 a 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.


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6 Responses to Inflatable kayak construction part 2: hull forms and rigidity

  1. Very impressive blog site and info. Very happy with my Solstice Flare…now I understand better why. I use it on flat lakes and running rivers, so the smaller/shorter/slower/rougher drawbacks are not a factor to me.
    I need to add some D rings for overnighters…what adhesive do you recommend? Seems all I can find is the HH-66 (Amazon and others). Seems the right stuff…as stated in the blog…prep will be a vital step.

    • Chris S says:

      Not heard of it before but HH-66 looks like good stuff – and the fact they sell it with D-rings makes you hope it’s well suited to that.
      Get a good roller too. Glad you like the blog.

  2. hi, wich is your opinion about sea eagle?

  3. Once beam floor tubes get skewed, how to reposition them? I had this issue with multiple kayaks that use floor tubes – many times tube in the middle gets bumped which makes it hard to sit on the floor

    • what you’re describing is a blown ibeam. you’re over inflating your inflatable kayak to the point that you are blowing out the ibeams. once it’s blown, you’re done, there’s no fixing that. drop stitch floors are nice in that they are replaceable, and unless you’re using a high end high pressure pump, you likely can’t over fill the floor. and if you do damage your floor in a drop stitch boat, you can replace it easily.

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