Inflatable Kayak Construction Part 2: Hull Forms and Rigidity

Part 1: Materials and Fabrication
See also: Drop stitch IKs (or more below)

Updated Summer 2020

An inflatable kayak differs from a hardshell in many ways. An IK is usually wider and sits higher in the water because the sides and floor are effectively several inches thick, not a few millimetres as on a SinK (‘sit-in hardshell kayak’).

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While length-for-length an IK is usually lighter than a SinK, it’s rarely as rigid. On a short, rock-bashing, creek boat this can either be unnoticeable or even a slight advantage 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 adds up to a do-it-all boat that’s manoeuvrable on rivers up to WW3 and fine for coast hopping up to Force 4. Such a boat can either carry a second paddler or a solo camping payload for a few days touring while rarely weighing more than 18kg or 40lbs and so can be carried over short distances.
Problem is, when an IK gets beyond a certain length it can sag in the middle with a single heavy paddler (me?!). As on my old Feathercraft Java (below), even with rocks piled in each end or my ‘uphill‘ Grabner, above left. There are various ways of making a long IK rigid, but first…

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Hull profiles
A hardshell sea kayak can have various hull profiles that, combined with other design elements defines the boat’s stability in various sea conditions. The picture on the left may be merely down to timing but illustrates how a very long SinK on the right runs more level than an IK bobbing about on rough water, partly because the hardshell is 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-er. Some sea kayaks with V-shaped hull profiles become more stable the rougher seas get but at the cost of stability in flat water.

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Left: Grabner Amigo – right Incept K40.

The flat and wide hull profile of a traditional IK is not so sophisticated, nor is the boxy (hard chined) profile of a full dropstitch (F D/S) IK (right). For better or worse, most IKs are as stable as a raft. The key is to find a balance between reassuring stability and performance-killing width. On an IK I find the optimum width is around 32–27″ (81–69cm). On rivers or at sea an IK hull is usually very stable until things get exceedingly rough. I’d guess that 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. However, it’s pretty obvious why the grey Incept was some 20% quicker through the water, as was my slender FC Java.
Note that the width of an Full D/S IK can be misleading as the flat sides taper in to the floor. Above right a Sea Eagle Razorlite listed in the table below as 76cm and which is fast, but some find a bit tippy.

The Incept was 3.5 inches narrower than the Amigo and has a more pointy bow (its stiff fabric and being 15% longer were also factors). And yet, this 27-inch (67cm) wide Incept could never be described as ‘tippy’ even in seas up to F5-6 (other problems did occur).
It all proves that an IK doesn’t have to be ridiculously wide (as left) to be stable. Many, many otherwise functional-looking IKs are up to a foot wider than the Incept. Even if you are extremely nervous about padding and stability, such width is excessive and makes paddling inefficient.

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Stability and centre of gravity (CoG)
Having said all that, an IK needs to be a bit wider to compensate for the fact, that compared to a SinK or a packraft, you’re sat higher above 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 affects stability, just as a 4WD is top-heavy in turns or on slopes compared to a McLaren F1. 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 below the water level: lower CoG = better stability without resorting to width.

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Then, when you factor in less common self-bailing IKs, with an even thicker floor to be above the general water level, 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 left and the Feathercraft Java, below.

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Also, your physique/size can also produce an impression of instability in otherwise well-liked boats that most paddlers find fine. At my XL size, I found my old Gumotex Sunny’s 30″/76cm beam was more than enough and at 27″ my Incept was also fine. But the 28-inch Java (above) 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 (also not ideal but the best compromise for white water).

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IK floors: I-beam and drop stitch
Traditinal IK hulls had three chambers: two round side tubes and a flatter, wider floor composed of many interlinked tubes (left). And in case you’re wondering, an inflated floor is an important element in an IK’s buoyancy. Obviously, round sided tubes are easy to make and take on the required form on inflation. Providing it’s well made, modest over-inflating is OK as the round profile distributes pressure equally.

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The flatter, lilo-like floor is another matter. To make this section the floor is joined top to bottom with I-beam fabric dividers resembling the steel beam on the right. It’s said this is the most labour-intensive and expensive part of traditional IK fabrication and explains why easy, slip-in bladders are preferred by most manufacturers, as on the Java, left. It saves time, effort and cost. Without I-beams or other constraints, once inflated the floor would balloon into a useless rounded form.

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But with I-beams too much pressure can pull the floor dividers 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, bigger Grabners 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, it is vulnerable to damage from excessive pressures which can occur when an IK gets hot when left out of the water. Although it had four separate bladders as opposed to an I-beam floor, I learned this lesson the hard way when my day-old Feathercraft Java went Krakatoa on me one sunny day in Colorado.
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 accidentally allowing it to overheat out of the water. I could leave it on a car roof or a beach all day and air would 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. On my Seawave I added PRVs to the two side tubes to get the same benefits.

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Other ways of making an IK hull stiff
Whether your IK is bladder (as left) or tubeless, one way to achieve a rigid hull is to use twin side beams; two thinner tubes stacked over each other as pictured left. There may not seem much in it, but two tubular sections resist longitudinal bending better than one big tube, and all with a negligible gain in weight. This design also has the advantage of making a slimmer boat compared to a fat, single side tube. Higher sides without width also keep out waves but do make more windage – the bane of IKs. Examples of twin side tube IKs include the Grabner Holidays, Incept Ks and the Sevylor above, the old Semperit Forelle (the original modern IK) and Gumotex Seakers. At the time, Gumotex weren’t able to make a 5-metrelong Seakers out of Nitrilon that was suitably rigid so the Seaker (below) used a Korean PVC-coated fabric called Mirasol, but ended up weighing a staggering 34kg – double the Incept.

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A cruder method includes fitting metal frames or spars. Advanced Elements offer an optional 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 flabby hull floor and so improve tracking and speed. A picture here, and a forum full of discussions somewhere here. As the picture right at the top of the page shows, the metal frames in the FC Java (see also green graphic above) didn’t keep that boat rigid, at least with my weight.

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The newer Neris Smart IKs (left) use more substantial metalwork based on their folding kayaks. Using metal frames is a valid way of gaining stuffiness, but Incept, Grabner and now dropstitch 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 a good idea. If nothing else it makes damage-free transportation more of a gamble.

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This sagging 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 mid-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 0.2 bar (3psi) Sunny flexed in rough water. Later I tried fitting some chopped-down cheapo paddles. 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 (more here). 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 flex in the waves.

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dsfabric

In the last few years drop stitch (D/S) panels have appeared on a newer IKs. This technology is derived from the popularity of inflatable iSuP boards (above) which clearly need more pressure than your average airbed or IK. Lord knows how they make it, but it’s a way of joining two sheets of coated fabric with countless loose nylon filaments, all the same length (left).

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When the two sheets are sealed to make a chamber and then pumped up, the space yarn acts like multiple ‘I-beams’, distributing the tension over the entire surface area. Result: pressure can be up to 10 psi or nearly 0.7 bar’ – four times than a tubed IK, making the boat much more rigid. More on DS IKs here.

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Incredibly, in the late 1950s Goodyear used a the same D/S technology to design an experimental inflatable aircraft, the Inflatoplane (above). A light and portable plane, not an inexpensive inflatable decoy as armies have used previously. It did actually fly but as a project 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. Now there are IKs like the Kzone Slider below which are fully made from DS panels.

SLIDER

They are only just finding ways of making a D/S panel that’s anything other than flat as an ironing board, but they’re getting there – the Decathlon Strenfit X500 (below) currently leads the way. While full DS will make a kayak very rigid, the smooth, flat underside and basic, box profile may make handling in rougher water tricky. One side benefit of the I-beam floors on ‘tubeless’ IKs is the channels formed by the parallel tubes create a keel effect.

sea385skeg

On the Sea Eagle 385 they’ve incorporated a drop-stitch inflatable front keel (left) that looks rather exposed and prone to damage. You’d also assume this makes turning difficult, although with enough paddle cranking and some edging any IK can be turned easily enough.

Now they can manufacture DS panels which are more sophisticated than the slab-like platform shown below, mimicking the complex forms and curves of a molded hardshell kayak. It marks a big step forward in IK design. Currently the Gumotex Rush (part–D/S hybrid) and especially the Decathlon X500 have upped the game. More than just 3 planks making a paddling trough, Gumotex have managed to integrate D/S panels into the bow and stern, giving a more hydrodynamic form on the sides.
Drop-stitch is the future of IK design and I wouldn’t be surprised if it somehow moves towards packraft floors too. Longer boats like my Nomad S1 could benefit from the added support of a separate D/S floor.

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13 thoughts on “Inflatable Kayak Construction Part 2: Hull Forms and Rigidity

  1. Pingback: Inflatable Kayak Is Safer Than You Think! | Kayaking Venture

  2. Dario Steccanella

    What do you think about Aquaglide?
    I have the Chelan Tandem and I feel very good.
    One of the few and very rare, almost unknown in Italy. Very stable and fast on the lake.

    Reply
  3. John Thompson

    Your site is really great. I think you and I think alike on a lot of these issues. I am torn between going for a drop stitch floor (the Sea Eagle 385) or your current kayak – the Gumotex Seawave. Is the Seawave floor really stiff enough? I recently had a Blueborn kayak with drop stitch floor and it was quite impressive. Except for the fact that the seams separated very easily. I tried two kayaks and both failed within days. They recommend 7 PSI which seems crazy. The second one I inflated to only 3 PSI and it still failed. So now I am in the market for a more reliable and higher quality one. After the Blueborn experience I am a little gunshy on buying another drop stitch, but it seems the Sea Eagle gets very good reviews and I have found very little online about having problems with the seams. It is also lighter and wider (more stable it seems) than the Seawave… So these are reasons I am leaning toward the Sea Eagle. And also I think it is easier/faster to set up than the Seawave. Maybe the Seawave is more durable? It is a different fabric…? Maybe the Seawave is faster through the water? I realize you haven’t personally tested the Sea Eagle, but any thoughts you might have on this issue would be much appreciated! Thanks!!

    Reply
    1. Chris S Post author

      Hi John, ah yes the Bluehorn, those orange ones – they did look a bit dodgy but were the first around.
      As you have found you need to have a top quality DS if it is to take the pressure.
      Too wide can be slow and for me the flat floor puts me off as it may not handle like a kayak but a raft – hence all those skegs.
      When the sea gets uneven they may get a handful.
      I hate tippy but the SW is very stable.
      Seawave fabric is different. Sea Eagle is PVC plastic – Gumotex is Nitrilon (like hypalon) rubber.
      I suppose SE may be faster as it takes less air, but needs more effort to reach pressure.
      As for speed, i would guess they are the same is calm conditions.
      All I know is you can’t go wrong with a SW – a traditional European-style IK.

      Try and test both before dishing out a grand ;-)

      Reply
      1. John Thompson

        Thanks for your quick reply. Good points about the handling. And I like the Nitrilon fabric better, but I am leaning towards the SE. One reason is simplicity. It seems a lot simpler, easier to setup/take down, pack, floor is replaceable. I am from California, but live in Italy, and am going back to CA in a few weeks for a visit, so thinking to pick up the SE when I am there. One thing I’ve always wanted to do is kayak in Venice, which I saw you did. I was there about a month ago with a small dinghy and motor which was a lot of fun also. If I end up getting the SE I will try to post a report here about my impressions. Thanks!

  4. Martin Proctor

    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.

    Reply
    1. Chris S Post author

      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.

      Reply
  5. Hrvoje Pernar

    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

    Reply
    1. Jason Hamrick

      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.

      Reply

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