Category Archives: Tech

Packboat Fabrics & Construction

TPU Inflatable Kayaks: the Missing Link

Pictures from Zelgear and Marcin S

These days IK are mostly made from PVC, be it the hull or the bladders. Just three main IK brands still using old school synthetic rubber: Gumotex (CZ), Grabner (AT) and NRS (US). PVC gets recycled, is made everywhere and so is cheap off the roll and easy to heat weld. But is it only me who finds something unpleasantly ‘plasticy’ about PVC: the stiffness, the texture, the smell and maybe the eco-stigma.

The only PVC IK I’ve ever owned punctured on the slightest thorn and went on to do that with the next owner. And this was supposedly quality Mirasol PVC from Germany (to be fair, a mate with an older K40 had no puncture problems whatsoever). I can’t imagine any Gumotex or Grabner I’ve had ever doing that. That’s why I persevere with synthetic rubber IKs, even if I believe it’s becoming an expensive dinosaur fabric.

Synthetic rubber coatings like Nitrilon and EDPM are derived from the original DuPont hypalon. Boats must be entirely hand glued which adds to costs. But, in the same way nothing man-made has yet managed to beat the properties of leather for crashing fast motorbikes, compared to PVC, synthetic rubber remains more durable and more resistant to UV, lighter, more supple, easier to glue and easier fold compactly. After 15 years there was no noticeable deterioration in my Sunny, (below) other than a decade and a half of paddling wear and tear. A synthetic rubber IK will easily outlive a similar PVC IK.

Packrafts, meanwhile, are mostly made from TPU (as well as PVC), a different sort of polymer coating which has many of the benefits of synthetic rubber: odour-free, smooth texture, light, UV resistant, supple (crease-free), not environmentally toxic. But, like PVC, it too can be heat welded. Since Alpacka got the ball rolling, there are now loads of brands banging out TPU packrafts left, right and centre. In this time the fabric and seam technology have proved themselves to be as durable as PVC or rubber, and capable of running higher pressures. As someone on the internet observed: ‘Thermoplastic Polyurethane (TPU) is the link between rubber and plastic’.
For inflatables TPU is clearly superior to PVC in all ways except price. “It has properties between the characteristics of plastic and rubber. So, it is flexible without plasticizers, and its flexibility does not affect the design or its strength and durability.” Link

In a way, my 3-metre MRS Nomad packayak (above) was as much a TPU kayak as a packraft. With just 2psi or so, it was able to hold its shape (or my centralised weight), but now costs nearly €1400 in the decked version.

Zelgear TPU IK

While researching the Zelgear Spark 450 preview I found a 2018 ZelGear catalog. It states their now discontinued 5.2m PVC Igla IK can be requested in TPU (or the similar and much stronger Vectran which Alpacka use for their top-of-the-range packrafts). There’s more here. The weight of this long boat: is said to be just 15kg. The cost? $2000 I was told.

You may wonder if relatively thin and flexible packraft TPU could support a 5-m IK? TPU coating is also said to be more elastic than PVC, but it can’t be any more elastic than rubber. And anyway, a stretch-free scrim (woven core) takes care of that; the coating is primarily for impermeability.

An IK needs to be a lot more rigid than a relatively short and squidgy packraft. A lot of that is down to the fabric as well as the psi. That’s one good thing about inflated PVC: it’s stiff. You’d think a TPY IK would require high pressures to support a long boat which would then require bombproof seams. But add a drop-stitch floor (left) in TPU to take the load and the tubed sides would be under less pressure, so to speak. This Zelgear blog post from 2018 mentions some “some technological issues are being resolved“. I’m told Zelgear are on it.
Pictures below by Marcin S from a boat show in 2018.

With all these Asian-made TPU packrafts knocking about, some using locally sourced fabric whose quality – in my experience – is as good as the Alpacka stuff, the cost of TPU fabric may drop to a level matching the few ‘hypalon’ IKs still available.

A few years ago I predicted that full drop-stitch IKs would become the new thing. This has happened and has driven IK design and sales a long way forward . But, PVC aside, I’m still not convinced by the boxy profiles and packed bulk of FD-S IKs. Until FD-S forms can evolve (as the Itiwit X500 has shown), I think drop-stitch floors (D-SF) are certainly the way to go, if an IK is to stay undecked, unlike the X500.

There will always be a demand for cheap vinyl or PVC IKs but I predict the next big thing in high-end IKs will be TPU, including removable D-S floors in TPU. TPU is now well proven with packrafts and blends the heat-welding benefits of PVC with nearly all the better attributes of ‘hypalons’.

Tested: Tyre pump adaptor for inflatable kayaks

You may have seen these bayonet/car tyre adapters on eBay in recent months (left). The bayonet end clamps into your IK’s raft valve (won’t work on Boston valves). The other end is a regular Schrader valve like on your car/bike wheel. Attach that to your 12-volt Halfords tyre compressor and you can inflate your IK from your car battery. No more of that effortful, back-breaking pumping!

Me, I’ve never seen the value of electric pumps for IKs. You can only use them near a power source and how hard is inflating an IK with a good barrel pump anyway? It seems some think it is

The difference between tyres and IKs:
• a car tyre is a low volume running high pressure (~30 litres @ ~30psi)
• an IK has high volume but runs low pressure (3 chambers of 50–160 litres @ ~3psi). Drop-stitch has less volume but runs much more.

That’s up to five times more volume in an IK but at a tenth of the pressure. I would guess the swept volume of my better-than-average car pump (left) is 3–5cc. My Bravo RED 4 barrel pump is 2 x 2000cc (it pumps on the up and the down stroke).
Even if my 12-volt compressor whizzes along at 1001rpm, it will still take a long, long time to fill a 160-litre IK floor. But for a fiver, I thought I’d prove myself right.

The easiest way was to pump my Seawave’s floor to the point the PRV purged at about 3psi. The actual psi is immaterial but it’s consistant.

No surprise: it took less than a minute to pump up the 160-litre floor with the barrel. With my car tyre pump it took over 7 minutes.
And if you want say 4psi in the sides, or a 10psi drop-stitch boat, the duration of the tyre pump (or effort with the barrel pump) rises exponentially. It will take forever with the car pump adapter and I think the tyre pump would auto shut-off or burn-out before it reached anywhere near 10psi.

I read about portable USB rechargable electric pumps like the Pumteck or Sunta (left) on amazon from just £15. These are great for pool toys, air beds and other low-pressure items which just need a shape, not rigidity.
The Pumteck claims an obscure pressure rating of 4.5 kPa which sounds impressive but translates to just 0.65 psi or 0.045 bar. That is slackraft presure; there is no worthwhile IK that runs such a low psi, so all it will do is save you the easy initial pumping. For a typical 3-psi IK you’ll still need some sort of manual pump to top off and on a D-S IK, forget it. If your back can’t handle a barrel pump, get a Bravo foot pump.

Drying your inflatable kayak

This video shows how to properly clean and dry a 3.8-metre Itiwit 2-3 seater, one of the most popular IKs of 2020 – or at least one which remained available when stock of so many others ran out.
It also shows the inner bladders and how they fit: something that’s rarely described on vendors’ websites.

Like so much Decathlon gear, their shell & bladder (S&B) IKs are a bargain. This orange Itiwit costs just £300, but the cleaning and drying process takes a while and realistically, is something best done over a couple of hours back home, if you have the space.
Quite rightly, after a sea paddle the owner was concerned about sand and other grit getting into the nooks and crannies around the floor, and even inside the sleeves which house the relatively fragile bladder ‘inner tubes’.

As it happens, the sidetube and removable floor sleeves plus their respective bladders (above) were merely wet inside. As on all IKs, the grit mostly settled down in the gutters inside where the floor joins the sides. This is why, with full drop-stitch (FD-S), drop-stitch floor (D-SF), or S&B IKs like this Itiwit, a removable floor makes proper rinsing, cleaning and drying so much easier.

Tubeless IKs like vinyl ultra-cheapies, most PVC Sea Eagles and IKs made from synthetic rubber, like the old Gumotex Sunny (left), merely need seats unclipped before a freshwater hosing and a wipe down. A river paddle on a warm day won’t even require a rinse: just wipe the boat dry and roll up till next time.

Testing unbranded mini PRVs

See also:

IK valves and PRVs
Fitting 4.8psi PRVs to a Seawave

I fitted over-rated 4.8psi Ceredi PRVs to my Seawave’s side tubes a few years back. I like the idea of not having to worry about the boat getting hot in the sun (and exploding), just as much as gaining some extra rigidity by fitting PRVs rated a little higher than recommended side tube pressures.

I bought a pair of unbranded Chinese PRVs off eBay (<£9 posted – about 30% of Ceredi/Leafield prices) to consider for my next IK. Some sold on eBay don’t even mention the purge pressure! These ones did: 4psi or 0.27bar, just a bit over the 0.25 of the boat I have in mind.

Out of the bag, the quality of the molding looked no worse than a Ceredi. I was a bit surprised they’re smaller than usual PRVs, (the listing gave these dimensions), but what does it matter as long as they work.
I worked out a way of testing them by removing the backing ‘nut’ and screwing the ‘male’ PRV housing onto a rubber motorcycle throttle grip. The other end of the grip I jubileed to the barrel pump nozzle.
Pumping the pump, the pressure built up and PRV ‘burped’ suddenly at an indicated 0.4 bar. But as my manometer needle zeros at 0.1 bar and not zero, we can probably subtract that 0.1 and call that 0.3 bar which is 4.34psi. Close enough to 4psi. Now I can fit these PRVs with confidence.

The hole which must be cut in the boat to fit this valve is 24mm ø, but to get the back nut inside the hull you’ll need to go in via the larger inflation valve aperture. So cut the PRV hole close to the inflation valve. Once loosely screwed together, the knurled outer housing can be tightened with some wrench or another.

Make Your Own Hypalon D-rings

See also:
Inflatable kayak glues and repairs
Repairing a Gumotex Seawave
MYO alternative to D-rings

You can get Chinese PVC D-rings dirt cheap on eBay but genuine hypalon D-rings (not PVC claiming to work on hypalon) cost a lot for what they are. Once you factor in the price of two-part glue, it adds up, especially if you have a few to fit.
It’s fairly easy to make your own D-rings for your IK to attach gear, thigh straps, footrest mounts and so on.

You can buy metal D-rings by the sack-load online, as well as round PVC or Hypalon patches. (‘Hypalon’ is pretty much the same synthetic rubber as Gumotex Nitrilon and Grabner Nordel). Or buy an off-cut (above right) for much less and cut your own. A D-ring doesn’t have to be round but it’s better if corners are rounded. You will notice how unusually hard it is to cut this stuff with scissors or a blade. The fibre core is tough: good for zero-elasticity in an IK.

Pictures below show how to make your own D-rings.
Go to this page for how to apply any patch, step-by-step.


Sticking to the Rules
I needed to fit some tube-top D-rings to properly support a second backrest in my Sunny 2020. I found a stray, opened tin of Bostik 2402 two-part in my kit bag, but with an expiry date of 2009. Back then I only owned this original Sunny and looking it up, 2402 turned out to be for rubber boats. Perhaps I bought it more recently but didn’t notice the expiry date. In the tin the glue was still liquid and unseparated, but the little bottle of Bostik D-10 hardener had long since evaporated. Digging around, I also found an opened bottle of PolyMarine hardener. Comparing chemicals showed they both contained Diphenylmethanediisocyanate, one of the few words that’s too long for a Scrabble board.
I mixed the wrong-brand hardener with the 11-year old glue 25: 1 and the bond looked as good as anything.

How to repair a ‘hypalon’ kayak

See also:
Inflatable kayak glues and repairs
Make Your Own D-rings
MYO alternative to D-rings

lm-pumper

Hypalon is a cool-sounding word and although not made anymore, has become a generic term for the similarly durable synthetic rubber-coated fabrics still in production, like Nordel and Nitrilon. Once upon a time all rafts and were made of hypalon, then less expensive Asian PVC came on the scene. More about IK fabrics.

sw-holes

The other day, while lashing the Seawave to a chopped-down trolley, the bag sagged under its own weight and rubbed on the sharp edge of the hard plastic wheels which wore through the pack and then the boat’s hull (left) ;-((

The trolley had worked fine with my UDB drybag in New Zealand (below left), but that was partly because you can fully inflate a UDB via its one-way oral valve, transforming it  from saggy sack to firm travel sausage.

Ironically, just two days before I damaged my Seawave I’d snagged a BNWT Orlieb RS140 (right) on ebay.
I’d been eyeing up this non-rigid wheeler duffle for a while as a versatile Seawave transporter plus a reliable on-water drybag/buoyancy aid.
With a bag like this, an IK or whatever you got can be transported easily across any wheelable terrain, or carried as a holdall or on its backpack straps if you’re strong enough.


With enough practice applying D-rings, let along bike and moto punctures over the decades, I was confident I could do a bomb-proof repair on my Nitrilon Seawave. In a way, I was even a little chuffed that my 5-year old IK was earning its first battle scars. Plus, in my experience rubber-based IKs like Gumotex, NRS and Grabner glue more reliably than PVC boats. Shiny packraft TPU is even easier: you can just tape it, but packrafts are low-psi boats not normally inflated with mechanical pumps. My adapted Seawave side tubes run 4 or 5 psi.

Things you will need

Patch
The right two-part glue (below left)
Solvent (MEK, Toluene) and rag
Sandpaper or abrasive foam sanding block (note: Toluene eats foam plastic sanding blocks)
Masking tape
Small brush or wipe-stick
Tyre repair roller (right)
Well ventilated space to do a good job

STEP 1 • Match up a patch from your collection, ideally identical fabric. For a small hole extend the patch at least an inch.
STEP 2 • Clean the punctured area and patch surface with solvent and wipe dry. This time all I had was brake cleaner spray, but ordered some toluene for next time.
STEP 3 • Sand down the two surfaces and then clean and wipe again with solvent. Avoid touching these cleaned surfaces with your fingers.
Some colour coming off is a good sign you’ve removed any sheen or patina.
STEP 4 • Position the unglued patch and mask the perimeter with tape to avoid excess residue and to help with positioning. If the patch is not perfectly symmetrical (like above) mark it – but make it bold – I still got it wrong!
STEP 5 • Mix up some two-part Hypalon (Nitrilon; EDPM) glue. It’s rare than one-part glue works as well, but Aquaseal has worked for me, gluing a skeg-patch to a Grabner.
I found some mini brushlettes in my repair kit box – they must have come free with some glue.
STEP 6 • Brush on the glue thinly to the two surfaces. With Polymarine you then to wait 30 mins for it to cure/dry, then apply another coat and wait till touch dry (5-10 mins). Here’s their full guide: http://www.polymarine.com/advice/hypalon-adhesive-how-to-repair-inflatable-boat-tubes
You can see I made typical errors: mixed up too much glue (but better too much than not enough)…
… and applied too much glue on the patch…
… but a just-right thin later on the hull.
STEP 7 • With the deflated boat repair positioned on a firm surface like a hard floor or better still, draped over a wooden stool, carefully lay the glued patch over the damaged area…
… then – STEP 8 • peel off the masking tape and ROLL DOWN HARD moving from one edge to the other and again at 90° and again diagonally with your Baltic pine-handled roller, making sure the edges have stuck down. It won’t hurt to roll again in 20 minutes and again after an hour to make sure the two parts have well and truly bonded till death do them part. And actually, only about 25% of the glue was wasted.
In 12 hours the repair is cured and ought to last the life of the boat. Never do that trolley thing again!

Pumps for inflatable kayaks and packrafts

Inflation valves and PRVs are here
My PoV on electric pump here

ikpumps.jpg

Your inflatable packboat needs a pump to get going and to top-up once on the water. These functions may be best performed by two different pumps. It seems the era of the bellows footpump (left) is over and even low-pressure IKs now come with some sort of barrel pump.
A plastic-bodied barrel or stirrup pump is light but bulky so not something you’d want to tour with. They’re usually used for pumping up high volume/low pressure things like rafts, lots of IKs or kite wings. Some pump air on both up and down strokes to fill your boat more quickly, but automatically or manually switch to downstroke-only to reach higher pressures. They work best on flat, firm ground where you can stand on the stirrup plates and get stuck in. The Bravo 4 RED pump above is still only about £20 and will pump up an IK in 5 minutes.

sw05

I got a Bravo 6 with my Seawave once but found it hard work – who knows why. The cheaper Bravo 4 does claim to be an ‘R.E.D’ (‘reduced effort device’) and I can confirm this isn’t some gimmicky acronym.
Like a Bravo foot pump, the other port on the Bravo’s handle can be used to deflate or suck air from an IK so it rolls up good and flat; you can see creases forming in the hull as you suck it down.

I left my Bravo 4 RED at home one time so bought a Sevylor RB2500G barrel pump (below left) for a tenner off ebay. Same size as the Bravo barrels, it did well for the awkward topping-up of my Semperit’s lilo plugs. It came with push-fit, lilo-plug and bayonet adaptors and sucks as well as pumps. But pumping up my Seawave from flat was exhausting towards the end: I actually got out of breath and had to rest. Morale of this fascinating story: get a Bravo 4 RED and the right adaptor for your boat.

Not all barrels have a built-in pressure gauge which is obviously dead handy in getting the right pressure without needing to faff about using a separate manometer (see below). It’s worth an extra tenner to get a built-in gauge, especially with D-S boats/
Or fit your own manometre: see bottom of the page.

kpump200

The K-Pump Mini (above right) is a handy top-up pump or compact 600-g travel pump. It took 15 minutes to fully inflate up my Seawave; the push-fit nozzle works on any IK with one-way spring valves. You have to press the body of the pump against the valve. Using it a lot one time, I got the feeling it might break something or wear out the seal (which needs regreasing once in a while). I also now use the K-Pump to top up my Nomad S1 packraft which is too big and long to inflate firmly with just its airbag. Fuller review of the K-Pump Mini here. Hard to find in the UK, the long, and slim US-made K-Pump 200 (right) may also be suited to DS applications.

Left, the Bestway Air Hammer is an upside-down barrel pump which comes in three sizes and costs from just £6 on eBay. If you don’t want to paddle with your full-size barrel pump, the smallest Air Hammer could work as a compact top-up pump, like the K-Pump below but a tenth of the price. The one of the left came with my RIBfin and is rated at 4psi

High-pressure pumps

More and more IKs now feature super-rigid, high-pressure drop-stitch hulls – either just floors or the entire hull which runs 2–5 times higher pressures than regular IKs. Your old Bravo footpump will blow its brains out trying to reach the typical 7-10psi.
Barrel pumps with slim and long bodies (as opposed to some of the shorter, stockier examples above) will put out less volume (D/S IKs have less volume anyway) but can attain higher pressures. You don’t need a super high-pressure iSup board pump. Some of these pumps may be double action, but at a certain psi will become single action to help gain higher pressures. I beleive the Bravo Alu 4 R.E.D (0.8 bar) works like that.
Whatever you get for your D/S IK, make sure it is rated to comfortably exceed your D/S boat’s pressure rating by say 50%.

Left: Bravo Alu RED <£20 • Middle: Bravo 110 >£40 • Right Itiwit (Decathlon) £20
bravohose

Suited to low-pressure (non-dropstitch) IKs the once-popular Bravo bellows foot pump looks a bit crap, but lasted well, was fairly travel-compact and was easy to use without doing your back in. Occasionally the yellow tube split near either end if packed too tightly, so needed taping up (left) or cutting down and got shorter and shorter over the years. as mention, the bellows era seems to have passed.

Left: Kokopelli Nano pump: fold-out feet, screw-off handles, switch for one-way pumping as pressures increase, manometer in the handle.
Right: twice the volume Bravo Alu 4 with auto one-way switching (so it says). Bayonet valve fitting on the Nano keeps the valve open and so is for static manometer readings. With the push-fit adapter on the blue pump DIY manometer only reads as you pump and open the valve.
Both are better than soggy footpumps.


After many years a crease in the back of the bellows wore through, though that’s also easily fixed with duct tape. It’s a shame the Bravo pump is a tight squeeze into the Gumotex drybag’s outer pocket. If you use a Bravo footpump very frequently it just plain wears out, so if you’re using the car to get to the water get a barrel pump.

Bayonet nozzles
Who would have thought there’s something to be said about bayonet nozzles? Well there is. There are two types: plain (below right, Gumotex) and crossbar-peg (left, Bravo). Both need spacer washers to fit snugly against your boat’s valve but the small peg inside the green one will press open your valve stem as you connect it. You will notice a similar peg on car tyre inflation hoses and also on a hand manometer (below). It could not get a reading without this peg partially opening the valve.
A nozzle with a peg means that
a: you’re not pushing the valve spring open each time you pump (easier pumping) and
b: if your pump has a manometer you will get a live reading as the pressure climbs which is the point of having a built-in manometer. The Bravo one goes from 3 quid; search: ‘Bravo Adjustable HP Valve adapter”.
The only drawback might be that you need to remove a pegged nozzle carefully with Push-Push (Gumotex) valves. Normally a little air escapes as you do this but if the valve gets locked open air will rush out. Remove slowly.

Pressure gauge (manometer)
Until I got a Grabner which has no PRVs but ran a relatively high, 0.3 bar (4.3 psi), I never bothered with a pressure gauge (manometer, left) and just pumped up by feel.
Since then I got a Gumotex Seawave and fitted PRVs to all chambers. That means I didn’t need a pressure gauge to get the right pressure, I simply kept pumping until each PRV hissed: the boat was then at operating pressure.
With high-pressure D/S IKs you probably do want a pressure gauge as the boat will perform best at the right pressure which may be higher than you’re used to.

Add a manometer dial to a plain barrel pump

I got myself a new Brave 4 Alu R.E.D barrel pump (left) rated to 0.8 bar (typical D/S pressure). It was under £20 without a gauge. The next similarly rated barrel with a gauge worked out at nearly fifty quid. That’s inflation for you.

Then I decided a gauge probably was a good idea in case my next IK doesn’t have full PRVs (quite likely; most don’t). I couldn’t find a way to fit my handheld manometer (as pictured far above) neatly, but on ebay saw manometer dials at various displays and with rear (behind) rather than bottom inlets for a fiver (right)
I chose one which displayed up to 1 bar / 14.5psi. Whatever boat I get next, D/S or otherwise, it won’t be higher than that, and anyway the Alu 4 is only rated to 11.6psi (0.8 bar).
The easiest place to drill the hole is into the hard plastic end of the hose by the handle (below). The brass thread will screw into the plastic hole easily enough, but a dab of glue does no harm. Now, finally I can measure as I pump.

Alternative to D-rings for IKs

IKavity

Michael S from BC came up with a good idea for securing stuff, seats or thigh straps to the floor of your IK without resorting to the faff gluing on D-rings – something that takes application and the right glue to do well.
He suggests the cavity formed between the floor and the sides when you pump an IK up can be used to jam in short tubes attached to tape loops. Example left is a Sea Eagle Full D-S, but I know Gumotex and other IKs I’ve owned have a similar space along the sides.

tieseawav

Pictured below are some Sherpak Quick Loop tie-downs which go from $15 a pair on amazon US. You can buy Thule ones for six times as much or search eBay for <Kayak Hood Trunk Tie Down Loops> sent from China for 7 quid. The idea is you shut them under your car’s bonnet, tailgate or doors (right) to help lash on stuff including boats.

But they could also be lodged in an IK’s floor/side cavities as you pump up, and of course can be positioned anywhere and slid forward or back. It’s possible the 1-inch diameter tubes shown may be too small and pull out, so make your own using larger conduit from a hardware store, or just a shore-side stick and washed-up rope.
Below: I made my own. To be tested. Neato mosquito as my Kiwi mate used to say.

Inflatable Kakak autopsy

Semperit main page

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.

sempauto - 16

hypalon fabric

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 left is clear: what’s outside and what’s inside an IK hull is not the same stuff. There’s no need to waste UV-resistant hypalon coating (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 wetsuits 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.

sempauto - 17

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.
So 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 - 14

Twin 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.

sempauto - 2

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.
When 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. 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, like thin cotton cloth. 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 - 4
sempauto - 10

Where mine failed
Inspecting 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.
I could have fixed that leak but, as mentioned, another would probably pop up whack-a-mole style somewhere else, quite possible while at sea in either my- or a new owner’s hands.

semp- - 6

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

Although I’m pretty sure they’d have lasted, I could easily pull off the Bostik patches by hand. Pulling off the big round Polymarine’d 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 (above 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, new or old.

ami-poly

I 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 correctly. But as you can see, this stuff sticks. If you absolutely, positively want it to stay stuck, use two-part adhesives.
I still don’t know if the second part curing agent merely speeds up the drying process, or is 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-glue
semp-bowwow
sempauto - 15

Other stuff
The distinctive marine plywood bow has lasted fine – no warping at all and the rivets are 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.