Tag Archives: self bailing inflatable kayak

Inflatable Kayaks: Hull Forms and Rigidity

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Gumotex Sunny/410C ~ self bailing and hull rigidity

Sunny main page

Gumotex Sunny

Paddling the <a class="wp-gallery mceItem" style="color: #000000;" title="Kayaking and packrafting in southern France Haute Allier in France in a Sunny required frequent visits to the bank. Not to get money out of the ATM but to drain the swamped boat.
And on Shark Bay in Western Australia one crossing of a very windy bay required frequent pumping out. The water came in over the sides as the boat flexed over the swell (below).

On a river, flipping the boat is the quickest way of doing this, but can make a mess of the packing. Tipping it up on-end works less well because of a triangular patch at each end. It’s a handle of sorts but also keeps some water in. I cut a small hole in the back so I could drag it up a steep bank to drain itself (see little fountain, left).

Making the Sunny a bailer?
I’ve considered drilling bailing holes (easily and reliably reversible with duct tape I found on the old Safari, left) but am pretty sure the floor of the Sunny is below the water line with me in it. Loads at either end help, as would the hull sticks or plank described below, along with a thicker seat pad. It could be something worth trying to not end up sitting in water. Lighter solo paddlers in a Sunny may get away with cutting bailing holes without doing all these bodges because the boat won’t sink so low in the water (see graphic below).


In the end, I decided I didn’t really need self-bailing for the sort of tame touring I did in the Sunny. If I was more into white water I’d get something like a self-bailing Safari. I never used my Java IK long enough to appreciate the benefits of its self-bailing feature and some later IKs (and packrafts) I’ve owned had zip on decks which are an alternative way to avoid water in the boat, although I used them even less. Away from flat water, a bit of splash gets in the boat most of the time but it takes a while before it’s sloshing about. The stiff and high-sided Incept was much less prone to swamping and the similar-to-Sunny Grabner seems the same. Longitudinal hull flex was the problem.

Making the Sunny/Solar 410C hull more rigid 

The Sunny is 3.85-m long but runs 0.2 bar (2.9psi). The replacement Solar 410C is even longer at 4.1metres but runs the same pressure. Since I originally wrote this I’ve realised that higher pressure IKs are indeed more rigid. I have read of people running a Solar 410 at 0.3 bar. I think this would take more than a regular Bravo foot pump could manage (or at least, my old example) and the push-fit valves (as opposed to more secure bayonet fitting) may pop the hose off. A way around that would be to use a push-fit hand pump like a K-Pump. I never tried it, but I have to say my experience with Gumotex IKs suggests that they feel sufficiently well built to take 50% more pressure. But to blow a seam in a tubeless IK like a Solar or a Sunny would be hard to repair.

Can you see any difference whatsoever in the pictures on the right? It’s supposed to show the Sunny with no load (top) – quite bent; the boat equalised with a heavy load – low and relatively level (middle); and at the bottom with a light load with some straight branches jammed in the sides (see below) – less bent than it would be. No, you probably can’t, they all look the same.

While paddling the Tarn Gorge in 2007 I tried ways to help stop the hull on my 13-foot Sunny sagging with my weight. The handy gap where the side tube meets the floor tube was just right for jamming a stick in.
Up to that point I had found that leaning far back on the seat and taking the weight off your butt and onto your heels and shoulders was a way to unsag the middle of the Sunny and could mean the difference between scraping through a shallow shingle rapid without punting or walking the boat through.
I was going to buy a pair of broom handles in France one time but forgot, so later by the Tarn river I found a couple of branches that were pretty straight over 5 feet and jammed them in between the floor and the side tubes more or less in the middle of the boat. My unscientific impression was that the Sunny was indeed more rigid, responsive and faster, leveling the boat out in the water. Later I found some light metal tubes (right) but never did anything about it. The fact that the river sticks popped out through some rapids shows how much the Sunny flexed in rough water.
A year or two later I was reminded what a good idea a pair of stiffening side floor poles could be on a Sunny and how easily that can be achieved. Of course, the poles will be more junk to carry around, but as I was considering using the old Sunny more in the sea it felt worth doing. I cannibalised an old TNP paddle and with a saw and a hot knife trimmed off the blades. Now I had two 1.55-metre long sticks although 1.75 would have been better as this is the distance where the starts to taper in to each end.

A series of attachment points needed to be glued to the 3-inch wide flat strip where floor meets the side tubes. I got as far as this (right) but then someone told about the Incept K40 which seemed a more seaworthy boat so I gave the Sunny away.
But – if I owned a Solar 410C and paddled solo and out at sea, it’s something I’d consider trying. It’s easy to do and harmless to try. There’s more on hull flexing here.