Drainpipe kayak

Quite a while back I came across an article about a man building a “kayak” out of PVC pipe, which looked fun. So I thought I had to give it a go, now being in the UK and not the US getting hold of random bits of PVC pipe in large sizes is a lot trickier, and also I wanted to use it in the Forth ( because why wouldn’t I ). Research was therefore called for, and whilst there are quite a few videos and articles about building such craft they’re all rather scant on detail.

From what I could find on line and collected on Pinterest, I decided I need to use 160mm pipe. Additional reasons for this were that it’s a readily available size in the UK and has a wide selection of fittings. Less than 160mm the price drops but so does the buoyancy, of course in the UK the only 160mm pipe I could find was drainage pipe which isn’t the cheapest or the lightest. On the other hand as I was planning to use the resultant craft in the forth a bit of robustness probably isn’t a bad thing.

To get out of the catch 22 situation of not being able to calculate the buoyancy needed without knowing the pipes used, and not being able to decide on design without knowing the desired buoyancy, I decided to assume I was going to copy the design from the initial article. That decided I was able to build a rough list of required components.

  • 160mm Drainage Pipe End Cap x 4
  • 160mm 45° Triple Socket Junction 110mm Branch x 2
  • 160mm 3m Underground Drain Pipe Single Socket x 2
  • 160mm Underground Drainage 87° Bend (Swept) x 4
  • 160mm Underground Drainage 45° Junction T/S x 2
  • 110mm Underground Drain x 3m S/S x 1
  • 110mm 87° Single Socket Bend (Swept) x 2

With that list in mind as a rough guide I was able to look up approximate component weights on the Polypipe website. Which gave me a working value of 36kg, and a desired cargo weight of 100kg – so I needed enough buoyancy to cope with 136kg minimum. Fortunately there are several sites that go through how to calculate the buoyancy of pipes. Unfortunately they don’t all agree on the results. Whilst in theory it’s just a case of calculate the volume of water displace and multiply by the density of water it’s nice to double check these things.

To make life simpler and to help build in a bit of leeway I decided to only consider the buoyancy of the diameter of pipe in the water and ignore that the fittings were larger and so forth. On that basis it looked as though I’d need about 7m of pipe, so two 3.5m hulls should do the job. It wouldn’t sit that far out of the water but it should float. 160mm pipe having a buoyancy of about 20kg/m.

Next step a 3d model to get a better idea of how to put it together and to double check that I was actually getting the right parts.

Kayak 3D model

As I knew I’d have spare 110mm pipe left over, adding an optional out rigger seemed a cunning idea. The model doesn’t show any of the non-pipe cross struts only an approximate position of the seat and several of the pipe sections aren’t quite to scale. Interestingly at this point I discovered that I had accidentally designed something of about the same dimensions as a real Sea kayak.

“A sea kayak usually ranges anywhere from 10–18 feet (3.0–5.5 meters) for solo craft, and up to 26 feet (7.9 meters) for tandem craft. Width may be as little as 21 in (53 cm), and may be up to 36 in (91 cm). “

The next step was to order a lot of pipe, adhesives, expanding foam sealant , balloons and cable ties.

A pile of pipes

Being aware of the fate of the Titanic and that a collection of connected drainage pipe is a single space, I planned to create “bulkheads” using expanding foam. Whilst the foam might not be 100% water tight it would at least mean that if a joint or pipe sprung a leak it wouldn’t be able to quickly fill the entire structure. My method to create these bulk heads was simple:
Block the pipe with a balloon
Add a disc of card
Cover card with expanding foam
Push another bit of card onto the foam
Hold in place with another balloon

Balloons to bulkheads

Once I had all the pipes cut to size and “bulkheads” created, I could start assembling the 3 parts of my vessel. It was at this point I realised I could have saved both weight and money by using 110mm pipe for both cross joints as it would have been more than strong enough and would possibly have looked better. To hold the various joints together I used a mixture of epoxy resin and grab adhesive :  epoxy for strength, grab adhesive to help prevent leaks. I also spotted a bit of rigid galvanised metal mesh in the garden and decided it would make an excellent cargo deck as well as strengthening the rear of the craft.

Fitting the cargo deck

It was at this point I discovered that I’d made my prow too long and it would no longer fit flat in the van, but it would still at least fit in the van. Once I’d worked out how to stack the parts and strapped it to an old sack trolley it was even really portable apart from getting it through doors.

Assembling the sections once I was at the coast was quite tricky and revealed a bad design decision on my part. I should have had the central section as a single block consisting of all the cross joints which the separate prow and stern parts would then plug into. Having the central parts of the hulls needing to be plugged into both prow and stern at the same time was not a good design choice. However we managed to get assemble it and get it into the water, this was a bit of an unfair trial as the winds were over 20 mph gusting over 30 mph – so the sea was a bit choppy. Despite such unreasonable testing conditions it did float and at about the level calculated ( 75% submerged )

The observant amongst you will notice the slightly worrying sag in the middle of the hulls. This was due to that design flaw mentioned earlier and the central connection not being as firm as might be desired – fortunately it held together until it was out of the water. Application of a large log before the next test as well as the addition of another pair of lateral supports resolved the problem before the next voyage. The balance of it is a bit off so the stern tends to sit a bit lower in the water ( not helped by a slight leak around the end caps, which needs to be fixed by some sealant ). It also needs an extra bar to rest you feet on when paddling, but despite all that it actually handles reasonably well though not the smallest turning circle. Lengthier voyages will probably wait for longer days and warmer weather, or at least until I have a buoyancy jacket as well as a flag. In the meantime a couple of bits of split pipe, spare planks and large castors make a reasonable trolley for it.

I was pleased at how well it handles and how stable it is, no need for the out-rigger and it really was a very simple build. Rather gratifyingly it turns out that apart from being about the same dimensions as a sea kayak it’s also about the same weight – just with probably less than a third of the buoyancy. If I were to make another drainpipe kayak I’d do a few things differently :

  • Use 160mm pipe through out the 160/110 t-joints ( used between stern and middle sections )are heavier than the 160 only versions and I didn’t really need the 110 pipe for anything else.
  • The central section would be a single block with everything glues in place, as the prow and stern hulls could be inserted separately and then the non-pipe cross supports added. Which would be a lot easier.
  • The central section would also be shorter to make the center of mass when sat on it more central.
  • If it needs to be possible to take apart I’d apply longer lateral supports from the outset.
  • A lot more care needs to be taken sealing the end caps.

On the whole though a fun build and no obvious significant problems so far – though if I attach an electric troll motor later in the year that may reveal more problems.

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