Designing a Packraft “Trailer”

Sometimes people ask me if there’s a way to increase the payload of the single-person DIY Packrafts. Unfortunately there isn’t, because payload is a function of tube volume, and that can’t easily be changed.

One workaround to increase your carrying capacity would be to build a “trailer” for your packraft. This could be a very simple inflatable cylinder (with an airtight zipper in the side) that you can partially fill with whatever extra stuff you need to carry. You could then attach it to the stern of the packraft with a rope (or to your body with a quick release mechanism) and drag it along behind you. If you make it streamlined and add a bungy to the rope, it won’t affect your paddling too much.

Often it’s hunters who want to carry more stuff, so as an example, let’s say you’re a hunter who wants to carry 150 pounds of meat. (I’m sorry if this example offends you – I’m not a hunter myself, but where I live hunting is how many people feed their families.)

I’d calculate the trailer’s dimensions in the following way (metric is easier):

  • 150 lbs = 68 kg (1 kg = 2.2 pounds)
  • Meat is approximately the same density as water, so let’s say 68 kg has a volume of 68 litres.
  • 1 litre = 1/1000 of a cubic meter, or 0.001 cubic meters
  • We want the trailer to float fairly high in the water, and we want to be able to pack things inside with room to spare, so let’s make the volume 400% of what is required to make it neutrally buoyant (I just picked this number out of the air)
  • 4 x 68 litres = 272 litres = 0.272 cubic meters
  • Let’s round that to 0.3 cubic meters
  • Now let’s make a cylinder that has a volume of 0.3 cubic meters. There’s an infinite number of ways to do this (long and skinny to short and fat), so we have to constrain it by either choosing a length, diameter, circumference, or radius.
  • Let’s choose a circumference of 1.4 m because that’s a bit less than the width of the fabric rolls and it will make a cylinder 45 cm in diameter (17.7″), which seems reasonable. (Diameter is 1.4 m divided by pi). That means the radius will be 22.5 cm, or 0.225 m.
  • Now we need to figure out the length of the cylinder to figure out how much fabric to buy. You can use the formula below or an online cylinder calculator.
  • The length of a cylinder (L) = the volume (V) divided by π (3.14) divided by the radius (r) squared.
    L=V/(π·r2)
  • Length = 0.3 cubic meters (volume) divided by pi (3.14) divided by 0.225 m divided by 0.225 m (radius squared)
    Length = 1.89 meters, or 6.2 feet, which seems like a reasonable size.
  • Instead of making an actual cylinder with circular end caps, we could just pinch the ends flat to make it easier to construct. This would decrease the volume somewhat, but that’s probably not a big deal. If you wanted to do that, you could buy 2 m of fabric to make the cylinder with pinched ends, and maybe an extra meter for reinforcing the seams and making attachment points, etc (though you’ll have scraps left over from the V3 kit you could use for that kind of thing).

The weight of the cylinder described above, including an airtight zipper and a Boston valve, would be about 900 grams, or 2 pounds.

You can substitute your own numbers into the calculation above to design a trailer to suit your needs. Note that for lighter gear (like sleeping bags, etc.), it might be more practical to design a trailer based on the volume of the gear, not the weight. The figure out the volume, pile it all into a box and measure the length, width, and depth, and then multiply the three numbers together (measure in meters to get the volume in cubic meters). Don’t forget to add some extra volume as a fudge factor so packing the trailer doesn’t become a game of Tetris.

 

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