Designing the Minimal Packraft

Inflating a design model of a simple packraft. 

One of my main requirements of the packraft design I am working on is that it be quite easy to construct. Alpaka rafts use 10 sections of fabric rolled into cylinders and 21 seams in their tube design. Any seam adds work to a DIY project and this previous attempt to DIY this sort of design has shown that these many curved seams can be difficult to seal. Flat pieces of fabric with seams that are two dimensional enough to be sealed against a table top (as in the Supai design) are easier to deal with.

Knowing a bit of mathematics, I think it is possible to find a design that uses a small number of two dimensional seams to produce a nice three dimensional shape when inflated.

The problem with this sort of design is that curves change and often fold or krinkle as a shape is inflated. Klymit's yet-to-be-released Light Water Dingy, seen in the video above, uses scallops on the the bow and stern to mitigate this. These scallops aren't a great design in terms of how the boat moves thorough the water but allow for very simple construction. My approach is to construct the bow and stern as separate flat pieces that naturally pull into the correct 3d shape when inflated.

Inflated curves behave similarly to these paper models. 

I've calculated that the degree to which circles (and some other conic sections) tighten through inflation can be approximated by assuming the surface of the inflated tube is a cone. I also discovered a wonderful superlight fly fishing tube build in which the author arrived at a similar result by experimenting using models made from Glad Press'n Seal Food Wrap. 

(He also had better luck with his testing of heat sealable nylon which has convinced me to evaluate some of the more expensive non-military surplus stuff from Seattle Fabric...more on that in another post but initial results are good.)

Raft model inflated above the pattern used to produce it. Non diabetics will have to find something other than an insulin syring to use as a valve. 

Press'n Seal is a bit of a pain to work with but my first raft model shown in the photos on this page validated my calculations. I was able to come up with a design that uses four pieces of material and 8 seams to produce a nice tapered packraft-like shape with smooth circular bow and stern curves. 

Working design and pattern of the "Minima" packraft.

My sketches are still a bit rough but a larger version of the design I built would make a nice ultralight boat for alpine lakes and other flat water. If anyone wants to build it I can provide you with specifics. 

I would like a boat that is a bit more capable and includes a raised bow and enlarged stern. The raised bow can be achieved by curving the bow seams in the pattern a bit. One might be able to achieve a better stern shape by using elipses, or other conic sections. The stern section is, however, already at the limit of what can be cut out of one 58-60" wide piece of fabric so a longer stern will require the introduction of at least one more seam. I plan to split the stern piece on the center line of the boat which will allow a nice pointed stern and more efficient layout of the pattern on fabric.

This first design would require almost 6.5 yards of 58-60" fabric as the curved pieces do not fit together efficiently   

I'll do a write up on the next iteration of the design as I figure out the details.

My working name for this first, simple design is the "Minima"which is mathematician for the point at which a minimum is achieved and also sounds like a small fish. 

The  more capable design I am working on will be called the "Extrema" which is a term that can mean a point that achieves either a minimum or  a maximum and sounds like something that comes out of a stream.

The model is pretty leaky and I got quite red faced keeping it inflated for photos. 

This post is part of a series you can see all the posts under the Homemade Packraft Tag.