Thursday, March 7, 2013

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.


Alejandro Martín said...

Amazing!! Congrats!!
Me and my friends are considering do our attempt. So I'd like to ask you for help.
We'd really appreciate it if you could send us the maps to start our "model".
Also I would like to ask you for suggestion and ideas as our idea is to build a model to raft a little bit (grade 2 o 3 at max).

Ryan Bressler said...

Hi Alejandro,

The blue images should work for small models to start or I can send you the SketchUp files if you email me (use the "email us" link on the right).

I'm still working on a design for a more whitewater capable raft with a protruding stern to prevent wheelies/bandersnatching but will try to finish that design and start building soon as summer is coming!

I think that sourcing the right fabric is going to be the real crux. Comercial rafts use custom runs of fabric with urethane coatings on both sides but I can't find a place to buy this stuff in small quantities. I'll do a post on fabric soon but I've been experimenting with taking heat sealable fabric (coated on one side) and coating the other side myself to get something that will stand up to some abrasion.

jeremy089786 said...

Hi Ryan,

Nice work having a crack at this! To make a really cool raft shape without too much hassle, could you:
1.) create the desired shape of the raft out of foam
2.) lay fabric strips over it until you get the correct configuration and then cut out the correctly shaped nylon
3.) glue or weld the fabric to the foam core
4.) burn out the foam core using something like acetone

I know they use something similar when making carbon fiber bicycles and thought that might help. If it works let me know and I will make one too!!

All the best,


jeremy089786 said...

Sorry to spam you!

You could also, do it the other way...

a.) cover half of a nice pre-existing raft in something like ABS plastic (I use plastimake). The idea is, when you take this off you have a mold of half the boat
c.) add the second half of the plastic mold to the boat with a nice flat seam around the middle or sides (by this point the boat should be completely covered in plastic
d.) take the moulds off and add you Nylon
e.)lightly attach the material to plastic (maybe PVA glue) and then cover seam 2 part urethane glue
f.) finally, stick the 2 parts together

I may be missing something crucial though (it surely can't be that easy!)

Finally, in the first post, you could use a water dissolvable compound (with something like a PVA base) that you could burn out with water rather than the foam.

Hopefully some food for thought.



Ryan Bressler said...

Thanks Jeremy. I think some sort of foam or something like that would work well for a more complicated shape but it would also be a pain (and toxic?) to shape. I need to find some more time to work on this but I think sewing or sealing against a flat surface are going to be hard to beat for speed of production provided I can find a design that works with them.

Alejandro Martín said...

Hey Ryan,
I find Jeremy's idea interesting but simplifying it, I mean:
maybe sculpt with clay or plastimake the shape you want, so you can "easily" get the measures, on a small scale, for the design you want.
After that, the thing is convert measurements to the real scale, cut the fabrics and glue them.
Maybe with one side coated is enough if you use one extra stripe on the coated side (2 cm or so) to glue the parts. I know is not the most elegant but is an easier solution.

Ryan Bressler said...

Hey Alejandro and Jermey, I had another thought. The minimal design I am working on relies on a bit of stretch in the fabric to keep construction simple.

The actual "relaxed" cross section of the tubes will be square (like the paper models) since they are composed of sections of cones but with internal pressure and a bit of stretch they will end up round. For wrapping over a solid you would either want a round design like alpacas or you could build a square cross section/conic curve solid and use something like my designs.

I've been playing around a bit with the math (and a cad program) to see if I can figure out how to use a square/conic design to get a pointy stern and I think I have something...We are in the middle of moving but after that i'm going to try and build my frist real prototype!

Alejandro Martín said...

Hi guys,
a friend brought me 420D double sided coated TPU from China! :). Seems quite strong, my only fear is that is so thick that maybe I won't be able to use an alpacka's inflate system (with a bag).
I'm now thinking about the design, I playing with a cad program, but I'm far from an expert, so I suppose it'll take some time.

Ryan Bressler said...


Do you have a link for the 420D chinese fabric? That for the floor and the 210D fabric mentioned in the comments on my valve article might be perfect.

I did some extended soak tests of one sided fabric I treated with tent sure with mixed results so I am looking for two sided fabric again if I can find it.

Also email me and I'll send you my google sketch up files if you want.

misterphil said...

really interesting thoughts. I've previously made a folding kayak - the guy who designed it has also done some inflatable kayaks - you might find his site useful:
Good luck - I'm following your progress with eager anticipation!

eeSmith said...

Have you tried using the 70D heat sealable nylon from Seattle Fabrics? ( )
They also carry a 30D heat sealable ripstop. The Supai Flatwater Canyon II uses 75D polyester.

Ryan Bressler said...

I tried some other thinner fabrics and found it hard to work with as it was easy to melt or stress the coating to the point where it wouldn't hold air. The stuff I had was some sort of cosmetic second and the seattle fabric stuff might have a better/thicker coating but you'll definitely need to use a bit of care.