SHig's Superlight Packin'tube Project

[SHigSpeed]

Okay, first off, no, I haven't yet built my pram. It's still in the works, but on the back burner for now.

So what's new? Backcountry hike-in fishing! I gave in and tried it and found that I really like it! Best off, there are LAKES in the mountains, and I love fishing lakes, and most of all I love fishing from a float tube.

So what's a boy to do? My FishCat IV DX weighs a healthy 14 lbs and when fully deflated and folded is the size of a 2200 in^3 pack! Luckily, I was able to snag a Del Canty Lunker Hunter tube which fits the bill for packing (mostly). 3 lbs, bladderless coated fabric, packs small (about the size of a pair of breathables rolled up) and floats reasonably well. Add the half-pound inflatable backrest from my Outcast and the top halves of my thighs stay dry - though my knees do press up against the inside of the donut somewhat unfomfortably, and it sure does "weebles wobbles - but they don't go down" quite a bit more than my v-boat. The inflator bag works efficiently and weighs another half pound.

Here's the setup next to Scott V's FCIV:



And me sitting in it:



So now what? Heck, let's build one that's better! I used to sew and competed with stuntkites of my own design, I'm marginally skilled with a blade and fabric, and the sheepskin says that I've completed all prerequisites that somehow prove that I can design and build stuff...

Well, I first did my homework. What's out there? Well, the DC was 'it", though they stopped making it 20 years ago. Since then, the donuts have gone out of vogue, and the U/V-boats are what's cool and what works. The Creek Company U-Boat 2000 weighs in at supposedly 6 lbs, though it plunks your ass into the water and is somewhat ungainly. The Creek ODC420 Lightweight is 8 lbs, and you sit high and dry. Add the inflatable seats and it folds down relatively small - maybe a 1800 in^3 daypack? Both of these guys are a pack cloth and vinyl bladder deal. Fianlly, the "Cadillac" of UL tubes right now is the Outcast Trinity. 6 lbs, urethane coated 420 denier fabric, bladderless, packs down into it's own 1600 in^3 pack. With pack and pump I think they claim 8 lbs?

BUT, if 'ol Del could make a 3 lb tube 20 years ago, I can beat that right?

So I first decided what the basic design would be - bladder in skin, or bladderless? At first I thought that bladder in skin would be easier being a bit leery about sewing and sealing a bag. I looked around for lightweight bladder materials, tried to get information on the mylar that they use to make balloons that is heat sealable and super light. Unfortunately, nobody wanted to talk to me. While I was waiting for information about the bladders, I came across heat-sealable urethane coated nylons. Similar stuff to what Outcast uses, but it's lighter (70 and 200 Denier). Think Thermarest skins. Apparently it's not too hard to work with.

At this point it was decided - bladderless, heat sealed fabric. No sewing at all for the main bladder(s). Inflator bag and mouth inflated. No pump required.

Next I had to decide on the shape of the boat. Should I go V-hull like the Fish/Fat Cats? U-boat u-tube style? Caddis pon'tube style? After spending a lot of time Google image searching all possible configurations, I settled on a simple u-tube for ease of manufacture. The main bladder would be two flat pieces of fabric edge welded with a perimeter butt seam. Did the calcs at 5 psi (about double what any normal human can blow by mouth) and the seams would only have to take 30 lbs per inch of seam of force. Apparently no problem. Did I mention that I managed to strike up a conversation with a guy who markets ultralight packrafts? He's been a great resource to bounce ideas off of.

In addition to the simple u-tube "frame", there'd be a nice, fat inflatable seat to add buoyancy and to lift me out of the water. With this seat, I'd have the second bladder to give me some redundancy in case something terrible went wrong with the main bladder. Even the smaller seat bladder will give me more than 90 lbs of buoyancy. Great comfort that...

Now to the drawing board! I wanted a 12" diameter main tube that ended up parallel after inflation. Did some drawings, and printed out a scale drawing and started playing with some Press 'n Seal wrap. This stuff is GREAT for getting in the ball park with goofy stuff like this.

First try:



Ooops! I guess if you model it flat, it won't blow up quite the same. Since the outer radius of the bladder "shrinks in" but the circumference of the flat profile is longer than the final inflated outer circumference, the thing closes up quite a bit!

Did some calcs and back to the drawing board...

Rev 2:



Ooops again! Guess one of my assumptions was wrong... Try, try again...



There! That's more like it...

Now on to a bigger model to see if things "scale" right:

Laying it out:



Blowing it up:



Looks good, but still a bit pinched in.

Also, you'll notice that the nice triangular shape of the "tits" on the drawing don't inflate well. It was at this point that I started playing around with this profile a bit:

Nope:



Nope:



Nope:



YES!!



Now that I had the geometry of the tube ends right. time for a full blown (no pun intended?) 1/2 scale model:




Pretty damn cool! Still a bit pinched in. Couple more degrees maybe? Nip there, tuck here...

To be continued...