I'm starting this topic to both log my experiments and to petition contributions and ideas from others.

The basic premise, to build a strong, inexpensive inflatable boat. If you look at the Zodiacs, you'll see they run into the thousands of dollars. Even the lower cost (smaller) inflatables at WestMarine sell for over a thousand and go up. My basic plan is to use some heavy canvas, and impregnate it with something to make it air tight. The canvas should add plenty of strength allowing it to hold a little bit of pressure and stay rigid. Everyone I've spoken to about this project has responded either that I'm crazy (possible), this is impossible (I'll never believe it), or both.

Lets start with the base material. I am really thinking of using a heavy canvas, like the kind available at the hardware store as painter's drop cloth. Purchased like that, I can get large pieces of canvas that are very sturdy and inexpensive. I've used this material to make bags, covers, and cushoins for my sail boat. So, I think its a good starting point.

Next up, a low cost way to make the canvas air tight and durable. An early thought was to use latex because its cheap and readily available. The problem is latex will not last very long exposed to the sun and elements. I have experimented with "PlastiDip" stuff used to plastic coat tools, and "liquid electrical tape" but, these two substances are hard to the saturate fabric with. Probably just as well, as these would be pretty expensive in the quantity required.

I really think that a liquid vinyl or some kind of vulcanizing compound would be ideal. However, I have no idea where to find such things.

Step 1: Trival Math

This is trivial but, still something to consider.
The greater the airpressure in the inflated chambers, the stiffer the vessel will be. Determining the air pressure to use will come from a combination of material strength and size of the chamber. This will be modified downward with a consideration of material strength decline with age, consideration of forces from use (people sitting on it, waves, motor, etc) and a safety fudge factor.

Generally in the realm of inflatable boats, 5psi is considered high pressure.

So, for a 24 inch diameter tube, we get a circumference rounded up to 76 inches. So, 1 psi would equate to 76 pounds of force trying to tear it apart (2 would be 152). Since large tubes will be very hard to contain lots of pressure, a compromise could be to stack narrower tubes that could each have a higher pressure.
This asks for some pro/con cosideration.

Stacked Tubes:

  PRO                        |   CON• Higher pressuere (stiffer) | • Greater weight• More air chambers safer if | • More complex construction  there is a puncture        | • More material/expense

The single tube is basically, the opposite of the above. If material costs are kept low, the cost part may not be a big deal.

A quick and dirty method of measuring the strength of the material is to cut a one inch wide strip to a length equal to the desired circumference, plus a little bit for hangers. Hang it up and start suspending weights from it until it breaks. Do it a few times and pick the low value for the weight that causes it to break.

Step 2: Basic Design

My thoughts for the initial design are (loosely) based on using the RB-15 while in the Army.
So, the floor, will be made of several sections of wood planks, that can be made rigid with sliding bars (ok, this part is the only resemblance to the RB15). The transom, will be supported/anchored to the floor using 2 triangular pieces of wood.
Instead of a wrap-around inflatable tube, my first attempt will have two long tubes tapered at each end, with two short conecting tubes for seats. The boards will sit right down the middle.

Step 3: A Word About Materials

One of the objectives of this project is to create something that can be built with readily available (and low cost) materials. This is one of the reasons that some choices may appear to be kindof odd.
I've received some really great comments and suggestions (some of which I am going to try but, have to wait for materials to arrive).

The idea behind the canvas is more about providing strength than acting as a substrate for the airtight material.

Some more experimentation over the weekend has shown that RTV silicon is almost perfect. Its strong, and seals well. However, It needs to be thinned to saturate the fabric. A bonus for a boat, is the silicon could survive an accidental fuel spill. Negatives are, I have no idea where to buy it buy the galllon, and its expensive. Another problem, is fixing leaks would be near impossible as once cured, new silicon doesn't bond well.

I've been busy with other things lately and then sick for about a week so, I haven't been able to do to much.
I've just discovered this instructable:
https://www.instructables.com/id/EGF6OU1EIGEP4HCZUE/
I think they have found the perfect substance to coat the fabric with, rubberized roof paint. I'm going to try to pick some up on the way home and give it a try. On the surface, it looks like it could be perfect. Strong, flexible, resistant to the sun. I'm really motivated now
:-)