Skin on frame scantlings

Here is a skin of frame I saved from the Internet a long time ago:



Pretty cool really, AlanX

 

Increasing the skin thickness improves impermeability and skin stiffness. Paper because it was cheaper then canvas and made a more rigid skin (think papier-mâché).

As to your original question, if you don't want to go trough the whole engineering you have to use an empiric scantling rule. I am not aware of any dealing specifically with SOF, so you have to use one designed for other traditional methods (carvel, batten seam, etc.) and adapt from there. This will result in a pretty heavy boat by multihull standards, but it can not be helped.
You will be building a fully longitudinal and probably also fully transverse framed boat, where the skin has no structural contribution whatsoever. It's only function is to keep the water out, and as a fairing. The closer the stringers the fairer the boat, needing a less stiff and strong waterproofing layer, until you arive at wood-canvas canoe construction where the stringer spacing is zero.

 

I think that cow hides soaked in water for long periods of time are likely to produce mold. Also, the tanning process uses toxic chemicals. If your daughter is very sensitive, you should first have a list of everything that affects her. Then make a list of materials you would be able to use. Whatever those materials are, they will be the constraints. The type of construction, whether SOF or welded aluminum will be determined by them.

 

To get to a successful craft you can have confidence in, an engineering degree would be quicker and easier. What you are trying to do would be and extrapolation. There is no existing craft that big or bigger so you need to go to fundamental application of science, which is engineering. You can do some calculations for feasibility, but I think that when you get to deflection failure modes it will take more than you can figure. Without deflection you can't figure fatigue or wear/chafe.
Ancient Polynesian craft have some of the structure worked out but they may be lacking in safety, comfort, and durability and skill requirements.

Environmentally friendly boatbuilding materials are an extremely active development field. If you bring skill and resources, the developers are likely to provide the engineering guidance.