I'm looking at making my first hammock. I can sew - I'm not magician, but I can certainly handle the minimal sewing needed to pull off a decent hammock.
I think I want a double layer hammock for holding a pad. I need it to be strong to support a lot of weight - potentially near 400 if me, my kid, and possibly my dog are all chillin' in there at the same time. Ed Speer uses a 3.5 ounce fabric in his "C" hammock to handle 350lbs.
My question is whether or not "stacking" fabrics produces the same strength as a single heavier fabric. For example, would 2 layers of 1.1 ounce fabric be stronger than 1 layer of 1.9 fabric? I've been messing with fabrics - mostly in clothing - for years but have never had to think about strength in this way. I'm thinking of a 1.9 main fabric with a 0.9 bottom layer. Just an idea - I'm not sold on it myself - but I'm curious about the strength aspect.
I think it would depend on the individual strengths of the different weighted materials. A more specific question (and perhaps more relevant) is, how consistent is the strength of ripstop by weight? That is, does the strength increase linearly with denier (which is what determines the weight)?
Another interesting question, pertaining to your double-layering plans, is whether or not two layers of ripstop is stronger than the sum of the strengths of each layer individually. I think probably not - a bridge or a laminate beam is stronger than the sum of its parts, but that's because the pieces have lateral bracing between them. This isn't the case with a "standard" two-layer hammock. The layers are only connected at the edges, which isn't where most of the stress lies anyway.
If the hammock was going to be holding that much weight (especially if there is going to be a child in the hammock) I would go with a heavy inner layer that you are pretty sure would hold you both, and then a thinner outer layer, possibly with a DWR coating.
You are assuming that by using 2 layers the forces and stresses are evenly distributed over both layers, but I don't know if that would be the case.
With a forum full of Physics gurus, engineers, and rocket scientists I'm sure you will get plenty of opinions on this matter. ;) :D