Re: 4oz vs 6oz Glass

Posted by Laszlo on Jun 9, 2006


It depends entirely on what you're using it for, that is, what you mean by strength. Is it resistance to applied forces, abrasion resistance or just adding mass to stiffen an assembly?

When resisting forces, what actually determines strength is the size and number of threads running in the direction of the force that is going to be resisted. If the weave is arranged so that the force runs directly along the direction of the weave, 1/2 the threads offer no resistance to it. Turning the cloth 45 degrees means that each thread now resists 70% or so of the force. So you go from 50% of the threads offering 100% resistance to 100% of the threads offering 70% resistance - a 40% imporovement which can make 4 oz pretty competitive with 6 oz in some cases. However, if the 6 oz is oriented correctly, it wins.

For abrasion resistance, the total thickness and number of threads is what matters and 6 oz wins. But if the abrasion risk is low, 4 oz may work fine.

In the case of adding mass, 6 oz wins. However, except for the endpours, the CLC designs are too sophisticated to rely on brute mass for strength. They tend to be monocoque composites where the strength comes from the plywood and glass working together to share the load. Plywood on frame boats tend to get their strength from brute mass, as do roto-molded and polyester resin boats. (That's why the latter need so much fiberglass matt. That's also why we can astound the "civilians" at a boat ramp when my 4'10" wife picks up her 16 foot kayak and starts walking off with it.)

Finally, if you just wnt to use up the leftovers and aren't overly concerned about weight, you can always use 2 layers of 4 oz instead of 1 layer of 6. As long as it's the same weave and thread orientation as the heavier cloth, it should work fine, exxcept that it'll be 33% more glass plus the epoxy it takes to wet it out and fill the weave. Please note that this is for small slow boats only. Do not substitute without the designer's blessing when building high performance structures, such as very powerful boats or aircraft. In those cases the extra weight may surpass the designer's margins and cause excessive stress.

Hope this helps,


In Response to: 4oz vs 6oz Glass by Mac on Jun 8, 2006