Re: fiberglass mat or sheet, which is stronger?
Note that the above replies are completely correct when using poly resin or vinylester, but the rules change a bit when using epoxy.
In any case, know that the strength of a glass composite is directional... as you might expect, when fibers are pulled in such a direction that they stretch out lengthwise, they're the strongest.
So with woven fabrics you need to pay attention to which way the weave goes as you laminate, it'll be strongest in the direction of the fibers. This is why biaxial fabrics are popular, because the weave goes in directions other than 90 degrees, so it's easy to laminate in a way that is strong (but still possible to do wrong).
The strongest possible layup of a composite would be done by pre-calculating all the forces on the eventual part, developing a mold to shape the part, then laying a continual strand of fiber onto the mold in such a way as to oppose those forces most directly... basically rotate the mold plug in the air and use a computer to control winding the fiber onto the mold, then bake the whole thing to cure it. This is how carbon composite aircraft parts are usually made.
For home boat work, you usually rely on overkill... amateur builders and repairers could get by with either mat or cloth, but in case they make a mistake, it pays to build in lots of extra strength, so the boat remains safe and in one piece. So even where we could use mat and poly resin, some of us use cloth or biaxial and epoxy.
Many are the opinions here about which glass is better, or stronger, or more waterproof?(I don't understand that one either) but I think the question isn't which is stronger, it's which fits a given situation.
Erik
