Postby Iggy » Thu Mar 24, 2011 6:59 am
I think you are misunderstanding the function of fiberglass over wood in your experiment. A 6oz cloth of fiberglass with 3 layers of epoxy is not applied for the sake of surface impact resistance, but rather for surface wear resistance. Those are two different things. In this case, the fiberglass overlay will add a layer of protection to the wood beneath it from some (not all) scrapes and friction-based wear like dragging over sand, gravel, etc.
I've designed fiberglass structural components and they can have some significant impact resistance, however they are MUCH thicker than 6oz cloth and 3 layers of epoxy. 0.1" thick or greater.. THEN you will see a significant impact resistance from fiberglass.
I would take your same rock test and try it against a thin sheet of steel or aluminum, and give it a good wack like before.. you will see and dent just as well and mark up the surface.
The reason I fiberglass my plywood is to help protect the thin veneer of wood from as much wear as possible, but its not going to save my boat from dents if I run it into a rock.
You have the optoin to add a thicker layer (or multiple layers) of fiberglass cloth.. which will require a lot more epoxy... and add some weight to the hull as well.