Fiberglassing Plywood for Impact and Stiffness

Originally posted in the FG and Composites section

Just looking for a little clarification on the utilization of fiberglass and it's application here.

In looking at the Gougeon Brothers data on impact testing and the location of reinforcing fabrics in wood/composite construction, their conclusion points out that the best location for fabrics is toward the backside of a panel (one application being on the interior of a hull skin). Does this suggest the following would be true.?

If a 2' x 2' piece of Okoume ( 6mm for example ) was bonded to a 2' x 2' x 3/4" frame, It would be easy to stomp on it with "X" amount of force and break through. If you were to cover only one side of the plywood (eg. 6oz. plain weave E-glass), Optimum impact resistance and stiffness would be achieved with the fiberglass applied to the underside of the plywood.

Correct?

Although simple "beam" test with a narrow strip of glassed plywood would verify my understanding, I am too lazy this morning, but energetic enough to give the most convoluted scenario for such a simple question.

 

I guess the optimum impact resistance is to put 10mm steel on the outside as well.

Jokes aside, I guess you are asking "what is the best value you can get from a combination of materials to give acceptable strength". The inside covering probably contributes a very large amount, but an outside covering plays a big part as well, in conjunction with the inside - and the greater the gap, the greater the strength. Once again though, you wouldnt be bulding a boat from 2inch plywood.

There are lots of very good discussions on the effect of one side V two side, plywood v foam construction if you want to do a search of these discussions.

The basic principle is that your local friendly Naval Architect can tell you how much of everything you need to keep the water and other sharper objects on the outside of your boat, for the size and performance of the craft you need, hopefully in the most cost effective manner.

In the case of the gougen bros - they cover both sides, not one, for optimum strength, as well as to preserve the timber.

In the balance of things, coverage of two sides gives you better value in time and materials, for a desired strength combined with protection of the ply, than one side alone.

 

What is known as a "balanced laminate" is one that is equal on both sides. It is a preferrable way to do things in composite boatbuilding, but certainly not essential.
As stated above , sheathing the outside as well obviously aids the waterproofing side of the story, and provides good abrasion resistance as well.

 

If using regular fabrics, impact and puncture resistance is increased with a laminate on the inside, because the skin works in tension. An exterior skin on plywood is primarily for abrasion resistance.

Plywood, in this application (with a single relatively thin 'glass skin) is the structure, with reinforcement from the internal sheathing. This isn't true of cored structures, using foam, balsa, or one of the honeycomb materials. These builds are a "sandwich" structure, which rely on fabric sheathings (and the distance between them) to provide the bulk of the panel strength. In these applications the laminate has to be engineered to support anticipated loading, including impact and abrasion resistance.

Yes, a balanced laminate is one with an equal number of layers.

From a practical stand point, sheathing both sides of plywood increases waterproofness. It's stiffness is also increased, but not much with thin sheathings. Thicker 'glass skins will impart additional strength, but in most plywood builds isn't necessary, if the wooden elements are engineered properly.