Cores?whatthehay?

Need some help understanding the need or use of core material.Say im going to build a 28 foot catamaran with a small center cabin,out of plywood,i see using some sort of epoxy over fiberglass cloth.So is the core material for sound deadening or insulation,i read about using it on decks.Help me get a clue on use of this material.

 

It's mostly for stiffening and a little bit for strength. Take a quarter inch foam core with FG on both sides. Now double the thickness of that core to a half of an inch and the piece will be about 8 times stiffer and about twice as strong.

Imagine placing a 2x4 of foam on a pair of saw horses. Cover both sides with a few layers of fiberglass and then have a seat on it. The top part of each layer of fiberglass is in compression, the bottom part of each sheet is in tension. It will remain this way until you reach the ultimate load of one of the layers of glass.

You can keep adding weight until the fiberglass fails. How will it fail? Well, fiberglass is usually slightly better in tension than compression. So the top side that you're sitting on (or the face of the bottom panel of FG that is facing the foam core) will fail first.


Now add in a thicker core...

The thicker the core, the more efficiently the load is distributed between the sheets. Essentially the core spreads the loads. Imagine you sitting on that fiberglass sheet causes one inch of deflection. Double the thickness of the core and now it only deflects a small fraction of an inch. Since it isn't deflecting as much, there isn't as much stress on the strands of fiberglass- either in tension or compression. In other words, the FG can take even more load now and it deflects less.

Where did the stress go? It's getting transferred to the core.

Is there a limit to how much thicker a core can get? Practically speaking? No, there really isn't. A 1 inch core is better than a half inch core. A 2 inch core is even better. But a 4 inch core is even better yet. Sure, eventually, you will reach a point where the core will fail, but that point is typically way out in the stratosphere as far as earth-bound craft are concerned... the tradeoff you really have to be concerned with are cost, loss of interior volume, and additional weight.


Find an aeronautical discussion of designing an I-beam spar. Read it sideways. Then you will understand.