Why are fiberglass decks made of plywood patchwork?

[Captain Jeff]

This one is bugging me: Why are fiberglass decks made out of a patchwork of plywood squares? It seems counter-intuitive from a strength standpoint. The only logic I can figure is that each square should be sealed in resin so that if one is compromised, the water and rot won't spread across the entire floor.

Here is the fuel tank cover decking on my center console. When they drilled an access hole for the fuel sender, it was never sealed correctly and eventually water made its way in and toward the stern. I am cutting off the lower skin and scraping out the old wood. Then replacing the ply and re-glassing the bottom. Any tips?

Thank You!

 

[tpenfield]

Plywood is used as a core material for fiberglass deck in many instances, as is balsa wood or even urethane foam. It provides the stiffnes that is needed by increasing the thickness of the decking, without substantially increasing the weight.

As you know, water penetration can be a problem, particularly with wood as the core material. Not so much if foam is the core.

Tips would be to post some pictures via photobucket or elsewhere, so we can see what you are seeing. I think your best approach would be to see what parts of the deck, if any, are easily removeable. You will want to work from the underside of the deck, so that the finished fiberglass/gelcoat side remains intact. sometimes that is not possible, so pictures would help.

 

[tpenfield]

Here is a link to someone who reworked some of the deck 'hatches' on a Grady White. It has coring similar to what you described.

http://www.greatgrady.com/forum/viewtopic.php?p=89337&sid=067c1210580cd1876e256e063b5ce1f2

 

[Woodonglass]

Panels are made like that to ensure flatness and overall stability. With Plywood, each lamination goes in the opposite direction of the previous one thus making it stronger. By alternating the orientation of the squares and the additional resin/glue you actually get a stronger and flatter panel when its fabricated under pressure as it is at the factory. Unless you have a vacuum press trying to do this DIY is not practical.

 

[Captain Jeff]

Panels are made like that to ensure flatness and overall stability. With Plywood, each lamination goes in the opposite direction of the previous one thus making it stronger. By alternating the orientation of the squares and the additional resin/glue you actually get a stronger and flatter panel when its fabricated under pressure as it is at the factory. Unless you have a vacuum press trying to do this DIY is not practical.

Thank you for the info. Do you recommend using a continuous piece of marine plywood for DIY?

 

[Captain Jeff]

I thought about making my own vacuum press with some heavy plastic and a shop vac but it will probably burn out the shop vac in short order.

 

[Woodonglass]

You might enjoy this... Build Your Own Vacuum Press
I'd just use the best piece of Ext. Grade Plywood of the same thickness that was there before and seal it. If you're using Poly then saturate it with resin and when it tacks up, coat it again with resin and lay 2 layers of 1.5 oz CSM on both the top and bottom. Cut the pieces about 2" larger and "Tug, Pull and Tear the overhanging edges so they're loose and hairy. This will make it easy to wrap em over the edges. If you're using Epoxy then 2-3 coats of Epoxy is all that's needed. No cloth...Nothing. Just the epoxy will suffice. NOTE: If you want to GelCoat over the repair Epoxy won't work. Gelcoat will not adhere well to Epoxy

 

[jigngrub]

You may be just scraping the tip of the rot iceberg, you should consider removing the entire deck and checking your below deck structure.

If your boat has floatation foam it may be saturated, if your foam is saturated you probably have stringer rot, if your stringers are rotten you may have transom rot. All this is pretty typical with fiberglass boats that have rotten decks.

 

[Ned L]

"~~patchwork of plywood squares". Hmmm, .... it sounds like you might be describing balsa core construction. If so, it is not plywood, it is as you said, small pieces (patchwork) of endgrain cut balsawood. Light weight and stiff.