Exploring coring on boats
The coring material in your composite glass reinforced plastic deck is the web in the I beam of deck strength until it degrades.
This degradation is caused by the universal solvent getting where it doesn't belong. The only thing water doesn't dissolve is water. So how does this solvent get in to do its dirty deed?
The boat builder just had to drill holes in your boat. The builder had reasons such as to attach toe rails, stanchion bases, cleats, skein chalks, grab rails, anchor windlasses, hawse holes, teak sheathing, antenna bases, rod holders, and on and on.
A great many decks are cored with vertical grain balsa wood that is light and strong when sandwiched between two skins of properly laid glass reinforced plastic of sufficient thickness for expected loading.
Most builders don't take the time because of expensive labor to install solid core where deck penetrations are made. Simply drill the hole, bed the fastening, and away we go. Now come the culprits: sun, salt, age, cycle loading and flexure.
The bedding compound begins to break down, and the wet stuff gets in. Now balsa core doesn't rot; it turns to rancid grits and the I beam's strength has fled. So what's a body to do?
Look for the tell-tale signs: stress cracks around fittings, flexing stanchions,
minor concavities around these fittings.
Surveyors use phonemic hammers to impact sound decks and hulls to detect
delamination and coring degradation. We use moisture meters to read relative
moisture, but mostly we rely on our years of experience with different vessels to
make our diagnosis and recommendations for repair.
What can the boat owner do? That depends on how far advanced the problem is. Most problems can be solved or slowed down by removal of fastenings, drying and simply re-bedding with an acceptable marine bedding compound or adhesive.
More serious problems require removal of degraded coring material by cutting upper or lower laminate skins and cropping out degraded coring. You then have to re-core, re-skin and perform an acceptable cosmetic repair.
When it gets this serious, you may need some help from the boatyard. I have seen decks so degraded that the repair was just not cost-effective relative to the value of the vessel.
If you are looking at a vessel considering a purchase and the deck feels like a trampoline, do yourself a favor and bounce on down the dock and look for a stiffer ride.
If you start to see the tell-tale signs, get out the tools, caulking gun and get down on your knees in the position of humility and go to it.
One of my fellow surveyors, C. Warren Moore of Washington, N.C., whom I highly regard, shared a trick with me. He reports that if the core degradation is not too far advanced around fastenings, try the following: Take out your trusty drill and chuck up an "L"-shaped Allen wrench. Insert the short leg into the hole and carefully, and with a steady hand, sweep between the laminates and vacuum out the degraded coring.
Use progressively longer wrenches until you see clean dry coring. Dry thoroughly and fill with thickened epoxy. I haven't tried this myself, but it sounds like a good trick to me.
There are some good books on GRP repair: the Gourgeon Brothers' easily understood
repair manual, Vaites's fiberglass boat repair manual and many others. The tools and
techniques are basically the same as well as the humility. May only your breakfast cereal
go snap, crackle, pop and not your decks. And remember, the Lord loves sailors.