Cored composite construction, in the simplest sense, is a sheet of some kind of coring material sandwiched between layers of fiber-reinforced plastic — usually fiberglass but sometimes carbon fiber or Kevlar.

The coring might be marine plywood, end-grain balsa wood, honeycomb sheet like Nidacore, PVC foam sheet like Divinycell, high-density reinforced polyurethane sheet like Coosa board, or any of a variety of other materials. Regardless, the goal is more rigidity for less weight than solid fiberglass. (Generally speaking, cored construction also costs boat builders less in both time and materials than a solid fiberglass layup.) 


Fiberglass Boats

Most fiberglass boats built during the last few decades employ cored composite construction in at least some parts. Decks, caps, liners, consoles, and hatches are most often cored to save weight and improve panel stiffness. Hull sides, from the chines up, are often cored as well. Hull bottoms may also be cored, but most are made of solid glass.

That means most of the accessories you add to your boat will likely be installed on cored glass. Electronics, rod holders, antenna mounts, drink holders, lighting, Bimini hardware, seating, and so on are almost all mounted on parts of the boat likely to be cored.

The Drawbacks of Cored Glass

Unfortunately, one of the drawbacks of cored composite is that attaching things to it with screws or bolts requires some extra steps. There are several reasons for that. First, water can reach the coring through fastener holes and cause significant damage. Second, self-tapping screws don’t hold well in most cored fiberglass, since the only thing the threads have to bite into is the outer “skin” of the fiberglass. And third, through-bolting can actually crush the coring material, weakening the panel.

 Water can reach the coring through fastener holes and cause significant damage.

Most of the coring materials used today are “waterproof;” in other words, they don’t absorb water themselves. Polypropylene honeycomb, closed-cell PVC foam, and high-density reinforced polyurethane all fall into that category. On the other hand, plywood and balsa, which are still used by some popular and well-respected boat builders, readily absorb water, leading to rot. Even with “waterproof” coring, water intrusion is damaging since water can weaken the bonds between the core and the layers of fiberglass it’s sandwiched between, leading to delamination and ultimately structural failure.

Through-Bolting to Cored Fiberglass

Because self-tapping screws don’t hold nearly as well in cored glass as in solid glass, through-bolting is always the preferred approach when attaching something to cored glass.

Some experts go so far as to say screws should never be used to fasten to cored glass. But in certain situations, there’s no choice. The vast majority of transom-mounted transducers, for example, are attached with self-tapping screws. 

Through-Bolting Troubles

Most transoms are simply too thick to make through-bolting practical and most are cored with higher-density materials like Coosa or plywood, which hold screws better than foam, honeycomb or balsa coring. Regardless, though, if you have access to the back of your mounting surface, through-bolting is the way to go.

The problem with through-bolting in cored glass is that the inner and/or outer layer of fiberglass can flex or give, allowing the core material to be crushed. This can result from use — a hard, pull on a cleat, for example — or even from simply over-tightening fasteners. In extreme cases, the washers and nuts can rip right through the cored composite.

There are a couple of ways to prevent this. First, spread the load over as large an area of the mounting surface as possible by using large, thick fender washers or, better yet, a backing plate made of stainless steel, aluminum, marine plywood, or King Starboard.

Second, you can create epoxy “plugs” around the fasteners to add strength and resist compression. There are a variety of ways to do this, but all basically involve making a hole larger than the diameter of your fastener, filling it with epoxy, and then drilling a hole for your fastener through the epoxy.

A Simple Method

The simplest method is to drill a hole roughly twice the diameter of the fastener through the outer layer of fiberglass and the coring but not the inner layer of fiberglass. A Forstner bit, which bores a flat-bottomed hole, is ideal. Then backfill the hole with thickened epoxy and let it cure to create a plug. Because self-tapping screws don’t hold nearly as well in cored glass as in solid glass, through-bolting is always the preferred approach when attaching something to cored glass...

If you don’t want to drill that large a hole in the outer layer of fiberglass, you can drill a hole sized for your fastener, and then ream out the coring around the hole using a bent nail or Allen wrench chucked into a drill. Then vacuum the debris out before backfilling with thickened epoxy.

Once the epoxy plug cures, drill a new hole in the epoxy for your fastener. This method reinforces the laminate to prevent crushing of the coring and, as an added benefit, also prevents water intrusion into the laminate.Thickened epoxy can be made by mixing epoxy resin to a mayonnaise-like consistency with colloidal silica such as Cab-O-Sil, microballoons such as West System 407, wood flour, or fiberglass strands. It can also be purchased in ready-to-use formulations like West System Six10 and Marine-Tex.

This method reinforces the laminate to prevent crushing of the coring

Note that plywood and Coosa-type coring are both fairly resistant to crushing. Only less dense materials like foam, honeycomb, and balsa need to be reinforced. Guarding against water intrusion is important with all types of coring, though.

Sealing Holes in Cored Fiberglass

Even when self-tapping screws are used instead of through-bolts, epoxy plugs should be used whenever possible to prevent water intrusion.

Drill a hole the appropriate size for your screw through the outer fiberglass skin and the coring but not the inner fiberglass skin, ream out the core around the hole, vacuum away debris, and backfill with thickened epoxy.

Then drill a pilot hole of the appropriate size for your screw in the epoxy plug.

Additionally, bedding deck hardware and accessories in marine sealant are important to prevent water intrusion. When saltwater seeps under fittings and encounters stainless steel in an oxygen-poor environment the result is a process called “crevice corrosion” — the cause of those rust stains you often see around cleats, stanchions, hinges, rod holders, and other deck hardware. This can be prevented by bedding — generously coating the bases of fittings and accessories in marine sealant before installing — to keep water out from underneath.

A variety of sealants will work, including silicon, polyurethane, polysulfide, elastomeric, and butyl tape products. Each has its own strengths and weaknesses, which deserve an article of their own.

Reality Check

There’s little debate that the method outlined above — reinforcing the laminate and waterproofing the core with thickened epoxy plugs and then through-bolting with a backing plate — is the best way to fasten hardware or accessories to cored fiberglass.

But the fact is that it often isn’t used — not by boat builders; not by professionals installing electronics, canvas, and so on; and not by most DIYers. To be fair, quality boats are typically built with areas of solid fiberglass where cleats, T-top legs, outrigger bases, stanchions, hinges, etc. are meant to be attached, so there’s no need for the original builders to bother with epoxy plugs.

But take a look under your cleats, Bimini mounts, electronics brackets, rod holders, deck fills, etc., and chances are you’ll find that some or all of them aren’t bedded correctly, aren’t reinforced to prevent crushing the laminate and/or are screwed on when they should be through-bolted.

DIY Installation

For DIY installation of accessories like rod holders, antenna mounts, drink holders, Bimini hardware, and electronics brackets, common sense and judgment should play a big part in your approach. Whatever your application, there are two requirements: that the core doesn’t get wet and that the fasteners don’t pull or break out. Depending on what you’re mounting and where you’re mounting it, those two requirements can be satisfied in a lot of different ways.

Make sure the core doesn’t get wet and that the fasteners don’t pull or break out. 

For a couple of #8 screws holding a drink holder on your console where it will be sheltered from spray and rain by the windshield and top, you can probably coat your threads in sealant, dab a little around the holes, and call it a day. For outrigger mounts on your hardtop that are subject to huge forces and constant exposure to windblown water, the approach will be very different.

Other Things to Consider

When it comes to fastening to cored glass in boats, the challenges are real, but so are the solutions. Whether you're mounting electronics, rod holders, or cleats, these insights will help you fasten your boat accessories with confidence, making sure they stay put.

Don’t Over-Tighten Fasteners! 

Cranking that nut or screw until you hear something starting to crack doesn’t make the attachment any stronger. Always, always finish fasteners with hand tools for a better feel and err on the side of loose.

Use the Right Size Drill Bits

With self-tapping screws in fiberglass, 1/64” matters. There are a number of charts available online showing what size drill bit to use with each screw size in soft and hard wood. Start with the “hard wood” size for fiberglass.

If at all possible, do a test in a scrap piece of fiberglass before drilling into your boat. It’s actually better to drill a bit too big than a bit too small. With too small a pilot hole, the screw will damage the fiberglass, compromising its strength. If you hear glass cracking, the hole is too small.

Chamfer Holes in Gelcoated Fiberglass

Gelcoat is hard, brittle, and prone to cracking with self-tapping screws. Cracks around fastener holes trap water, leading to crevice corrosion. Always chamfer the edges of your hole with a countersink bit or a larger drill bit.

Go slow and use a light touch, especially with a larger drill bit; all you want to do is remove enough gelcoat around the hole so that the screw won’t crack the gel going in. Even for through bolting, chamfering holes allow sealant to better protect fasteners

If through-Bolting Isn’t An Option, Consider An Adhesive

The common advice is to avoid high-strength adhesive sealants like 3M 5200 for anything you might want to take apart in the future. But if you can’t through-bolt and you’re concerned screws aren’t strong enough, 5200 will help (except on Starboard, which it doesn’t adhere to well).

It’s not uncommon to hear about something like a T-top or poling platform staying in place for years held by nothing but 5200.

Sealants Have a Finite Life

All marine sealants crack, shrink, break or deform over time. That’s part of the reason experts say you should use epoxy plugs to waterproof the coring around your fastener holes: if the bedding sealant fails, the epoxy will still keep the water out.

Consider removing deck fittings and accessories every three to five years to scrape away old sealant and re-bed them.

Use Unthickened Epoxy To Seal and “Prime”

In applications where epoxy plugs may not be truly necessary, exposed coring inside fastener holes or cutouts can be sealed by “painting” on a heavy coat of epoxy resin. For bolt holes, seal the bottom of the hole, fill it with resin, let the resin soak into the coring for a few minutes without curing, then push the excess out.

Experts also recommend thoroughly wetting the inside of a hole with unthickened epoxy resin before applying thickened epoxy to create a plug.

Don’t Seal Backing Plates or Fender Washers

If water is leaking into or through fastener holes, you want to see evidence of it so you can fix it. You also want it to have a way to drain on through to minimize soaking into coring. So leave the underside of your installation unsealed.