"W haddya think it is?" Dave asks as he watches the thin stream of brown particles flow out of the hole we've just drilled into his hull. "I think it’s what used to be the core of your hull," I say, trying to break the bad news gently as we continue to watch. It seems as though we’ve drilled our hole into a big box of brown sugar, but in reality, we’re seeing the remnants of a wooden hull core that’s thoroughly rotten.

In the old days, when fiberglass boats were solid 'glass throughout, it used to be so simple to add equipment: Just drill holes of the right size, slather on some bedding compound, pop in the fastenings and tighten everything down.

Today's boats, however, are far more complex, and a boat owner needs to have a working understanding of what can (and can't) be done to his boat. Modern builders rely on a variety of coring materials — usually balsa, foam or plywood — which, sandwiched between an outer and an inner layer of fiberglass, forms what's known as "core construction." Not only is a cored boat lighter (for better performance) as well as less expensive to build (it requires less fiberglass), but it also benefits from increased strength and insulation qualities. The downside, of course, is it makes installing aftermarket equipment much harder for the boat owner.

Intelligent Installations

There are a few things you should consider before beginning such a project. Before you drill any holes, carefully think through the entire process. Know what’s inside (or outside) the area where you’re drilling, because you don't want to accidentally puncture a fuel tank, drill into the icebox, or sever any electrical wiring. 

Consider the entire project as a whole, rather than as several small tasks. If, for example, you're installing a depth sounder, you'll need both a visible area to mount the instrument, and a good place to install the transducer through the hull. Equally important, you'll want the cable connecting the two units to be easily routed out
of sight, and you'll need access to the electrical system to make the hookup. All of these factors may point toward or away from various locations.

When installing fittings through the hull — such as the transducer for a depth sounder or the sensor for a speedometer — position them so they won't be damaged when you trailer your boat. And if you're adding gear such as handrails or pulpits that will have large loads, be sure to put a husky metal or wood backing plate underneath the deck to spread out the loads over a large area. 

 

Cored Boat Basics

With a cored boat, your first task is to determine the type of coring, since the type may affect your installation procedure. For example, foam coring is susceptible to crushing, and you can deform the core by tightening down too hard on the fastenings. Balsa coring is prone to rot where water seeps into it, and so is marine plywood — even the good grades. In each situation, your primary task is to create a solid, watertight installation.

If you need to make a small hole in balsa or foam core, drill a hole that’s oversized by at least 1/8 of an inch. Tape a piece of wax paper stiffened with cardboard on the bottom of this hole and fill the area with an epoxy-based filler paste. Once this has cured, you can pull off the wax paper, sand this plug smooth, and then drill a properly sized hole through the center of the epoxy plug. This plug, along with a good bedding compound, will keep water from migrating into the core, in addition to preventing the core from being crushed when you tighten the fastener snugly.

When drilling a large hole into an underwater hull section, start by drilling a hole twice the size you need from the inside, but only through the inner liner and the core. This will permit you to remove the liner and coring material so you can replace them with a beefier backup plate. This can either be a piece of marine plywood swathed in fiberglass for rot protection, or even better, you can simply fill up the opening in the liner with a putty made of epoxy resin and chopped fibers. Once you’ve finished, drill the final hole and tighten the fasteners securely.

Sealant Solutions

Every installation, whether it’s on deck or underwater, needs a good bedding compound to make it watertight. Remember, water can sneak through any opening larger than an H20 molecule, usually so it can form a droplet that will fall on your forehead as you read in your bunk on a rainy afternoon.

There are three basic types of sealants: polysulfides, polyurethanes and silicones. Make sure you understand their strengths and weaknesses before choosing a sealant for your project. Polysulfides, such as BoatLIFE's Life Calk, are the most common and best all-around sealant, as long as you don't need strong adhesive properties. Don't use them on acrylic or polycarbonate windows. Polyurethanes, such as 3M's 5200, are sealants/adhesives that may take as long as a week to cure, but once set up, they cling tenaciously and are extremely difficult to remove. Silicone sealants are often used in homes to seal bathtubs and sinks where a fast curing time is needed, but their prime use aboard boats is to seal plastic windows, which may be attacked by the sulfides. Use only silicone sealants formulated for boat use, because the household versions won’t stand up to the rigorous conditions.

Both the polysulfides and the polyurethanes can be used underwater, but the polyurethanes are less flexible, so unless you need their adhesive properties, you can eliminate them as a bedding compound. Silicones should never be used below the waterline, and unlike both of the polys, cannot be sanded or painted. 

 

Here are a few helpful instructions to make your installations watertight: 

• Unless you're using a polyurethane for strength, don't tighten the fasteners fully until the bedding compound is cured, or you'll simply squeeze it all out. Tighten down until you start to squeeze out the compound, wait until it cures, and then crank down on the fastening, which now has a thick rubbery gasket under the fitting. This is particularly important with silicones, which have so little adhesion they require the mechanical fastening to create and maintain the seal.

• Don't be stingy with bedding compound — you can always cut the excess away later with a razor blade.

• Be sure the holes you drill are the proper size — too large and they'll allow the fastenings to "work" and create leaks.

• Before resealing an older fitting, carefully scrape off all remnants of the old bedding compound, and then roughen the surface with sandpaper for good adhesion.

• If you have a window or porthole leak, don’t try smearing sealant around the outside to stop the leak. Remove the window, clean it thoroughly and reinstall it properly — you'll save a lot of time in the long run.