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Installing hardware on cored decks

Mark Corke

You’d think that installing new deck hardware, such as a cleat or a sheet winch, would be a simple procedure: Decide where the fitting will go, drill a few holes and bolt it in place. Sadly, it often is not that simple, and a little planning and care will save you some sleepless nights and perhaps a hefty repair bill later.

When boatbuilders first began using fiberglass, almost all boats were built of solid laminates. As the years passed and builders began to understand the material better, they incorporated coring into decks. Cored decks — now the norm rather than the exception — are lighter, stiffer and have significantly better thermal and acoustic insulation. However, water that finds its way into the core can cause myriad problems, so it’s critical that hardware is installed properly.

Cored decks are a sandwich of layers: the outer layer of fiberglass, which may have a non-skid pattern or other features and is the part everyone sees; a core, which may be one of several materials but often is end-grain balsa; and a thinner interior layer of fiberglass, which often forms the cabin interior overhead.
When a boat is manufactured, solid sections usually are built into the deck molding so that when a winch, cleat, pad eye or other fitting is bolted in place the integrity of the sandwich construction is not compromised. Most problems arise after a boat leaves the factory and well-intentioned owners — or boatyards that should know better — attach new items to the deck without proper preparation and precaution.
Balsa is an ideal core material in many ways; it is comparatively cheap, very lightweight, stiff and offers some measure of thermal and acoustic insulation. Its one major downfall is that it will suck up water at an alarming rate. Ensuring that deck fittings are correctly installed and bedded will do much to prevent water from getting into the core material.
Fittings often are simply bedded into some form of mastic compound that is expected to keep the water out. This might work for a time, but eventually all bedding compounds break down to some degree. As soon as the smallest of cracks develop in the bedding, water will find its way into the core.
Water that comes into contact with a balsa core may not initially have much effect because the water will be absorbed into the wood. As more and more water finds its way into the core over time, it combines with the air in the cells of the balsa. Mold starts to grow, and the core eventually breaks down.
Problems are further exacerbated when moisture in the core freezes. The ice that forms will expand the deck, forcing the core and laminates apart and allowing more space for water to enter as the temperature rises. As the temperature drops once more, everything freezes again and so on, each time doing a little more damage.
Some core materials, such as Core-Cell, are manufactured from a closed-cell foam that will not absorb water. However, if water finds its way into a small crack in the deck or a poorly bedded fitting, the water can cause delamination between the core and the fiberglass. I’ll discuss delaminated cored decks and how to repair them in a future story.
Even though core material is very stiff for its weight, bolting down a winch or cleat willy-nilly is a bad idea because the core will be crushed and the deck deformed. Fitting anything to the deck needs to be done properly and takes time. This initially may seem like overkill, but you can rest easy knowing you have done everything possible to avoid problems and preserve the structural integrity of the deck.
There are many ways to install deck hardware, but the methods I mention here have served me well for more years than I care to remember.

1. Decide where the fitting is to go, paying attention to such things as sheet leads. If the fitting is heavy, such as a winch, it probably will stay in place by weight alone. Small deck cleats might require a piece of double-sided tape if you need to stand back to check the placement.

2. When you are happy with the placement, mark the position of the bolt holes with a Sharpie marker or pencil so you can easily see where to drill the mounting holes.

3. Using the Sharpie marks as a guide, drill 1/16-inch pilot holes through the deck. This serves two purposes. First, you can go inside the boat or look under the deck and be sure the holes and bolts will not interfere with anything. Second, drilling the larger holes will be much easier and more accurate because the drill has something to follow. If you have to make adjustments after drilling the small holes, filling them with epoxy is no big deal and will make them almost unnoticeable

4. If everything checks out, carefully drill a hole twice the diameter of the bolt hole through the top layer of fiberglass and core only. (You ’ll feel it when you’re through core and reach the inner fiberglass skin.) This hole will be filled with epoxy and will seal the core and offer solid posts that won’t compress as the fastener holding the fitting is tightened. For the neatest appearance the epoxy should not be visible around the edge of the fitting after it is installed. If it looks as if this will happen — a few measurements can confirm it — you might want to drill the correct size clearance hole for the bolt, though still going only through the outer laminate and core. Then cut the head off a nail, bend the last inch 90 degrees, and tighten it in the chuck of the drill. Feed this into the hole and grind away, removing the core as an undercut.

5. With the holes drilled, put tape on the underside to prevent epoxy from running through the small pilot hole.

6. Mix some unthickened epoxy and brush a little into the hole. You’re not filling the hole at this stage, just wetting the surface as a sort of primer. With this done, mix colloidal silica into the epoxy until it is the consistency of thick mayonnaise.

7. Force the mixture into the hole, using a small scraper knife or other suitable tool. Make sure there is no trapped air and that the epoxy is level with the surrounding deck surface. Allow it to cure for 24 hours.

8. Remove the tape from the underside of the deck. Using the pilot-hole mark, drill a small hole back up through the cured epoxy. Then, from the top, drill back down with the correct size bit for the bolt hole.

9. When all of the holes are drilled, insert the bolts and have a dry run. If everything lines up and looks good, add suitable caulking to the underside of the fitting, reinstall it and the bolts, and tighten the nuts so they are snug but don’t overtighten. Avoid using 3M 5200, which is too adhesive and will make removal difficult if that becomes necessary at some point. It is a good idea to install a backing plate on the underside if the loads are likely to be more than modest.

Masking tip

Here is a tip that can save you hours and make cleanup a snap. With the fitting sitting over the bolt holes, draw a pencil line around its base, then remove the fitting and put it to one side. Apply tape so that it covers the pencil marks.
Place the fitting in the correct position and cut around the base with an X-Acto knife or razor blade. Remove the fitting, then peel up the tape on the inside of the cut line. You can then use mastic for the fitting down and any that squeezes out will be removed with the tape. A perfect job and very little mess.
Using the methods described above to reinforce and seal the deck provides epoxy “pillars” for the fitting to sit on, which not only protects the deck from being crushed but also means there is no conduit for water to find its way into the core, even if the bedding compound breaks down.

Bedding compounds
Enter any marine store and you will see a bewildering array of mastics and sealants. Knowing which to choose can be a problem. The most common products are either polyurethane or a polysulfide, each of which is excellent but only if used for its intended purpose.
Polyurethane is an adhesive compound, of which 3M 5200 is perhaps the best known. It should not be used if there is a chance you will need to dismantle the joint later. 5200 is used by many manufacturers for the hull-to-deck joint, and for this application it is perfect. Polyurethanes also hold up well under water and will bond to most materials. They can attack some plastics but not Marelon, so it’s OK to use them for bedding plastic under water fittings made of this material.
Polysulfides, such as BoatLife products, remain permanently flexible and are my preferred choice for most jobs. They can be used above or below the waterline, and the joint can be disassembled later. Polysulfides will attack polycarbonates such as Lexan or PVC and should not be used to bond them. For this reason, most window frames and other components containing plastic are bonded with a silicone product. Polysulfides are available in a variety of colors. I have always found brown a good match for teak and mahogany, but for the best seal with teak use the primer first.
Dolfinite bedding compound is slightly more old-school. It can be hard to find, but I like it for applications in which I am bedding wood to wood. That’s its primary purpose, but it also can be used for bedding hardware. It has a strong aroma, and every time I pop the lid it reminds me of old boatyards, much the way fresh varnish does.
I’m getting a bit off topic here, but Dolfinite remains flexible for years, will not dry out and will not destroy parts if you need to take them apart someday. It comes in a can that can be resealed, so it will not go hard, as half-used tubes are prone to do. For general bedding it could save you money in the long run and it’s available in white, gray and brown.

This article originally appeared in the August 2011 issue.