Sailboat deck

This is an area of fiberglass sailboat construction that many owners ultimately become interested in, either because deck hardware installations on their boat start leaking, or because they decide to replace and upgrade hardware. Unfortunately, it is also an area where some builders often try to streamline their methods to save time and money, particularly when it comes to installing hardware such as winches, cleats, genoa tracks, travelers, stanchion bases, and the like.

As we’ve discussed earlier in this series, almost all fiberglass decks are cored these days, which presents two problems any time a deck is penetrated to receive a hardware fastener. First, the core must not be crushed; second, it must not be exposed to any moisture. Given the enormous number of fasteners needed to secure deck hardware and the enormous loads some hardware carries, it should come as no surprise that hardware installation is both critical and troublesome.

Damage to a deck, once hardware fasteners start leaking, can be substantial. First small areas of deck core may become saturated; over time these can become large areas, requiring major repairs. (To get an idea of what’s involved, check out these excellent blog posts by Paul Calder: Deck Repairs, Part 1 and Part 2)

Rotted deck core
Rotted deck core exposed (Photo by Paul Calder)

To prevent such nightmares, the traditional practice is to treat hardware fasteners individually. This can be done by drilling out an over-sized hole for each fastener, filling it with a plug of epoxy paste, waiting for the epoxy to cure, then re-drilling a proper-sized hole through the plug. The plug both seals the core, protecting its internal face against moisture intrusion, and acts as a compression post that prevents the deck skins from being squashed together. Another alternative is to drill a proper-sized hole for the fastener, ream out the core around the perimeter of the hole with a bent nail or router, then fill the cavity with thickened epoxy filler. Again, the epoxy both seals the core and resists compression as the fastener is tightened down. Yet another alternative, rarely seen, is to drill a slightly over-sized hole, seal the core perimeter with epoxy resin, then insert a metal compression tube to bear the load of the tightened fastener.

Deck hardware fasteners
Fastener hole treatments–variations

Few, if any, builders took such obsessive precautions in the early days of fiberglass boat production. Older boats thus often have deck hardware installed with fasteners piercing unsealed balsa-cored decks with no compression fittings of any kind. The assumption then was that the core would not be crushed if nuts on bolts were not over-tightened, nor would it get wet if the hardware was well bedded with sealant. But heavily loaded hardware may in fact crush a core even if its fasteners are only finger-tight, and even the best sealant, liberally applied, will eventually break down over time and allow moisture to creep in. In the end, it is often some poor boatowner (or a yard crew hired at great expense) who many years later must properly seal and secure the fastener holes.

The best and most efficient way to install deck hardware on a production basis is to properly prepare the areas in question when the deck is molded in the first place. For example, the deck can be molded with only solid laminate in areas receiving hardware. If this is done, transitions from cored to uncored laminate within the deck should be made gradually, with the core gently tapering down in thickness so hard spots are not created. Another more common method is to replace compressible core material under hardware with another firmer material. In the past plywood was often used for such inserts, because it is cheap and is not easily compressed. Plywood does, however, readily absorb moisture, so each fastener hole, again, should be sealed with epoxy. Another more common alternative core material these days is high-density plastic foam, which is lighter and less absorbent than plywood, but also more expensive. Yet another sensible alternative, which hopefully will become more popular over time, is StarBoard, a lightweight solid plastic faux wood that cannot be compressed and is entirely waterproof.

Deck hardware installation
Solid deck inserts

All fasteners should be threaded through-bolts with nuts on the end, as illustrated. This is the only reasonable option, unless the hardware in question is never subject to any load, in which case self-tapping screws may be used if they are well bedded with sealant.

Besides being through-bolted, all loaded hardware should be supported underneath the deck by substantial backing plates with generous margins extending beyond the hardware’s footprint abovedeck. This spreads loads over a much larger area of the deck. It is best if these plates are metal–either stainless steel or aluminum–although plywood is commonly used and is usually acceptable if large fender washers are also installed. Sheets of StarBoard are another excellent alternative. Builders, unfortunately, often omit backing plates and instead install fender washers that may not spread loads sufficiently. I myself have seen cleats secured with fender washers torn clean out of decks by docklines strained by passing boat wakes.

Failed deck hardware installation
Stanchion post torn loose: no backing plate!
Deck hardware installation
Backing up hardware: washers vs. plates

There is another method for installing deck hardware without washers or backing plates that is now popular with many builders. First, during the deck layup, aluminum plates are glassed into areas where hardware will be installed. Then holes are drilled and tapped into the embedded plate (this can even be done by computerized robots) and hardware is fastened in place with stainless-steel machine screws. The great advantage is the entire installation can be made from above the deck, which simplifies deck design, as no allowance need be made for access under the deck. It also saves manpower, as a second worker is not needed below deck to install washers or backing plates and thread nuts on bolts.

Deck hardware installation
Embedded mounting plate

I have to believe that the long-term viability of such installations is suspect. The embedded aluminum plate, if appropriately sized, can act like a backing plate and help spread loads, but the load may be carried by only a portion of the laminate. Moisture may also cause problems. With through-bolted hardware, if moisture gets into fastener holes it eventually emerges in the interior of the boat, having done no significant damage as long as there is no unsealed deck core to absorb it. The resulting leak can be discovered and redressed by rebedding the hardware with fresh sealant.

But with an embedded aluminum plate, moisture may be trapped in the laminate, eventually causing a problem. It will certainly cause corrosion where the aluminum plate and stainless-steel screw are threaded together. Expanding powdery waste from the corroding aluminum (you often see this on aluminum masts and booms where stainless-steel fasteners are installed) may someday cause significant delamination.

This can be prevented by removing the hardware and rebedding it on a regular basis, before the sealant fails. Removing hardware will be hard, however, if the aluminum plate and stainless-steel screws have seized together, as often happens. Then it will be necessary to drill out the screws. To replace the hardware, the holes will have to be re-tapped to receive a slightly larger screw. This can probably be done only once or twice before the screws are too large to fit the hardware’s fastener holes. All of which only discourages an owner from trying to remove the hardware in the first place.

To forestall these problems any boat with deck hardware screwed into embedded plates should have its hardware mounted on elevated bosses or plinths that shed water and so make it harder for moisture to find its way into fastener holes. Indeed, this is very desireable on any boat, but unfortunately is rarely done.

Coping with Deck Liners (or Not)

Comprehensive one-piece deck liners that cover all the overhead in a cabin interior are common on production sailboats. Such liners usually restrict access to fasteners under the deck, making it impossible to remove hardware without first cutting away part of the liner. The best practice when a liner is fitted is for the builder to cut small access hatches with removable covers so fasteners can be reached easily. Often this is done after the fact by owners seeking to remove old hardware or install new hardware in new locations.

Alternatively, fasteners may pierce the liner, spanning the gap between the liner and deck. In this case, some sort of compression fitting should be installed so the liner is not warped or crushed by the fasteners. It is best, too, if a backing plate is also installed, though this is rarely done.

Ultimately, it is best if there is no deck liner. On well-built boats you’ll find instead a series of removable overhead panels in open areas of the cabin. These are often held in place with Velcro strips, which makes it easy to access under-deck fasteners, though the Velcro eventually wears out and needs replacing. Overhead panels can also be secured with trim strips and/or small screws, which are more permanent and reliable, but take much more time to unfasten if panels must be removed.

On simple boats (or on lightweight race boats), there may be no liner or overhead panels at all, in which case all the under-deck fasteners will be exposed to view. It may not look very pretty, but from a maintenance point of view this is highly desirable. Any leaking fasteners will be immediately apparent; they will also be immediately accessible.

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1 Response
  1. What’s best way to fit deck fittings with an old non cored half inch grp deck? How do you maintain a moisture seal with epoxy on such a deck fitting? If using stainless steel backing plates do you need a timber pad as well? What thickness stainless steel plate should be used on an old 8m cruising cat? Finally, should the timber backing pad be beveled to gradually transfer any stress to the grp deck – I’m thinking of using epoxy coated 19mm marine ply for the timber backing pads and steel plate instead of washers with through bolted fittings. Many thanks for your thoughts.

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