Improperly mounted hardware is a constant source of frustration for boat owners. Sometimes it's water leaking in through a bolt hole, sometimes it's rust bleeding onto the deck, sometimes it's a cleat that tears off its mount under load.
Here's how to mount hardware on cored fibreglass decks correctly, so you won't have to deal with it again- and a few examples of why things go wrong otherwise.
The Right Way
(Can't see the pictures? Click here, then go update your browser- if you're running a browser old enough to not recognize SVG, it's old enough to be a big security risk.)
A good hardware installation features:
- Solid fibreglass replacing the core underneath the hardware. On very high-load fittings such as windlasses, winches and mooring cleats, the solid area should extend well beyond the footprint of the hardware.
- Bedding compound between the hardware and the deck to stop water from seeping under the fitting.
- A hefty metal backing plate to spread the load of the bolts over a large region. I have seen some builders laminate the plate into the last layer of fibreglass, which works just as well but makes it a bit harder to repair if it rusts out later.
Sounds simple, right? Let's look at some common ways that it goes wrong, and what happens in those cases.
The Cheapskate Production Line Way
Cheap boats are cheap because corners get cut during construction. See if you can spot a few errors in the installation shown above. I'll wait.
Ready? The flaws are:
- Bolt holes go through core. Any water that gets in will get into the core. Wet core loses its strength and eventually turns to mush, allowing the deck to sag and hardware to rip out. This ends with a very expensive deck replacement- if not a total write-off of the boat.
- No bedding sealant, so water will have an easy time getting in.
- Washers are too small. Pull hard on this cleat and the washers will punch right through the inner skin, crush the core, and tear the outer skin from the deck. Composites don't handle point loads very well; they hold up much better when the loads are spread over large areas.
If a new boat you're interested in has a lot of hardware installed like this, just walk away. You'll spend far more time and money patching leaks than you would have spent to get something better in the first place.
The Overkill-In-The-Wrong-Places Way
This looks better, right? It's just like our "good" example, but with more sealant and without the solid glass.
Err, no. This installation is doomed to a very short life once the sealant starts to age.
- Sealant on the backside will trap water. Outer seals always, always fail at some point. In this case, their failure will allow water into the core. The inner sealant will stop that water from draining, so you won't know anything is wrong.
- The bolts will corrode through in short order. Water trapped against metal in an enclosed space is a recipe for rapid corrosion.Stainless steel in a damp, stagnant, oxygen-poor environment does not last very long. Your first clue that anything's wrong will be a slight rust weep if you're lucky, or a cleat whipping across the pier if you're not.
Never seal the back side of a through-deck fastener. If water does get through the outer seal, you want it to drain, and you want it to drip where you'll see it. That way, you know to fix it before things turn bad. (If you're buying a boat and its fasteners are back-sealed, you'd better remove a few of them to check for problems before closing the deal.)
No matter how much sealant you lather on, the core itself still has to be sealed against moisture with a chunk of solid fibreglass resin. Exposed core in the bolt holes is a recipe for expensive failure down the road.
The Good Enough For Low Load Way
Hardware that won't see high loads may not need the full, beefy solution. If you're mounting a VHF antenna, a cupholder or some other low-load fitting, this technique can work. You drill the holes oversize, ream out a bit of core, back the holes with masking tape, and pack them full of thickened epoxy. Then you re-drill the holes and mount the hardware.
It's considerably stronger than the cheapskate method, because the epoxy rings don't compress like core does and they'll share the load between the two skins. Don't use this for anything that'll see high loads, though. The stresses are still concentrated around the bolt holes- you need a backing plate to prevent that. If you mount a cleat like this and drive off from the dock without untying the line, the cleat will probably rip out of the deck, bringing the epoxy rings with it.