Mounting hardware on cored decks: Right and wrong ways

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.




This is a really nice

This is a really nice article. Thanks!

Would it be reasonable to beef up the "good enough for low load way" with a backing plate?

Backing plates

Matthew's picture

Yes, a backing plate would certainly help in the final case, making it almost as good as the ideal case (although I would still go with solid laminate, or at least a few extra layers of laminate, around the really high stress areas like winch bases and mooring cleats). A large, stiff backing plate, which spreads a concentrated load over a large area is ALWAYS better than washers- and, similarly, large stiff washers are much better than small thin ones. The only downside to backing plates is that it takes extra time to measure and drill them.

Hardware mounting

Matt, this is excellent information that is very well presented to all.

The dilemma with our project was to try to match the strength of the hardware. Therefore we replaced the core with ply around the hardware location, added a extra large ply backing area, both penetrated with 25mm glass/epoxy rods at each screw location. A 10mm plate was buried in a matching composite area under the large backing area. The epoxy rods were clearance drilled down to the plate, which was set drilled and tapped for the 12mm cap socket bolts resulting in a bulky but smooth are under the deck with no protruding nuts.

If you have the time please see:

Re: Hardware mounting

Matthew's picture

If you need to mount hardware to a cored deck without any protrusions (nuts, etc.) on the back side, Tom's way is probably the best way to do it. It's labour intensive but, if done properly, is obscenely strong. Liberal use of anti-corrosion agents (tef-gel, etc.) would be prudent when inserting the fasteners, just in case water does seep in at some point- it's important to ensure that there's no way for water to sit, stagnant, in contact with the metal.

Tom's Scrumble Project site is an excellent few weeks' worth of reading for anyone considering a large self-build boat. He and Judy have meticulously documented every detail of their big Oram cat's build process, the ups and the downs- it will give you a good feel for how much work is involved in a project like this, as well as being a useful reference for all sorts of tricky construction details.

Mounting Hardware on Teak Decks

When mounting hardware on a teak deck, do you cut back the teak and mount on the fiberglass beneath or mount directly on the teak. Does it make a difference if it is a high load piece of hardware like a cleat or a low load like a pad eye?

Teak decks

Matthew's picture

Hi Stan,

Cutting back the teak to mount on the fibreglass underneath would, I think, put the hardware in a recess that would tend to trap water. That seems like a recipe for corrosion and eventual leaks. I'd rather see the hardware mounted flush with, or slightly proud of, the surface of the deck.

If I were designing a moulded fibreglass deck to take a teak overlay, I'd probably include cleat mounting pads of solid fibreglass that stick up to the level of the surface of the teak. That way, you can repair the wood finish without removing and re-bedding the hardware.

Waterproofing hardware on teak decks

A very good article, we have always drilled, epoxied & redrilled for mounting anything on deck. Normally we measure for a large thick SS backing plate and find a machine shop to do the actual cutting and drilling for us. I always keep an eye peeled for scrap SS pieces others have discarded and use these for my backing plate stock. We also use SS nylon locking nuts and Loc-Tite (Overkill - I know) but I feel better when the ---- hits the fan.

Nice articles here,


Matthew's picture

Nylocks plus Loc-Tite? I wouldn't call that overkill. That's just being cautious. And yes, it's really quite amazing what you can find in the scrapyard.

I forgot to ask this question in my last post.

I am looking at purchasing another boat and it has teak decks.
How would be the best way to re-bed the existing hardware on the teak?

Teak decks

Matthew's picture

As far as I know, it's perfectly OK to mount hardware on top of teak.

The holes through the fibreglass should, if it's a cored structure, be reinforced and sealed as described earlier. The portion of the hole that's surrounded by teak would be drilled a little bit oversize, just enough that the bolts aren't in direct contact with the teak. You'd seal the hardware against the surface of the teak, as usual, and the shaft of the bolt would be left bare (unsealed) so that if the seal does fail, the water will drip inside the boat- where you can see it- instead of pooling around the bolt where it can do damage without being noticed.

laminate thickness

I am using a pop up cleat on a boat with 2 1" diameter fastener holes. The load is 5,400 lbs (ABYC rule for 32 foot boat). How do I determine how thick the fiberglass should be?

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