Yacht decks are typically composed of layers, with the top and bottom layers being paint or gelcoat over a fiberglass skin, and the thicker inner layer being core. The core thickens the deck to increase stiffness, generally using a low density material to keep the resulting sandwich lightweight. These core materials are typically not designed to be wet, so any water intrusion is bad news. A wet core in a fiberglass structure gets softer and can delaminate from the fiberglass skins. The result is a soft feeling deck, which can make the boat slower as it flexes and lead to further structural issues like fiberglass cracking and failure.
The culprit behind wet decks is often improperly installed hardware. If the deck feels soft, it’s usually near a piece of gear that was installed in a hurry. If non through bolted fasteners like self tapping screws are used, the hardware will loosen over time. Similarly, inappropriate backing plates and or washers under the deck mean the deck will crush over time, loosening the hardware as well. Once the hardware is loose, water finds it’s way in and softens to core. Poor or no sealant is another issue, as hardware needs a sealant beeding to keep water out. Something flexible like a polysulfide or rubber is best, as it keeps a tight seal even if the hardware isn’t installed perfectly tight.
So. Assuming the hardware is installed correctly, and the proper sealant used, what else can one do to keep a deck intact? The goal being to first keep water up on deck where it belongs, you can also ensure that if water does make it’s way under deck gear, it won’t reach the core. I like to surround each fastener with epoxy where it goes through the deck, so that any water around the screw isn’t allowed to get to the core.
Here is a quick walk through on how the core was sealed on a 1970 Shields deck, while installing a 2:1 jib sheet system.
The hardware in this case is a Harken cam cleat with an angled riser to get the sheet led to the trimmer at a comfortable angle. You can see I’ve done a quick mask around the area, as there will be several glues and sealants used, which we want to avoid having to clean out of gelcoat. The holes are drilled, in this case at an angle to match the cleat riser.The next step is to drill a much larger hole partially through the deck. This larger hole is going to determine the size of the epoxy plugs that surround the screws. In this case I used a 3/8″ bit to surroud #10 (or 3/16″) screws. When drilling the oversize hole, go through the top skin of fiberglass and the core only; don’t drill through the bottom skin. This lets us have a solid layer of skin on the bottom, to better support the backing plate.
The epoxy plugs work best if they’re larger than the top hole in the deck. This means that the cured epoxy will make a small flange under the top layer of the deck, holding it in place. To make this work I use a small bit in a drill, held at an angle, to “till” the core out of the deck around the hole. When complete, the void under the deck will be around 3/4″ around. This ensures both plenty of epoxy surrounding the screw, and that the plug won’t pull out of otherwise move from the hole. I’ve also heard of people using a angled allen key to scrape the hole out, but I find the drill faster. Caveat: you must be extremely steady with the drill, to avoid going through the bottom skin of the deck, or carving up the top skin. After I’ve done the routing out of the hole, I like to wash the hole and surrounding area with acetone. Give it plenty of time to dry.
Here is the underside of the deck. I’ve taped over the bottom of each hole with two layers of masking tape, after washing the area with acetone to ensure a good tape bond. You really, REALLY, want the tape to hold here. If it doesn’t, you’ll be happily injecting epoxy through deck, right into the cockpit or cabin, probably on top of the owners bunk or a brand new sail or something.
All that work, and the actual epoxy part of the job takes about 5 seconds! I think West 610 is the best for this purpose, but for smaller jobs will often use the Loctite 5 minute self mixing epoxy. Both are far easier than mixing epoxy and loading a syringe. It’s best to fill the bottom of each hole, move on to the next one and then come back and top it off, as there is typically some soaking of epoxy into the core, which shows up as the epoxy settling. Once this is done, double check the seals on the bottom of the deck and walk away. Really, go tune the rig or something, as this will take time to set.
Is it set? Really? Are you sure? Tune the rig again if not 100% convinced. Once it’s set, I will drill the holes for the hardware again. Once the holes are in place, test the fit again. Once happy with fit, I like to use a countersink bit on the holes, to make a small depression in the surface of the deck. This void will fill up with sealant, giving a better seal than simply a micron thick layer between the hardware and deck. Depending on the part I’ll sometimes use the countersink bit on the bottom of the hardware as well, to make a raised dome of sealant to similarly keep water up and out of the hole.
Once the holes are drilled, and the tape removed from the bottom of the deck, add your
sealant. I really like butyl rubber best, but for some fittings-like this angled cleat riser-it’s not the best fit. In this case, it’s because the bottom of the riser has open voids to make it lighter, which means there isn’t a lot of surface area for butyl to spread. For this I used Sika 291, which is pretty good as an all around bedding compound. When installed bedded hardware, tighten the screws only just enough to get the part touching the deck, and then let it cure. Have you tuned the rig yet? Why not do it again? Once the sealant is cured, you can tighten the machine screws down all the way. This gives you a thicker patch of sealant. If you go to full tightness right away, the sealant can be thin, or squeezed out entirely. I never use sealant on the bottom of the deck; if water does find it’s way in, I woudl rather have it dripping out of a screw hole (alerting someone to the leak) than trapped inside the deck