Graphic by Paul Esterle Aluminum is also used often for backing plates. In quarter-inch thicknesses, it can easily be cut and drilled. The downside is potential corrosion between the stainless steel through bolts and the aluminum. This can be mitigated by placing a hard nylon washer under the stainless steel washer and the nut on the bottom.

Stainless steel sheet can also be used for backing plates, usually in eighth-inch thicknesses. It is much harder to cut and drill but eliminates any possibility of corrosion between dissimilar metals.

Washers – even large fender washers – are not considered appropriate backing plates, especially for high stress applications like mooring cleats.

SEAL IT UP

With all components created and trial fitted, it is time to select a sealant and begin final assembly. There are three basic types of sealants commonly available: silicones, polyurethanes and polysulfides. Silicone sealants, in my opinion, shouldn't be used for installing hardware. It isn't a very good adhesive and doesn't bond to anything very well. It also "poisons" fiberglass, making future painting of that area difficult.

Polyurethanes, such as the 3M's popular 5200, are tenacious adhesives and good sealants. That adhesion characteristic is the problem. Deck fittings should be rebedded every five to seven years. Use 5200 and it will pull up the gel coat before coming loose. A better solution is a polysulfide, such as 3M's 101. It is still a good adhesive and a good sealant, but it is much easier to remove fittings for re-bedding.

Anyone who has used these modern sealants can attest how messy they can be. However, proper preparation can make for a mess-free installation. The first step is to apply masking tape where the cleat will be mounted. Tape several inches back from the actual location (how far back depends on how messy you are).

Graphic by Paul Esterle Put the cleat in its place and trace around it. Remove the tape from under where the cleat will be mounted (see illustration). Place the fasteners in the cleat, and then tape over the top side to keep them in place. Place either a rubber washer or an o-ring on the fastener below the cleat.

Apply a layer or sealant where the cleat will go. Don't scrimp, use plenty. This is the last place you want a sealant failure because you used too little. Place the cleat in place and push it down. Sealant will squeeze out from under the cleat, but that's okay. Apply sealant on the backing plate and push it in place. Add the washers and nuts, and then tighten.

Do not over-tighten! The worst thing you can do is squeeze out all that expensive sealant, leaving a very thin layer in place. That thin layer will soon rupture and leak from the differential expansion of the fiberglass and metal. The washer or o-ring is there to keep you from tightening too much. Clean up the sealant that squeezed out and remove the tape.

Let the sealant cure for the recommended time. You can now go back and tighten up the fasteners slightly. That will further compress the sealant layer, making for an even better seal.

But only tighten the nuts below. Do not allow the head of the fastener to turn, because that will break the sealant bond around that fastener and form a ready path for leaks.

There you have it, Cleats 101. Evaluate the cleats on your boat, and every other piece of deck hardware. If they are undersized or not properly installed, you know what to do.