Backing Plates

How well has the hardware has been installed on your boat?

Article by Paul Esterle

Sooner or later, you are going to approach a dock faster than you intended. Or you might have to pull your boat onto a trailer at a steeper than normal ramp. You may have to make an emergency grab for a handhold to prevent going over the side or keep yourself from a serious fall or injury. In all these cases, you are about to test how well the hardware in question has been installed on your boat.

In other cases, you may be adding a new piece of hardware; a cleat, eye, stanchion or bimini. In those cases you will want to ensure you are doing the job properly and safely.

Welcome to the critical world of backing plates.

Backing plates are installed on the inside of fiberglass moldings such as a deck or cabin top. They are there to spread the load on the fitting or item of hardware. If your fitting has no backing plate or a poor one, you run the risk of actually pulling the hardware’s fastener through the deck or cabin top. I’d like to say that all hardware on all boats is installed properly, but I would be wrong.

In a worst case scenario, the hardware is simply screwed to the deck with large self-tapping screws. Loading the hardware can pull the screws right out of the fiberglass molding. In other cases, the hardware is through-bolted but there are only washers under the nuts. The bolts are stronger than screws, but the multiple fasteners don’t support each other. Pulling sideways on a cleat, for example, will load the far side fasteners, pulling them through the deck before loading the near side fasteners,

In a quality installation, there will be a backing plate underneath the fitting, inside of the molded fiberglass. It will be large enough that all fasteners pass through it and the fitting itself will be through bolted with bolts or machine screws and nuts. It would be a very good idea to inspect all your fittings and evaluate the installation. Retrofit backing plates where needed.

Materials

In a new installation or retrofit of an old one, the choice of backing plate material is the first step.

A common choice is marine plywood. The plywood should have no voids and multiple thinner plies. Typical thicknesses are 1/2" or 3/4" hardwood plywood. The edges of the plywood should be tapered to prevent forming a hard spot in the fiberglass around the edges of the plate. The plywood should be coated with several coats of epoxy to seal out any moisture.

Most people remember to do the sealing of the plywood surfaces but then drill fastener holes through the plywood. A leaking fastener will then allow water inside the backing plate delaminating or rotting it. Fastener holes should be drilled oversize and then filled with epoxy filler. The fastener hole is then drilled through that epoxy plug, resulting in a well-sealed backing plate.

Aluminum backing plates are sometimes used. The aluminum is inexpensive and easy to cut. I get most of my aluminum stock from an online metals supplier. They have it in a variety of thicknesses and widths. Most will cut to order for you so you can order the aluminum the proper length and width for use as a backing plate. Say 1/4" thick by 3" wide and 6” long. It’s not that much more expensive when purchased that way and is ready for your use.

Grind the edges to a taper similar to the plywood plates and drill for the fasteners. One problem with using aluminum for backing plates is that the fasteners will be stainless steel and will corrode the aluminum. Use a material like Tuf-Gel to eliminate contact between the two metals.

Using stainless steel for your backing plates will eliminate the corrosion problem but the stainless steel is much harder to cut. Use a fine tooth metal blade in your saber saw and cut slowly. Use plenty of oil or other lubricant to keep the blade cool and lubricated. You can probably go a little thinner with the stainless steel stock, say 3/16" as opposed to 1/4". Again, an online supplier can cut it to length for you.

Some people consider using a polymer material such as StarBoard as a backing plate. It is easy to cut, won’t absorb water and doesn’t require epoxy sealing. It does have one drawback, however. It cold flows under pressure. Through bolted, it will deform over time. I recommend not using it.

That brings us to my favorite backing plate material, G-10 fiberglass. G-10 is available in a wide range of sizes and thicknesses. It is made from multiple layers of glass cloth and epoxy or polyester resin. It won’t compress or cold flow. You can cut it, slowly, with standard wood working tools; although I make sure I only use carbide bits and blades.

It won’t absorb water so doesn’t need to be finished. I use 3/8" or 1/2" thick G-10 which I find on eBay. You can also order it from suppliers such as McMaster-Carr. I do taper the edges like plywood backing plates.

Prep Work

At this point, a lot of people simply bolt the backing plate under the fitting and let it go at that. You need to explore the surface you are mounting the fitting or retrofitting an old one to. Most decks use a core material between an inner and outer layer of fiberglass, If there is core material in the area you have an old fitting, or are going to install a new one, you really should remove that core material and replace it with layers of solid fiberglass or a plywood or metal insert. Through-bolting a fitting through core material will likely crush the core. Also, any future leaks around fasteners will lead to soft or delaminated core.

You should also take a close look at the surface your plate will be resting against. It will probably be rough, uneven fiberglass and the backing plates may only bear on a few high points. The backing plate should be bedded in something that will bridge those high spots and give the backing plate a good bearing surface. You could sand the surface level but access in most cases will be too limited to make that practical. They need to be bedded in something.

One solution is to butter the backing plate with epoxy filler and lightly bolt it in place. The bolts should only be tight enough to spread out the epoxy evenly. Don’t forget to coat the bolts with Vaseline so you don’t epoxy them in place. Once the epoxy has cured you can finish the installation.

You could also use a sealant such as 3M5200 or 3M4200. Butter the plate as before and lightly bolt in place. Be sure to allow enough time for the sealant to thoroughly cure. That means at least 7 days for regular sealant and 48 hours for fast cure.

Installation

With a firm and level bed for the backing plate, you can now finish the installation. Use plenty of sealant under the plate if you haven’t already used sealant. I like to place a small neoprene washer or O-ring on the fastener under the fitting. This gives you something to tighten down on when bolting the fitting in place.

Use plenty of sealant around the head of the fastener. Once in place, tighten up the nuts from underneath. Do not overtighten! The worst thing you can do is squeeze out all the sealant, leaving a thin layer that will soon rupture and leak.

Likewise do not turn the bolt or screw to tighten, tighten the nuts only. Rotating the fastener will break the sealant bond you worked so hard to establish. Let the sealant cure for the required time, and then come back later to tighten those nuts. Do it right the first time and you won’t experience any leaks.

Finally

So there you have it, the proper method for installing backing plates, whether new or a retrofitting. Take your time, use the right materials and properly prep the installation site. Your boat will thank you. •SCA•

First appeared in issue #86