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The Critical World Of Backing Plates

Avoid serious boat damage and personal injuries by ensuring your hardware is properly installed. Here's how.

Damaged backing plate

This cleat lacked a backing plate — it was installed only with nuts and washers. When tightened, the nuts compressed the balsa core.

After major storms, you hear a lot about broken lines on boats. But a related issue that members of the GEICO | BoatUS Marine Insurance Catastrophe (CAT) Team also routinely find are cleats torn from decks. When you see TV storm coverage showing boats being pummeled in marinas, remember that a lunging 10,000-pound boat puts a tremendous load on any cleat used to tie it off.

So why do so many cleats fail? Inadequate installation, to put it bluntly. In the worst-case scenario, the cleat is simply screwed in place with large self-tapping screws. In other cases, the cleat is thru-bolted but has only a nut and washer on the underside. Bolts are stronger than screws, but multiple fasteners don't always 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. I'd like to say that all hardware on all boats is installed properly, but I would be wrong. And a bigger problem is that you can't necessarily tell how a cleat is installed; from the outside, they all look OK. What you need to look for is a backing plate installed under the cleat.

Damaged cleat

Here’s another example of what can happen when a cleat is not properly installed using a backing plate.

Backing plates are installed on the inside of fiberglass moldings such as a deck or cabin top. They spread the load on the fitting or hardware. How does this help? The shock load on that hardware is dispersed over a much wider area. For example, a cleat thru-bolted with two 5/16-inch bolts with washers under the nut provides just over 2 square inches of surface area to resist pull-through. A simple 3-by-6-inch backing plate provides 18 square inches to resist that pull-through, almost nine times as much.

Lack of backing plates isn't limited to cleats, either. Handholds and stanchions can also be placed under heavy loads when people grab or fall against them. Making an emergency grab for a handhold to prevent going over the side or keep yourself from a serious fall or injury is not the time to find out how well the hardware in question has been installed on your boat! So check those for proper backing plates, too.

Whether you're checking hardware that's already installed or perhaps adding a new cleat, stanchion, or bimini, it's critical to ensure the job is done properly and safely. Here's what you need to know.

Backing Plate Materials

In a new installation or retrofit, the choice of backing-plate material is the first step. There are four main types of backing plate materials: plywood, aluminum, stainless steel, and G10 fiberglass board.

Boat with backing plates

The average boat has plenty of hardware, all requiring backing plates.

A common choice for making backing plates is marine plywood. The plywood should have no voids and multiple thinner plies. Above all, it should be made with waterproof glue. Typical thicknesses are 1/2-inch or 3/4-inch hardwood plywood. There is a special case for using thinner plywood, but I'll get to that later. 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 seal the plywood surfaces but then drill fastener holes. A leaking fastener allows water inside the backing plate, delaminating or rotting it. Fastener holes should be drilled oversize and then filled with epoxy. I place the plywood backing plate in position and mark the hole location on the plywood. Using a hole saw or spade bit, I drill out the hole locations. Tape the bottom side of the plate to close off the holes, place on a level surface, and fill the holes with epoxy filler.

Tip

Not all backing plates are square. A large hole saw makes very neat backing plates for winches and stanchions.

There are a host of epoxy fillers on the market, but my choice is West Marine 105 resin, slow or fast hardener, and 406 colloidal silica filler. The oversized hole is filled and allowed to cure. The fastener hole is then drilled through the epoxy plug, resulting in a well-sealed backing plate.

The second type of backing plate material is aluminum. Aluminum won't rot or delaminate, and it can be worked with common woodworking power tools such as skill- and jigsaws. Use 1/4-inch minimum, 3/8-inch if needed. A common alloy is 6061-T6 extruded aluminum bar stock. A 3-by-6-by-3/8-inch piece will set you back less than $7 at current prices online. Most online suppliers will cut it to your exact length, eliminating any need for cutting. Grind the edges to a taper similar to the plywood plates.

A third option is stainless steel. Stainless is harder to cut and drill, but it's stiffer and stronger than aluminum. If you have to cut it, use a fine-toothed metal blade in your saber saw and cut slowly using plenty of oil or other lubricant to keep the blade cool. Use the same procedure when drilling. Quarter-inch stainless should be more than adequate, however, it is more expensive than aluminum. A 3-by-6-by-1/4-inch piece costs about $18. Again, online suppliers will cut it to length for you at only a marginally higher price.

G10 backing plate material

G10 is an ideal backing material.

The final option is a material called G10 fiberglass board. This material consists of layers of glass cloth bonded together with epoxy resin. The result is a stiff, strong material that is corrosion-proof, won't delaminate, and can be worked with most power woodworking tools. In the majority of cases, a 1/4-inch thickness is sufficient. If more strength is needed, two 1/4-inch layers can be bonded together. A 3-by-6-by-1/4-inch will cost $11.99 from a marine supply catalog. I much prefer this material due to its ease of use and lack of any corrosion possibilities.

Some people use 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 big drawback, though: It cold flows under pressure. Thru-bolted, it will deform over time and loosen up. I recommend not using it.

Prep Work

At this point, a lot of people simply bolt the backing plate under the fitting and let it go at that. But to do it correctly, you need to explore the surface you're mounting the fitting on. Most decks use a core material between an inner and outer layer of fiberglass. If there is core material in the area you're installing into, remove it and replace it with layers of solid resin/fiberglass or a plywood or stainless metal insert or plate — something that won't crush. Thru-bolting a fitting through core material will likely crush the core. Also any future leaks around fasteners will lead to soft or delaminated core.

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 instead be bedded in something that will bridge those high spots and give it a good bearing surface. You could sand the surface level, but access is often too limited to make that practical.

Large fender washers

Install using larger fender washers

Large fender washers also assist in spreading the load.

The best solution is to butter the backing plate with epoxy filler and lightly bolt it in place. The bolts should only be tight enough to evenly spread the epoxy. Don't forget to coat the bolts with petroleum jelly so you don't epoxy them in place. I use the same West epoxy mixture used above but mix it to a thicker consistency. Once the epoxy has cured, proceed to mount the fitting.

Installation

With a firm and level bed for the backing plate and the backing plate epoxied in place, you can now finish the installation. Start by taping around the area where the fitting will be mounted. Place the fasteners in the fitting and set it into position. Trace around the outside edge of the fitting. Remove the fitting and cut away the tape underneath. This will make cleaning up any squeeze-out much easier.

Backing plate installation

Careful installation will ensure strength and longevity with no leaks.

There are several choices for sealant. 3M5200 and 3M4200 are aggressive adhesives as well as sealants and might require something like DeBond if you ever have to rebed the fitting. Other choices might be LifeCaulk or butyl tape. Apply a generous layer of sealant where the fitting will be mounted.

Visit BoatUS.com/Expert-Advice and search "boat sealants" to find out which sealant is right for your project.

Apply sealant around the head of the fasteners and place them in the fitting. Apply more sealant around the fasteners on the bottom of the fitting and press a small neoprene washer on the fastener (available in the miscellaneous parts drawers in big box stores). This gives you something to tighten down on when bolting the fitting. Once in place, tighten up the nuts from underneath, but don't overtighten! The worst thing you can do is squeeze out all the sealant leaving a thin layer that will rupture and leak.

Likewise, don't 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.

Backing plate edge treatment illustration

Preparing the mounting surface illustration

Cross-section of installed cleat illustration

Stern cleat installation illustration

Curved backing plate illustration

Working With Curved Surfaces

Now for that special case I mentioned earlier. To this point, we've been talking about installing a flat backing plate against a flat surface. What about when the backing plate needs to be curved, such as a cleat mounted in the curved bow deck? In this situation, I'd use three layers of 1/4-inch plywood. Coat each layer with epoxy adhesive, then bolt the sandwich in place. Place plastic film between the first layer of plywood and the fiberglass surface. This will allow you to remove the curved backing plate, once cured, for sanding and epoxy coating. Then install as described above.

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

Dock Cleats Need Backing Plates, Too

It's critical to pay attention to how dock cleats are attached. Like the cleats on your boat, dock cleats should always be thru-bolted to the dock with backing plates, never just attached with wood screws or lag bolts. Because of the proximity to the water, dock cleat backing plates should ideally be made of stainless steel to avoid rust. Aluminum may corrode severely, and wood is likely to split or rot.

The planks to which the cleats are attached should be thru-bolted to the underlying support beams, also with suitable backing plates, to prevent decking from being torn off when cleats are subjected to heavy loads. If stainless steel plates are not feasible for some reason, planks should be thru-bolted using heavy extra-large stainless steel washers, suitable for the potential loading, under each nut.

— Mark Corke

Author

Paul Esterle

Contributor, BoatUS Magazine

Paul Esterle has built and refurbished numerous boats and is a prolific boating writer and author of many excellent boating books and articles. Visit his website ­thevirtualboatyard.com to see some of his projects as well as links to his books and videos.