A reader recently wrote in, asking about backing plates: if they were necessary and what material to use in making them.
All of our boats have “things” bolted on, cleats, chocks, biminis, T-Tops, rod holders, anchor brackets, winches and handrails for a few examples. Some of these things will have to withstand a significant amount of force or weight being applied to them and so need to be sturdy and well mounted.
Some boat builders are great at properly installing this hardware while others don’t do as good a job. Moreover, many items are added after the boat leaves the factory, either by the dealership or by the boat owners themselves. It is in your best interest to inspect this hardware and correct any weak areas.
Backing plates are flat plates placed under a component being bolted to a boat. The plate is inside, hidden from normal view. It spreads the load applied to the item to a larger area, increasing the load capacity of the hardware mounting and protecting the surrounding fiberglass hull from fatigue cracks, or worse yet, having hardware pulled off the deck.
These plates can be cut from any strong material, such as aluminum, stainless steel, plywood or StarBoard (R). If you choose to use plywood for the backing plate, consider at least half-inch thick plywood or better yet, three-quarter inch thick stock. The plywood should be exterior grade, with no voids in the inner plies.
The plywood should be well sealed with epoxy to keep from soaking up water and rotting. When installing, drill the fastener holes oversize and fill them with epoxy. Re-drill the holes through the solid epoxy. This will prevent water from soaking into the interior of the backing plate and rotting the plywood.
The edges of the backing plate should be tapered at least at a forty-five degree angle. This helps eliminate a hard spot at the very edge of the backing plate. If the fiberglass is somewhat flexible and the edges of the plate are square, there is a likelihood of stress cracks developing around that junction.
Stainless steel backing plates are a good choice since the fasteners used to mount the hardware are also stainless (or should be). The problem with stainless steel is that it’s hard to cut and drill. Use a new metal cutting blade in your saber saw and keep the blade cool with WD-40 or the like. When drilling, start with a new, sharp, small drill and work up in size to the finished diameter needed. The steel should be at least an eighth of an inch thick to resist flexing.
Aluminum backing plates are common as the aluminum is much easier to work with than stainless steel. The thickness should be at least a quarter of an inch, again to resist flexing. Coat the fasteners with an insulating compound, like Tuf-Gel, to eliminate the corrosion from dissimilar metals being in contact.
StarBoard (R) makes good backing plates as it can be worked with regular woodworking tools, doesn’t need sealed or finished and won’t rot. It is a little flexible, so use a bigger and/or thicker size than you would for metal. Taper the edges as done with the other backing plates.
Recently, I've started using a material called "G-10" for my backing plates. G-10 is an epoxy/fiberglass laminate and is dense and very strong. It is easier to cut than stainless steel and doesn't pose any corrosion issues. I've been using 1/4" thick pieces I've found, at a good price, on Ebay.
There aren’t any hard and fast rules about the correct sizing of backing plate (at least that I can find). If in doubt, make it bigger, The plate should fit tight against the underside of the structure it is reinforcing, If there are bumps or irregularities in that surface, apply a layer or epoxy filler and temporarily bolt the backing plate in place. Once the epoxy has cured, you can proceed to mount the hardware. The item being mounted should be through-bolted, with large fender washers and locking nuts.The hardware will need to be properly bedded in place with sealant. The most common problem in the bedding process is to tighten the fasteners too tight, squeezing out all the sealant. This practically guarantees that the bond between the sealant, the hardware and the deck will soon rupture and be a possible source for leaks.
A better method is to place a small rubber washer under each fastener. Then tighten down the fastener until the hardware bottoms out on the washer. This will allow a decent thickness of sealant to remain under the hardware.
Take the time to inspect your deck (and other) hardware before problems crop up. Repairing pulled out hardware or cracked decks is much more expensive than installing backing plates. Not to mention that failure of a critical piece of gear can put you over the side or your boat on the rocks.
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