Backing Plate Material?

Anyone got the foggiest idea what the strength requirements / point-loading requirements of the various plate materials might be? And more specific and useful, whether this makes much difference for our real-real world applications where we want to spread the load out under cleats, stanchions, pulpits, blocks, and etc. ?

Over the decades I have used 1/4" alum, 3/16" SS, and similar thickness in epoxy ("G10/FR4") plate materials. Often using 3/8" for the G10.

Factory stuff often consists of an Aluminum or SS plate the same size as the item on top, even to the same outline.

So, I put a much larger plate of some kind underneath, and then the factory b/u plate when I re-bolt... (Which is what I did, using G10, for all the stanchions and pulpits last year.) I wonder how much overkill is really enough?

Case in point: while working upside down in the bow the other day, prepping for an oversize G10 plate to go under a new anchor roller, I noticed that the bow cleats are backed up by 1/4" alum. about an inch by four inches. No corrosion, and all appears solid -- this in under the cored part of the outer edge of the bow on each side. Future plan is to replace both those 8" cleats with some 10" cleats and make it a bit... stronger...

In the case of the new anchor roller, the cantilever loads are very very high, and it would seem to call for a lot of load-spreading underneath. The cleat loads are mostly in shear, though.

Your "material" thoughts??

Thanks so much.

Loren in PDX

Well Loren you talk a lot about strength but what about weight and cost? All that extra SS adds up. Raises the COG also. If we take your methodology to the nth degree, we end up with a tank
I would think that 1/4" aluminum would work fine but now you have to consider corrosion between the 2 metals. How about Monel or carbon fiber? Now we are building expensive boats instead of heavy tanks.

Has anyone seen a backing plate fail from loading? If so, was it reasonable?

I haven't done any stress-testing but my boat came equipped with a few aluminum backing plates and a whole bunch of NO backing plates whatsoever.

I have no doubt that the aluminum ones are structurally strong enough, but they eventually corrode and look bad, even if they're painted--a fact which I have tested and confirmed.

The NO backing plates were clearly inadequate. I made a pile of 3/16" stainless ones twelve years ago and while they were tough to drill they are 100% solid and as nice today as they were the day they were installed.

If I had a cheap source of carbon fiber I might consider using it on a future project but otherwise I think stainless is the way to go.

Carbon and Stainless do not like each other. G-10 or starboard is all I use. Starboard is nice because it absorbs some of the shock loads I think. No scientific data to back that up though. Aluminum and satinless well we all know the trouble there. All stainless is great probably just expensive and harder to work with yourself if you dont have the tools. As for your questions Loren I would try McMaster Carr. They may have the specs you are looking for or can refer you to an engineer who could tell you the PSI ratings of these materials.

I once bought a small book at a hardware store that had all sorts of useful numbers for that kind of calculation, and it was not at all expensive. I will try to find it for the title, but I doubt, in your case, it will be very useful.

You would hope that those who design anchor rollers would build it with a surface area such that a balsa deck would not be damaged by typical forces in use. Since you are only talking about reinforcing under the deck, not on top of it, you will be limited by the roller design, so calculations will have limited benefit. The other, bigger, reason is that while you can estimate the force on a roller under normal conditions, shock loads, inherent in boating, are what will do the damage.

Below is my thought process for backing my hardware - the musings of a man rebuilding a boat armed with a physics degree, rather than boat building expertise - and worth what you paid for it.

Increasing the size of the backing plate will increase the area of the deck absorbing a stress load, lowering the stress, protecting the deck from damage. It will do less for a shear load, since the top of the deck will also take the force. Increasing the thickness of the backing plate will protect the backing plate from damage, but nothing else.

But the key point is that the forces being exerted by the attached hardware must be absorbed somewhere. To use winches as an example, when the jib suddenly fills, the load placed on the jib sheet is momentarily huge. As the load is transmitted through the winch to its backing, every component, the clew of the jib, sheet, blocks, sailtrack, winch and backing, deck core, will give a little, mostly the jib sheet by stretching, but also compression of the plywood coring of the cubby hole where the winch is mounted. In the case of the expensive boat with high tech lines that have very little stretch, and polymer coring with very little flex nor compression, every component will have to withstand higher forces - i.e. be built out of stronger, more expensive, components, and still probably have a shorter service life. I have heard stories of winches disintegrating when they became the weak link in the chain.

When I pulled apart my deck, there were obvious signs where the plywood coring had been crushed by the load on the winches which were backed with washers (I do not know how Ericson did it, but that is how the PLO left it). Most of the hardware on the balsa deck had the same problem. None of the stanchions (cored by Ericson with plywood) with full backing plates showed any sign of damage - except those that had been ripped out of the boat completely in the hurricane.

When I rebuild my boat, the coring where the hardware will be mounted will be of material that gives less than plywood (the new materials are not much more expensive, and do not rot). For the winches, stanchions etc. I will back them with a sandwich of plywood and then a stainless steel plate, of comparible size to the harware. That way, the loads caused by the sail suddenly filling in a strong wind, or the 'crew' trying to pull the boat to the dock with a stanchion, or the stress of raising a stuck anchor in a swell, should be absorbed in the plywood, which will be the first point to give, and also easy to replace should it become necessary.

Gareth
Freyja E35 #241 1972

Backing Plate considerations

Limiting factor in most of these applications whether they be tension or shear loads are going to be the fasteners/ bolts. The component manufacturer has designed in the size and number of fasteners so has pre-determined that max. load the component will ultimately withstand.
My preference would be for stainless backing plates matching the fastener material to reduce corrosion. In practical terms though, a well bedded fitting with aluminum backing plates of reasonable size the same thickness as the diameter of the fasteners would seem reasonable.
For strictly uplift loads you might also consider a beam/ like laminated ply sealed in epoxy or teak lumber cut to shape spanning from deck corner to deck corner. This would transfer loads out to the deck hull joint where there's more inherent strength. Assuming the span you're talking with a bow roller well forward is relatively narrow. This would reduce weight certainly compared to stainless steel plate and is much easier to work with.