Engineering backing plates

[JonnyBoats]

When making backing plates for deck hardware, are there any formulas or rules for determining the ideal size, type and thickness of backing plates?

[Hirilondë]

So much of determining size for backing plates is the deck/hull make up. When in doubt of what I need I just make it the same shape as the hardware base and a little bigger.

[Tim]

We've touched on this before. I think we determined that there's actually an engineering point of "no additional return" in terms of sizing--that is, larger only helps to a certain point, after which going larger has no particular increase in effectiveness.

I think consideration of the actual, realistic loads (no, not numbers--just the real way the hardware may be strained) has a lot to do with thinking about how large or beefy to make a backing plate.

Certainly anything that will receive a load in tension--that is, directly against the backing plate--will require more strength than something that receives only a shear load, since the direction of pull is directly attempting to pull the hardware right out of the deck (or whatever). Shear loads, with the stress roughly perpendicular to the fastener direction, still benefit from backing plates in terms of distributing the load, but the strains are much less. Dynamic loading requires more strength than fixed, static loads. So mooring hardware and anchor hardware requires large and strong backing to help spread the stresses of periodic heav loading, followed often by a release of the load completely. Mooring and anchor hardware is also a shear-type installation. Not many cleats would hold as much as they do if the load on the cleat was actually in tension.

Things with many fasteners often don't really benefit from backing plates at all--long tracks, for example. The loading is spread over so many closely-spaced fasteners that fender washers are typically adequate. Of course, it never hurts to have backing plates for any hardware.

I would rarely see a need to go much larger than just oversize of the hardware base itself--say, 1/4" to 1/2" larger in all dimensions, or enough so that any washers you use to secure the fasters bear completely upon the backing plate, unless available space makes this impracticable.

One doesn't need formulae; common sense and experience rule the day here. But then again, I almost always feel this to be the case, so maybe I ought not to post in the "nerdery" forum!

[Figment]

Yes, we went around this one last year. Click here for a very nerdy discussion that pretty much agrees with Tim's Common Sense above.

Depends on the deck.
Depends on the plate material.
Depends on the nature of the anticipated load.

[JonnyBoats]

Interesting thread.

One thing I did not see discussed is the thickness of the backing plate.

Also I _assume_ that just as with bulkheads where one does not want to create hard spots on the hull, there must be something similar with backing plates and fittings?

[Ceasar Choppy]

Interesting thread.

One thing I did not see discussed is the thickness of the backing plate.

Also I _assume_ that just as with bulkheads where one does not want to create hard spots on the hull, there must be something similar with backing plates and fittings?

Unless your deck is flexing and soggy (two obviously undesireable features), I don't think you need to worry too much about "hardspots," like you might if you were tabbing something to the hull. Hardspots are usually cause by improperly tabbed bulkheads or stiffeners and there are different loads and stresses associated with those types of fixtures than with something mounted to your deck.

The only reason you would need to go thick would be for strength, so it would depend on the material. Metal backingplates don't need to be as thick as FG backingplates or wood.

Going back to the discussion of fiberglass backingplates, I've used 3/8" thick electrical grade FG backing plates on my forward cleats and chocks on my 39' boat. I would consider 3/8" to be as big as you would ever want to go unless you were using something like toilet paper perhaps. :)

[Figment]

John, let's narrow the field of discussion. What material would you LIKE to use for backing plates?

[JonnyBoats]

John, let's narrow the field of discussion. What material would you LIKE to use for backing plates?

Well at the moment I am mounting a Sampson-Lawerence Seatiger windlass which has 4 5/8 inch bolts coming through the deck. It is mounted over the V berth, so it wouldn't be a bad thing to make it look good. I _assume_ that large fender washers on the bolts are not sufficient. I have 300 feet of 3/8 inch BBB chain and 45 lb CQR Bruce and CQR anchors as working anchors. The boat weighs 18,000 lbs before I load any gear, so in a storm I imagine there could be quite a load on those bolts if the chain snubber broke for some reason ( I know that the windlass should not take the load under normal conditions).

A stainless backing plate would probably look the best, but then again it might cost alot to have made even if I could get a piece of plate cheap.

Suggestions?

[Figment]

If ever there were an application for one hella-stout backing plate, that'd be it!

My gut says: 1/8" stainless plate, 4" larger than the windlass base.

To refine what my gut thinks, how high is the top of the gypsy from the deck, and how far is the gypsy offset from the centerline of the windlass?

[Hirilondë]

I know that the windlass should not take the load under normal conditions

Maxwell says right in their manuals not to use your windlass to secure your chain at anchor. But I don't think it is because of the strain on your deck. If you don't install it well you will rip it out of the deck pulling the anchor up. It is the long term fluctuations in strain that will damage the windlass itself.

If ever there were an application for one hella-stout backing plate, that'd be it!

Yeah, really! You can't go wrong spending a few bucks on some good stainless stock for a backing plate here. You can buy 12" x 12" pieces from McMaster-Carr and you can cut it with a jig saw and a good metal blade.

[kabauze]

Another great and fairly cheap source for small orders of stainless bar & plate is Online Metals. For small pieces of stock, they are cheaper than anything else my machinist can find.

[JonnyBoats]

Well here is what I did for backing plates. The windlass is mounted with 4 5/8 inch bolts, 2 in front and two in back. I purchased a 4"x23" 1/4" thick 316L piece of stainless plate at a surpluss yard for $24. Then I took it to a shop with a plate shear and had it cut in half lengthwise yielding 2 11.5"x4" backing plates. Cutting cost $5.

Next I took them to a machinest who drilled and chamfered the four holes and rounded all the corners. He charged me $25.

So for $54 dollars I have a pair of solid backing plates that are utterly georgous! Since they will be ver our heads when sleeping in the V berth, the price seems well worth it to have something that looks good as well as being functional.

My next question is how to properly bed them against the underside of the foredeck? I _assume_ that I should use epoxy mixed up with a a high density filler (such as West 404), correct?

Also when bedding them do I want to use some sort of release agent so that the backing plates will be removable?

[Hirilondë]

404 is designed for adhering to metals, so if you are trying to cast a smooth surface to mate your backer to with the intent of it being removable I wouldn't use it. Unless the underside of your deck is so irregular that a molded surface is really needed I would just bed the stainless on with Sikaflex or some other mildly adhesive bedding of choice.