Sizing Backing Plates

cliffg · Post by **cliffg** » Thu Jul 20, 2006 10:27 am

Is there any rule of thumb about how big a backing plate should be? Obviously at least as big as whatever it is backing up, but should it be 1/2 again as big, a little bit bigger, a lot bigger?

Any other advice is welcome?

Post by **Tim** » Thu Jul 20, 2006 11:54 am

Oh, I don't know about a specific rule of thumb--I'd say "whatever looks right", but that't not really that helpful! Pperhaps an overall size around 30%-50% larger than the largest dimension of the fitting makes sense. This means that for a 3" diameter fitting, you might end up with a 4-1/2" diameter backing plate. Or thereabouts.

You want enough backing plate so that your fasteners and any washers aren't too close to the edges, so allow for something like 1/2" beyond the outer edges of your washers...generally speaking.

The more stress the fitting is on, particularly for something where the anticipated load might be close to straight up, the larger the backing plate. Shear loads require less of a backing plate because the stresses aren't pulling directly against the fitting and are not trying to pull the fitting out of the deck quite so much.

For something like an outboard bracket, I'd go with a very large plate since these tend to create a lot of stresses in many different directions.

And of course, sometimes you're limited by the existing space, so you do the best you can.

Hope this helps!

Figment · Post by **Figment** » Thu Jul 20, 2006 12:05 pm

A lot of backing plates I've seen are way oversized. That is to say that the material from which the plate is made cannot effectively transfer the load that far away from the fastener.

(based on L/1440 allowable deflection, which seems appropriate to the purpose of a backing plate for a winch or lifeline staunchion.)

Plywood can't really transfer a serious load farther than 3x its thickness.
Steel can go 10x or 12x its thickness.
I don't have any good numbers for aluminum, but off the top of my head I'd say 5x thickness.
I dunno how stiff "starboard" is, but I'd guess it to be less so than plywood. 2.5x thickness perhaps?

cliffg · Post by **cliffg** » Thu Jul 20, 2006 1:39 pm

I'm not sure I understand about transferring the load away from the fasteners. In my case I'd guess I'll make the backing plate as big as practical.

Thanks as always for the help.

Post by **dasein668** » Thu Jul 20, 2006 3:17 pm

Mike's trying to get himself banned to the nerdery section again.

I think what he's saying is that, depending on the material used for a backing plate, there is a point at which making the backing plate larger will have no effect on spreading the load. As I read it:

If you are fastening a single fastener through the deck with a half inch plywood backing plate and a one inch diameter fender washer, the optimal size for that backing plate would be 4 inches in diameter (.5 inches thick x 3 = 1.5 plus .5 inches for the radius of the washer = 2 inches radius, 4 inches diameter). If it was larger, it would not lessen the point load on the deck appreciably more than at 4 inches.

Is that right, Mike?

I'd tend to err on the side of too large, though. I don't think there would be a real drawback besides wasted material.

Figment · Post by **Figment** » Thu Jul 20, 2006 5:24 pm

Right.

It's probably worth saying that L/1440 is pretty stiff.
This presupposes that the deck (or other substrate) has sufficient strength on its own to support the load, and that the purpose of the backing plate is simply to prevent the fasteners from working themselves loose under cyclic load.

(NERD ALERT!!!!!)

Post by **Tim** » Thu Jul 20, 2006 8:37 pm

Figment wrote:A lot of backing plates I've seen are way oversized. That is to say that the material from which the plate is made cannot effectively transfer the load that far away from the fastener.

(based on L/1440 allowable deflection, which seems appropriate to the purpose of a backing plate for a winch or lifeline staunchion.)

Plywood can't really transfer a serious load farther than 3x its thickness.
Steel can go 10x or 12x its thickness.
I don't have any good numbers for aluminum, but off the top of my head I'd say 5x thickness.
I dunno how stiff "starboard" is, but I'd guess it to be less so than plywood. 2.5x thickness perhaps?

Showoff.

Figment · Post by **Figment** » Fri Jul 21, 2006 10:10 am

hehe.

Of course, after I had posted that first reply I noticed that in the time I spent typing, you had simply answered the question, but I wasn't about to delete it after noodling all of that out.

I'm just a menace.

cliffg · Post by **cliffg** » Fri Jul 21, 2006 10:18 am

Guys, I am a retired high school history teacher and all this is way over my head. I'm not sure I am qualified to ask questions on this board!!!!!

Seriously, thanks for all the responses. I will make the backing plate a couple of times bigger than the hardware being attached, space permitting. After all it's the transom and there isn't endless room. However, I will take into account the information provided and try (using my pea sized brain) to apply it.

Post by **Tim** » Fri Jul 21, 2006 12:42 pm

Cliff,

Don't let the nerdy tech stuff get to you. I don't know about any of that either and have never heard of L/1440 or whatever! There are lots of ways to "nerdify" things about boats and sailing if one so chooses.

I'm a seat of the pants, hands-on guy, and I go with experience and what looks good for most things. Trust me: you don't need an engineering background to do this work--unless you truly do need engineering work for some project, in which case I would go out and ask one of them smart guys what I needed to know. Every person should know their limitations, but limitations aren't always limiting. (I'm on close speaking terms with Yogi Berra.)

When in doubt, just make things bigger and stronger than you think is necessary. Conservative construction never hurt anyone. Make that backing plate as large as you want, and no harm will come.

Figment · Post by **Figment** » Fri Jul 21, 2006 3:10 pm

Right on.

I hadn't intended to suggest that there is anything wrong with a huge backing plate. Rather that if it looks big enough to do the job, it's probably big enough and then some.

And because I don't know who really is reading this board and how seriously they might be taking my babblings, I should probably declare that those numbers above are just my personal semi-engineered thoughts. I've no idea how they compare to what the ABYC or any other knowlegeable body might have to say on the topic.