Backing Plate

I am about to mount my new Forespar gimballed stove to the forward end of the keel box cover on my micro-cruiser. The fibreglass is just over 1/8 (4mm) thick. The bracket supplied with the stove takes 1/4" (6mm) bolts.

Are there rules of thumb for the size and thickness of the backing plate for this application? Is there a reference that I can check?

 

Back it up with 12mm plywood and it should be fine

 

...or else get a piece of King Starboard in the same 1/2", lasts forever and no rot etc.

Maki it about twice the size of the mounts for the stove.

 

That's pretty beefy glass thickness. You may even get away with finishing washers depending...

-Tom

 

You guys are guessing. I'm looking for a reference.

Gerr uses a plate thickness of 0.3 - 0.4 x bolt diameter in Elements of Boat Strength. Now I'm trying to interpret his references to plate diameter. And how do I know that the glass is heavy enough?

 

We are not guessing. We are telling you what, as professionals, we would do to install it. You are not designing the Titanic, just hanging a bracket.

 

Gosh- can't a guy have his dreams?


To OP- everyone posted here from "rules of thumb" they have developed by years and years of hanging all sorts of bits to boats.

Given what you are mounting your case is a simple one- the wall thickness if such that there is not much chance of tear tear out if even a modest fender washer is used. In addition the location of the bracket on the end cap of the base eliminates the chance that the panel will oil-can from the weight hanging off it.

So.... you don't need to support the panel or protect from tear out in any meaningful way- the two reasons for sweating backing plates.

If you feel you need to protect from an unusual load like someone falling against the stove while underway- use a plate such as 1/8" Al, or 3/8" ply. I doubt it's needed as the stove will likely fail before there is any problem with the mounting.

 

Correction to my last: "Fender" washer is what I meant to say, not "finishing" washer.

Geez, I'm slippin up here... (thanks bntii).

If you want a reference, ask the stove manufacturer for a recommondation. I'm sure they'll give you some radiculously over-kill answer just to cover their liable butts, however, you may sleep better at night (or, in the middle of the afternoon).

-Tom

 

For every thickness of whatever layup of glass and how many bolts of what size, I don't have a formula either - other than "stronger than others think it should be". I use a bar of aluminum for such things. 1/4 x 1 1/2 will do nicely. It costs a bit but will never, ever pull out. The fender washers will hold fine, even the import paper-thin stuff I have seen lately. Heck, number 14 wood screws would hold that.

 

Ok. In the absence of a rule of thumb, I performed a number-crunch. Please follow along and let me know if I have gone wrong.

The stove bracket takes 1/4" bolts and I'll use stainless steel hardware, for sure. The TENSILE strength of stainless steel is somewhere around 100,000 psi. So, a 1/4" bolt will have a tensile strength of about 4500 lbs.

So how large a backing plate is required to create a pull-out strength of 4500 lbs?

The composite in the vicinity of the outside edge of the backing plate will be in shear. I found a reference that quoted the SHEAR strength of a composite at 30,000 psi. Therefore, the area in shear must be at least 0.15 sq.in.. Since the keel box cover is about 1/8" thick, the circumference of the shear area should be 1.2". That is a circle of only 3/8" diameter.

A 1/4" flat washer may be adequate, but a 1/4" x 1-1/4" fender washer should be more than adequate. Very good, Submarine Tom and bntii.

I thought the 1/8" thick fibreglass seemed a little thin to support the cantelevered load. The 3-dimensional shape of the keel box cover makes me a little more comfortable. I'll install the stove using fender washers and I will watch for cracking. I should have time to correct before a catastrophic failure. After all, I'm not designing the Titanic.

 

All that calculating has nothing to do with where the real problem lies. You could support that whole thing, kettle of water, and all, with 1/16" bolts and 1/4' washers with no chance of pull out...that ain't the problem. The failure you might expect here, even with 1-1/4" washers, would be from the glass around the bracket flexing continuously and maybe only very slightly. The experienced know that glass fails pretty quickly when you let this happen. Big pull out strength before repetitive stress will not guarantee you safety from failure. Distributing the load to the nearby perpendicular panels makes a lot of sense if you have the room inside the box to do it. A piece of well encapsulate plywood carried out to the edges of the mounting panel will go far to ensuring the stove will stay mounted for a very long time. Then you can use hardware that is adequate and not ridiculous overkill.
This is not a guess, this is years of experience.
Engineering rules are very useful when correctly applied by those experienced enough to know where they apply. Some things on boats undergo cyclic loads that far exceed what our intuition would tell us. I would listen to these guys...it sounds like they have busted as much gear as I have!

 

The only rule of thumb sounds to be the "whatever is in the scrap drawer" rule.

Actually, I think we are very close to developing our own design rule. Gerr's book describes rules for backing plates for engine mounts, for chainplates and for deck hardware. We should be able to apply one of those for this application. However, Gerr's rules are based on hull scantling numbers. I don't know if there is a scantling number for keel box covers.

I believe that the fender washer is an adequate backing plate. It also fits Gerr's rule that thickness equals 0.3 to 0.4 x bolt diameter. The discussions around 3/8" - 1/2" encapsulated plywood bear more resemblance to cored hull thickness and relates to my concern that 1/8" glass may be a little thin for this mounting.

We may have already been in agreement but we were confusing ourselves with differences in the terminology.

So, what thickness should the keel box cover be? Cored or uncored? Maybe we are moving towards a rule of 1x the bolt diameter. I'd be comfortable if the keel box cover had been 1/4" solid glass.

 

Whenever you drill a hole to mount something, the plate should be increased locally by 1.5X the plate thickness and the diameter of reinforcement shall be greater than the fender washer to be used. Fender washer is about 4X the bolt diameter.

 

Whoa !!! this is a swing cooker mount !!!!!!!!!!!!!!!

as far as I know A plywood backing block is the way to go.

Fastenings for any equipment mounting may be loaded in sheer, compression or tension. Fender washers alone do little when in compression and sheer.

The goal of all hardware mountings is to have the fastenings fail before the structure fails. The best way to address this is with a backing block to spread compression, increase sheer strength and with flat washers for tension.

Make your ply backing block with as much surface contact area as possible.... I use double the surface area of the hardware .... take a router and round over its inside edge to avoid point loading around its perimeter, then set in thickened epoxy to compensate for any surface irregularities. .

If thru bolting is impractical I have good luck by bolting into ply backing block via threaded brass inserts set into epoxy.

Now get to work...coffee time is just around the corner.

 

A Rule Is Born

We all were right. We all contributed. Check this out.

I suggested that maybe the solid glass thickness should be the same as 1.0x the bolt diameter. In the case of my Forespar stove, the glass thickness should be 1/4". Gerr's rule for sandwich construction is that core thickness be 1.5x to 2.2x the solid glass thickness. Therefore, a plywood core should be 3/8" to 1/2" thick for this application.

Deja Vu!

Gonzo, bntii, and I were all calling the same numbers! We just had different construction modes in mind. Of course, to be a true sandwich construction, 5 to 7 layers of 6 oz glass should be applied over the plywood core.

I thought rxcomposite had something with his 1.5x plate thickness rule, but the notion didn't pass a logic test. Suppose the keel cover was already 3/4" thick. I can't imagine increasing it to 1-1/8" thick to support a 1/4" bolt.