Stacking keel backing plates?

[RaymondR]

Not only is the modulus of elasticity vastly different, the failure mode of frp under compressive stress is also vastly different. Metals tend to just distort when the yield point is exceeded, frp,being far more brittle, tends to lose most of it's strength once the resin is fractured.

Consequently stressing the frp to anywhere near it's compressive stress limit is to be strictly avoided.

[Lee Jerry]

Originally Posted by wyb2

Appreciate the thoughtful response


What standard?

All torque standards are based an some assumptions about the joint, like base and bolt material. If you were using a nylon bolt because you needed electrical isolation, you wouldn’t just look up the torque spec in some industrial design handbook that assumes steel hardware.

The standard for the hardware you're using, not some random thing you're not using. C'mon man...

It changes the relationship between the two, as you basically go on to explain.


The modulus of FRP is significantly lower than the modulus of steel. FRP is around 17Gpa or 2.5 Mpsi (megapounds per square inch aka million psi). Steel is around 10x that.


Yes. Load and stress and modulus are all different things though.


Not exactly. The point of the backing plates is to enlist more area of clamped material to compensate for its lower strength, not lower modulus. Strength is how much stress (psi) the material can take without failure. Modulus is a measure of how much the material compresses/deforms under stress. Modulus is basically a measure of softness/hardness that tells you nothing about strength.

Yes, it's for all three things: compressive strength, stiffness and shear strength (which hasn't even been discussed).

My point is basically that those last two sentences operate on the assumption that the bolt and clamped material have a similar modulus. With metal hardware on metal components, this is generally true.

But if the base material is much softer, so relieving stress requires much more expansion, then loads become additive.

For steel bolting steel, if you have 5000 lbs of preload, and you apply 2000 lbs of load in tension, you inherently relieve 2000 lbs of clamping force, so your resulting bolt load is 5000 + 2000 - 2000 = 5000 lbs. I think this is basically what you are saying, and it’s correct

For steel bolting FRP, if you have 5000 lbs of tension, and you apply 2000 lbs of tension load, you may only relieve 200 lbs of clamping force, because the FRP needs more room to expand to relieve stress. In this case your resulting bolt load is 5000 + 2000 - 200 = 6800 lbs. The preload gets exceeded without the parts ever getting unclamped.

A column of the same cross section as the bolt may only relieve 200 lb, but (due to the backing plate) there are more (similar, if you will) columns of FRP surrounding it that will also relieve 200 lb each such that the total is 2000 lb. So, like a bunch of springs in parallel, the softer FRP acts similar (enough, if not exactly) to the stiffer bolt. You may notice that the modulus is PER SQUARE INCH. So increase the area of the FRP 10X to get the same (or at least similar) stiffness as the steel.

My approach is conservative. I’m ignoring a mitigating factor of the FRP relaxing over time (creep), which I think does happen. I’m using a conservative value for SS yield strength. I’m sure tons of people have just looked up 130 ft lbs (or whatever for their bolt size) and never had a problem. But I think it’s more because there is plenty of room for error when you are bolting on a X,000 lbs keel with hardware that could lift XXX,000 lbs, and less because it’s the ‘correct’ torque value.

The problem is your approach is only conservative in one respect (bolt strength) at the expense of other considerations, particularly clamping force. The latter is more important, IMHO. I'll stick with my recommendation to use the standard.

[wyb2]

Originally Posted by Lee Jerry

The standard for the hardware you're using, not some random thing you're not using. C'mon man...
.

Ok, fair enough. What standard are you using for this case then? Maybe there’s a standard for 316 SS hardware clamping fiberglass in a structural application that I don’t know about. I don’t build or design boats for a living, there’s lots I don’t know.

But if the standard you are using is a google result for ‘SS bolt torque chart’ like this one…

https://www.engineersedge.com/materi...teel_13353.htm

…it is assuming you are clamping metal.

Originally Posted by Lee Jerry

Yes, it's for all three things: compressive strength, stiffness and shear strength (which hasn't even been discussed).



A column of the same cross section as the bolt may only relieve 200 lb, but (due to the backing plate) there are more (similar, if you will) columns of FRP surrounding it that will also relieve 200 lb each such that the total is 2000 lb. So, like a bunch of springs in parallel, the softer FRP acts similar (enough, if not exactly) to the stiffer bolt. You may notice that the modulus is PER SQUARE INCH. So increase the area of the FRP 10X to get the same (or at least similar) stiffness as the steel.

You have a good point here, I probably went too far saying the backing plate has nothing to do with modulus. The application where I originally learned about the difference involved gaskets. We were easily engaging 1,000x the surface area of the bolt, but the modulus was probably 100,000x lower than steel.

The only thing I’m not sure about is, a backing plate would have to extremely stiff to impart a relatively evenly distributed stress into the FRP. Of course it’s distributing it somewhat, otherwise it wouldn’t be doing anything.

Using your analogy, if we translate the FRP compression into a bunch of parallel springs, imagine how floppy the actual spring that is a flat plate would seem.

I guess the devil is in the details, I don’t have a perfect answer here.

Originally Posted by Lee Jerry

The problem is your approach is only conservative in one respect (bolt strength) at the expense of other considerations, particularly clamping force. The latter is more important, IMHO. I'll stick with my recommendation to use the standard.

Correct, I am assuming that (using the numbers in this case) 5000 lbs of clamping force in each bolt is more than enough for a 1000 lb keel. With a dozen bolts, I think it would probably be plenty for a 3 or 4000 lb keel. But, yes, it’s an assumption. Happy to be proven wrong.

[GopherBaroque]

For your consideration for sizing backing plates, consider these tests-to-failure of backing plates vs washers. More doesn't necessarily mean much better.
https://www.boltonhooks.com/proof-testing/

[Seacod]

Originally Posted by Lou-In-NJ

I can't find ballast weight on your particular boat, but since Sailboat Data lists your total displacement at 4400 lbs., I'd guess your keel s nearly 1000 lbs. So I would have the same trepidations as you; I would get a single 7mm (1/4") plate, and have it extend at least an inch (25mm) further in all directions than what the two square plates currently cover. Spreading the forces out is the key increasing the margin of safety and reliability.

Definitely support this approach since broad distribution of load is the key guiding principle here.