Backing plates replacement - Techniques and sizes

[FishCake]

Hello fellow sailors,
I hope this message finds you all well. I'm reaching out to seek some guidance as I embark on a project to upgrade the backing plates for the keel bolts on my sailboat.
Let me provide you with some pertinent details about my vessel:
Year: 1982
Length: 30 feet
Material: Fiberglass
Keel: Lead fin keel
Weight: 1800kg
Keel arrangement: Fin keel, keel-stepped mast
Bolt diameter: 19mm
Number of bolts: 5
After some research, it appears that the recommended dimensions for the backing plates are 20cm x 20cm with a thickness of 10mm (5 times the diameter of the bolt for the radius of the plate and 1/2 diameter of the bolt for the thickness). However, I'm encountering a logistical challenge as I don't see how I can fit such large plates into my bilge.
I've begun the process of obtaining quotes from local companies for custom-made plates, and I'm leaning towards using SS 316 due to its excellent corrosion resistance. However, I'm open to any recommendations or alternative materials that the community may suggest.
Additionally, I would greatly appreciate any advice or recommendations you might have regarding the bedding process.
Your expertise and advice would be immensely valuable to me as I navigate through this project.
Thank you all sincerely for your support and assistance.
✨

[thinwater]

Sometimes laying in more fiberglass can help with fitting problems and reduce the need. Obviously, the layed-in fiberglass will conform to any curve. If the fiberglass distributes the load well enough, all the backing plate (washer) needs to do is distribute the local compression load.


A big backing plate is not a substitute for enough glass.

[rslifkin]

For cleats and other hardware I've used G10 fiberglass/epoxy sheet bedded in thickened epoxy for backing plates. It's significantly easier to cut and drill than stainless steel, so it's easier to shape and size a plate to be as big as possible in a confined space.

[BenBowSirocco]

RSLIFTKIN is correct: for a bilge, (read: underwater) there is basically nothing better than G10... Although I personally do not think the expense of it is necessary and would recommend any of the other hydraulically pressed premade sheet goods that cost significantly less.
Choose a plate that 1/2" thick and do them one at a time. Bond the plates into place after grinding the bilge and plate raw with 80 grit (leave nothing shiny or painted) and cleaning them with acetone using "Six10" from West Systems or make your own with some epoxy and fumed silica mixed to the consistency of good peanut butter. Use a notched spreader to create a uniform bonding thickness.



https://www.mcmaster.com/products/fi...rs-and-strips/


If you are stuck on stainless, certainly check out "Send Cut Send". They can be significantly better price wise on small custom plates and will show up pre-drilled. Bonding the stainless is a similar process: abrade the stainless plates with 80 grit and install them with epoxy or 3M 5200 onto your abraded bilge after cleaning both with acetone. Again: use a notched spreader to get an even thickness if you so desire.



Last thing I might say: your boat made it this far with what it has. So it might be easier to just replace what the factory did..

[deltaten]

Yer figures sound like a bit of overkill. Could they have come from a structural steel construction manual? I'd go with what Thin said; more glass, then larger than washers but smaller than a one piece over all plate....mebbe 2"+ or 3" X3/16" or 1/4" .

The bolts are in tension. No shear load and never likely to be side loaded/in deflection. Resultant t load is weight of keel (in water?) divided by the five bolts. I get 792 pounds each bolt, so.....that's 88 pounds per sq in/over the (assumed) 9 sq in each. As long as the substrate (Thin's add'l glass) resist deflection, not an unreasonable "pressure"

Just my humble opinion...not a marine architect or engineer. HTH

[s/v Jedi]

The washers under my keel bolts (7 ton keel) are maybe 3” x 3” square and less than 1/4” thick. Solid fiberglass underneath that of course.

[coopec43]

I'm sorry if I missed it but have you told us the design of the yacht?

[Lee Jerry]

Originally Posted by BenBowSirocco

Last thing I might say: your boat made it this far with what it has. So it might be easier to just replace what the factory did..

This!

Have you had any issues with the original plates? Did you have to make any repairs to the underlying structure? If not, why reinvent the wheel with something that has worked for 40+ years?


In terms of material, the G10 and SS are great options, but I'll add: don't discount aluminum. It may not last quite as long as the other two (depending on environment), but is readily available (in US, anyway) and much easier to work with than SS. (Get an "aluminum cutting blade" - cuts like butter, standard drill bits, power sander.) It would need to be thicker than an SS plate.

With respect to bedding, the plates just need a flat surface perpendicular to the bolts. (You should have this under the original size plates; you seem to be running into issues with bigger ones.) I don't think a thick "bed of goo" should be necessary (for a location expected to have a large bolt). Just a thin layer of something (4200, Six10, or whatever) to get good contact all around. A large plate that is not sitting flat is bad, much worse than a smaller (but not too small) plate with proper contact. Too large a plate can also move the force away from the intended load carrying structure (in general, probably not in this case), for example off of an area of thicker material onto thinner one or from solid material onto the cored section.

If the fiberglass distributes the load well enough, all the backing plate (washer) needs to do is distribute the local compression load.

A big backing plate is not a substitute for enough glass.

There is also a shear load around the perimeter and (maybe / probably) a bending load in skin(s).

Because of the lower compressive strength, even really thick glass needs backing plates - the backing plate should be inversely sized to the ratio of the compressive moduli (glass vs bolt material; compared to bolting steel, the "same" material as the fasteners, only requires washers regardless of thickness.) I think larger plates (can) help with the shear and bending loads more.

[Bowdrie]

If you have enough exposed thread above the nuts, it's a great idea to have thick G10 pads.
Something about an inch thick, or whatever you can get.
If you have to, it's really not hard to laminate up some squares of fiberglass.
A simple fixture with a little bottle jack that can apply some good pressure and you can make your own "high pressure laminate".
Working with, say ~6" squares, you can even use a bunch of C-clamps.
Then cut the 6" square in four 3" squares for the pads.
Yes. it's messy/gooey/time consuming, and not particularly cheap, but it works and you'll have learned a skill.
That gets the nuts/washers/plates above the bilge a bit.
It's always that last few cups of water that sits forever in contact with the nuts/washers that cause all the corrosion.
I would add that at least for me, I find the practice of builders sticking a sharp-edged metal plate directly on an uneven bilge surface with some goo-in-a-tube and calling it good to be slipshod construction.