Correct installation of backing plates under a sloped surface

[iansan5653]

The deck on my Freedom 21 is cored with balsa up until about an inch away from the edge, where it gradually transitions into solid ~1/2" fiberglass. My stanchions have two bolts through the deck at this point and two through the gunwale outside.

Prior to me owning this boat, all of the stanchions were bolted with just standard size washers and no backing plates. Even though there's no core here, 40 years of this had started to pull the washers into the glass and has created some concerning cracks around the fasteners. You can see that in this pic (ignore the missing nut on the left - forgot to take a picture before I removed it):


Obviously this wasn't great, so I installed some new fasteners with oversized washers:


But as you can see, the point where the holes are is sloped relative to the deck, so the washers don't sit flat relative to the nuts. I think this is much better than the previous arrangement, but I expect those washers to bend eventually - especially because my jib sheet blocks attach directly to the stanchion bases.

I will make a backing plate from FRP to distribute the load, but that won't solve the slope problem.

How can I fill in the thinner part so it's all flat? If the boat was upside down it would be a piece of cake - I'd just form a box around the spot and fill it with thickened epoxy. But doing that on ceiling feels like I'm asking for a huge mess.

[beetle]

The easiest way to create a perfect surface for a backing plate to rest up against the deck is to cast a thickened-epoxy surface in place. It's relatively easy and not messy if you're careful. General approach is to create the backing plate, apply thickened epoxy to the plate, bolt plate into place, let epoxy cure, remove plate, clean up, and final install of the backing plate. To actually do this, create the backing plate and make sure it fits on the bolts. When the plate is correct, mark the deck where the plate will go and remove the plate. Apply tape surrounding the mark so you can easily remove excess epoxy from areas beyond the plate. The plate needs to de-bond from the epoxy, I use plastic packing tape applied to the plate as epoxy won't stick to the plastic. Also tape the edges of the plate - not just the side that will touch the epoxy. Cut holes in the tape for the bolts to pass thru. When the plates ready to go, make up some super-thickened epoxy mash/spooge - I use West 404 as it's a strong thickener. Apply a thick layer of epoxy to the taped side of the backing plate. Also lay or tape a plastic tarp or sheet below where the plate will be - this catches epoxy that falls out. Put more epoxy in the center of the plate than on the edges - this way you minimize chances of creating a central air bubble in the epoxy casting. You also want the bolts to not be grabbed by the epoxy - a liberal application petroleum jelly (vaseline) on the bolts will work. Run the bolts in from the deck-side, slide the backing plate up into place, you want to see epoxy spooge squish out from all sides of the plate. Run the washers and nuts up from the underside and tighten until the plate is where you want it to be. Remove excess spooge from around the plate, I use a stir stick for that. Wait until the next day, let the epoxy cure. Remove the nuts and washers, withdraw the bolts (you'll need to unscrew them as the threads will be cast into the epoxy), remove the plate. You'll likely need to tap/bang the plate with a hammer and punch to get the epoxy to release from the packing tape. You don't want to hit the epoxy, just the edge of the plate. Remove the packing tape from the plate, inspect the backing plate, do any additional clean up you want - and now you have a perfect cast match to the backing plate that will support the deck and bolts. - rob

[iansan5653]

Thanks Rob, that doesn't sound too bad. The only part I'm not sure about is how you stop all the epoxy from falling out of the edges. If the space was very small I imagine it would work, but this is going to need to be around 3/4" thick at the thickest point. I guess I need to build some wall around the plate to block in the void. Maybe out a rigid piece of plastic or foam.

[GordMay]

Beveled [wedge] washers are angled [often 2 in 12 slope] on their bearing surface, to compensate for non-parallel bearing surfaces.

[iansan5653]

Whoa, I hadn't ever thought to look for something like that. It seems like a much simpler solution. I could still do the backing plate at an angle and then use the wedge washers between the plate and the nut. Probably wouldn't be perfect but more than strong enough for a 22' daysailer -- and no epoxying!

Thanks!

Now the question is whether it's better to use 18-8 stainless vs. aluminum wedge washers, since they don't seem to make 316 - https://www.mcmaster.com/wedge-washe...g-for-i-beams/. Probably not too bad to just go with aluminum.

[bensolomon]

Originally Posted by beetle

The easiest way to create a perfect surface for a backing plate to rest up against the deck is to cast a thickened-epoxy surface in place. It's relatively easy and not messy if you're careful. General approach is to create the backing plate, apply thickened epoxy to the plate, bolt plate into place, let epoxy cure, remove plate, clean up, and final install of the backing plate. To actually do this, create the backing plate and make sure it fits on the bolts. When the plate is correct, mark the deck where the plate will go and remove the plate. Apply tape surrounding the mark so you can easily remove excess epoxy from areas beyond the plate. The plate needs to de-bond from the epoxy, I use plastic packing tape applied to the plate as epoxy won't stick to the plastic. Also tape the edges of the plate - not just the side that will touch the epoxy. Cut holes in the tape for the bolts to pass thru. When the plates ready to go, make up some super-thickened epoxy mash/spooge - I use West 404 as it's a strong thickener. Apply a thick layer of epoxy to the taped side of the backing plate. Also lay or tape a plastic tarp or sheet below where the plate will be - this catches epoxy that falls out. Put more epoxy in the center of the plate than on the edges - this way you minimize chances of creating a central air bubble in the epoxy casting. You also want the bolts to not be grabbed by the epoxy - a liberal application petroleum jelly (vaseline) on the bolts will work. Run the bolts in from the deck-side, slide the backing plate up into place, you want to see epoxy spooge squish out from all sides of the plate. Run the washers and nuts up from the underside and tighten until the plate is where you want it to be. Remove excess spooge from around the plate, I use a stir stick for that. Wait until the next day, let the epoxy cure. Remove the nuts and washers, withdraw the bolts (you'll need to unscrew them as the threads will be cast into the epoxy), remove the plate. You'll likely need to tap/bang the plate with a hammer and punch to get the epoxy to release from the packing tape. You don't want to hit the epoxy, just the edge of the plate. Remove the packing tape from the plate, inspect the backing plate, do any additional clean up you want - and now you have a perfect cast match to the backing plate that will support the deck and bolts. - rob

Do you have to go through the cleaning process, or can you just tighten up the nuts against the backing plate after the newly made epoxy bed cures?

[Cheechako]

You can make a thick slurry of epoxy, but much easier to just use Kitty Hair or something like that. It's fiberglass resin with glass in it.
You can buy sheet bees wax for these types of projects in various thicknesses and mold it to hold your slurry any way you want. Resin does not stick to wax. Or if using the Kitty Hair you can just use wax paper from your supermarket. Fill the cavity with kitty hair, have a backing block, put wax paper on the block and brace it in position until it hardens. Easy peasy.

Using gloves you can mold Kitty Hair with your hands if you want.. CLean the surface with acetone as best you can prior.

[beetle]

Originally Posted by bensolomon

Do you have to go through the cleaning process, or can you just tighten up the nuts against the backing plate after the newly made epoxy bed cures?

You do want to take the plate and bolts off, mostly because the bolts won't have sealant in place when making the casting, and you'd also want to check the result to verify the work came out the way you wanted. Plus you don't really need to leave the tape in place - might as well get it out of there.

Then apply sealant to the bolts and tighten everything down.

- rob

[thinwater]

Are you sure the bolts are not in the tapered edge of the core? I think they are. The brown stains suggest rotted core. You should probably drill a shallow test hole (not through the deck) from below to confirm.


Washers do NOT pull into fiberglass, they pull into failed core (zoom into the left hand bolt in the first image). You may want to do some core replacement first.

[j.g.evans]

I have installed wood and AL backing plates on irregular and difficult to access places using automotive Gorilla Hair. This is thick and sticky fiber reinforced polyester resin. It is sticky enough to hold the plate during the cure. The cure is fast, less than 20-30 minutes.

Epoxy is better, but very messy and much more expensive. The polyester should be OK if the area stays dry.

[iansan5653]

Originally Posted by thinwater

Washers do NOT pull into fiberglass, they pull into failed core (zoom into the left hand bolt in the first image). You may want to do some core replacement first.

I actually thought the same and was not looking forward to replacing the core here, but was relieved to find that this is not the case. Note that in the image you refer to the washer is actually removed already, not pulled in deeply. I have carefully checked the holes with lights and with a sharp bent nail and I don't feel or see any wood at all. I think they did pull in slightly but just from some degrading of the thick fiberglass over the last 40 years.

[iansan5653]

I think the wedge washers will be a much cleaner solution for this problem than building up for a backing plate. I know the force will be more concentrated, but it's still significantly better than what was there before. On a bigger boat I'd do the backing plate for sure.

Follow up question though: I am using 316 stainless for the bolts, nuts, and other washers. Of course the ideal would be 316 stainless for the wedge washers as well, but they are only available in galvanized iron, aluminum, or 18-8 stainless. I don't like using 18-8 stainless anywhere, and aluminum in full contact with stainless feels like a bad idea. My instinct is to get the galvanized iron - is that the best choice?

[C420sailor]

I’d thicken the epoxy with colloidal silica and/or shredded glass. And use a thick stainless backing plate.

Washers would probably be okay for a stanchion, especially through thick glass. But with sails tugging on that too? Plate.

Ain’t no kill like overkill.

[LittleWing77]

Originally Posted by thinwater

Are you sure the bolts are not in the tapered edge of the core? I think they are. The brown stains suggest rotted core. You should probably drill a shallow test hole not through the deck, but from below, to confirm.

Washers do NOT pull into fiberglass, they pull into failed core (zoom into the left hand bolt in the first image). You may want to do some core replacement first.

+1

Ugh! All those pictures say to me is: (1) leak = continued water ingress; and (2) compromised structure.

You're jumping ahead of yourself, Ian, by focusing on backing plates. If you don't fix the underlying problem, all of the backing plates in the world won't help the structural integrity of your boat's deck + hull....

LW77