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Old 14-06-2013, 09:36   #1
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Backing Plate Questions

We recently acquired a 35-year old sailboat, and we're working on getting it shipshape. One issue is that the vast majority of through-deck mountings leak. I'm guessing that the deck hardware has never, ever been re-bedded since it left the factory.

Upon checking the underside of the deck, seems there are no backing plates on any of the through-deck bolts. Not even fender washers. Just small, standard-sized washers. Yikes. I'm afraid to pull very hard on the stanchions...

Two issues:

1. I know that SS backing plates are the Cadillac way to go, but I understand that cutting SS plate is not for the DIYer. Even drilling is more difficult. A knowledgeable sailor recommended just going with aluminum plate instead. Much easier to cut and drill. Of course, now we're talking about dissimilar metals. I could address this by applying some Tef-Gel, or perhaps putting a plastic sleeve in the hole and a plastic washer under the nut to avoid stainless-to-aluminum contact. (Note: boat is almost always in freshwater, have not seen much evidence of galvanic corrosion) Any recommendations? If SS, any suggestions where/how to get plate stock cut to size? Btw, since almost all through-deck locations are hidden from view, appearance is not a factor here.

2. In many instances using backing plates will be problematic, as the underside of the deck at the mounting location is contoured, not flat. Not just a little curvature, but in some cases complex surfaces. I was thinking of using some modeling clay to make an impressions of these surfaces, then using the clay models as templates to shape blocks of hardwood. In other words, construct filler pieces that would go between the backing plate and the underside of the deck. Is there a better way to do this? Epoxy? Anyone else ever have this problem?
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Old 14-06-2013, 09:46   #2
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Re: Backing Plate Questions

I would stick with stainless. Not that hard to work with. I had similar issue with my boat and have been making SS backing plates. You can cut with an angle grinder no problem. Drilling the holes takes slightly more care.

Best to use a drill press where you can apply constant, firm pressure and drill at a low speed. Cutting oil can help cut and keep the piece cooler. Try to drill so you get a constant curl of metal from the hole. The perfect result is almost like making a little coiled spring of metal.

Or you can take the plates to a local metal shop to have them make holes for you. Not too outrageously expensive.

PS

All this advise is based on your keeping the boat for a few years. If you plan on upgrading in a year or two then any old piece of junk will work. Who cares about the next owner.
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Old 14-06-2013, 09:54   #3
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Re: Backing Plate Questions

Aluminum will work fine and be easier to work with. Also consider G10 fiberglass board if you are worried about corrosion, but that is difficult to work with also.

Frankly, for stanchions and most other gear on a 28' boat, large fender washers will be OK. they will be distributing the load a long way around the point load.

For the complex curved parts, can you fill them with thickened epoxy similar to your thoughts of using wood? Make the epoxy thick like modeling clay and push it up there. Be quick - it will set fast.

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Old 14-06-2013, 10:03   #4
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Re: Backing Plate Questions

A couple of points here:

1) I agree that you should only go nuts on this job if you are keeping the boat for a decade OR if safety is an immediate issue.

2) Drill and fill, coatings and bushings get you around the dissimilar metals issue. The point of a backing plate is to spread the load. If you use aluminum bar or plate stock, you can bend and work it without special knowledge. Nylon bushings and fender washers or even Tef-Gel or similar goos can isolate the SS thru-bolts pretty effectively...after all, the deck will be sealed...no salt water down there is sort of the point, right?

3) The bigger job than the backing plate in my view is remediating or replacing the soaked or rotted core you will find. Some places might simply be too tight to do this job from below, meaning you have to consider the cosmetics of the deck work.

4) Do not rule out encapsulated wood as a backing plate. Again, it's a matter of overdrilling holes, putting in epoxy "filler" and drilling through that epoxy to get the bolts through, but if you isolate the deck core (encapsulated marine ply or solid glass) and isolate the holes in the backing plate, you can get identically good results to SS and aluminum.

I've got a 40 year old C&C. I've done this job, albeit in fresh water. It is labour-intensive and fiddly, but you can actually end up with a stronger boat that is "better than factory" because you are replacing dead balsa with epoxied ply or solid glass and proper load-spreading plates that should have been SOP in the first place back when the boat was made.
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Old 14-06-2013, 10:35   #5
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Re: Backing Plate Questions

I'll second G10. You can get it from McMaster-Carr. Use the carbon blade on a jig saw and it cuts fairly easily.
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Old 14-06-2013, 11:11   #6
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Re: Backing Plate Questions

Another vote for G10 , with a dollop of short strand reinforced filler [kitty hair] to bed it to the underside of the deck and fill any irregularities.
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Old 19-06-2013, 15:37   #7
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Re: Backing Plate Questions

Thanks for the responses. I was not familiar with G10, but it certainly looks like a good solution. For those who have used it, any recommendation as thickness when used as a backing plate system?
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Old 19-06-2013, 15:59   #8
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Re: Backing Plate Questions

Quote:
Originally Posted by voxxmd View Post
Thanks for the responses. I was not familiar with G10, but it certainly looks like a good solution. For those who have used it, any recommendation as thickness when used as a backing plate system?
I used G10 when I made a backing plate for our bowsprit where it bolts through the deck. I used regular old blades on my circular saw and a plain old drill bit and it worked just fine. A lot of folks say it is hard to work with but I found it was about like a piece of hardwood and drilling too. For our application we used 1/4" but that is probably way overkill for smaller hardware.
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