Deck Hardware Backing Plates

[JimLor]

You may notice from another post that I am chasing leaks on our new to us Pearson 28. I also noticed the other day that our mid-ships port side cleat (very substantial cleat) isn't properly secured on the bottom side. There isn't any backing plate and the holes for the 2 front bolts run into a bump on the liner and the nuts can't be (and aren't) on the bolts! So, I intend to remove the cleat, use the 2 aft holes as the new forward holes and drill 2 new holes for the aft bolts.

My question is what makes for a good backing plate as to material and thickness? Where to buy it would be helpful too.

I'm also going to rebed the stanchions and will replace the original fiberglas backing plates with a more substantial plate. I figured stainless steel until I looked at a site where a 2x4 sheet of 1/4" was $500!!

I appreciate any and all comments.
Jim

 

[Lee33-2]

Jim, what year is your 28?

 

[JimLor]

She's a 1986 Hull #86.

 

[rhr1956]

I was at West Marine on Saturday and had the same discussion about material for backing plates with the know-it-all guy there. (I say that with affection..he really is a wealth of knowledge.) He said he uses 1/4" aluminum for backing plates because that's what he has. He said it's really over kill...3/16" plenty. You can get a 1X2 foot piece of 3/16" for $48. http://www.metalsdepot.com/Cart3/viewCart1.phtml?LimAcc=&aident==

 

[JimLor]

Thanks. I gotta ask, assuming stainless steel bolts and nuts, what about corrosion with dissimilar metals?

 

[rhr1956]

The aluminum backing plate would be the "less noble" metal which would potentially corrode, not the stainless. The following is required for corrosion to occur. The 3rd point would be avoided, I think, with proper sealing/bedding of the hardware installation.

Condition 1. Metals must be far apart on the galvanic series
The galvanic or electrochemical series ranks metals according to their potential, generally measured with respect to the Standard Calomel Electrode (S.C.E.). The results are often viewed as a galvanic corrosion chart or galvanic corrosion table similar to that on the third page.
This chart says that the "anodic" or "less noble" metals at the negative end of the series - at the right of this diagram, such as magnesium, zinc and aluminium - are more likely to be attacked than those at the "cathodic" or "noble" end of the series such as gold and graphite.
The critical point is the difference in potential of the two materials being considered as a joined pair. A difference of hundreds of millivolts is likely to result in galvanic corrosion, but only a few tens of millivolts is unlikely to be a problem.

Condition 2. The metals must be in electrical contact

The two different metals must be in electrical contact with each other. This is of course very common. The two metals can be bolted, welded or clamped together, or even just resting against each other. Condition 3. The metal junction must be bridged by an electrolyte
An electrolyte is simply an electrically conducting fluid. Almost any fluid falls into this category, with distilled water as an exception. Even rain water is likely to become sufficiently conducting after contact with common environmental contaminants.
If the conductivity of the liquid is high (a common example is sea water) the galvanic corrosion of the less noble metal will be spread over a larger area; in low conductivity liquids the corrosion will be localised to the part of the less noble metal near to the junction.

 

[JetStream]

I really think that any metal backing plate is overkill - besides the potential corrosion issue. Remember what you are trying to do here. The backing plate is just to spread the load more than a simple washer. I've seen some really nasty aluminum plates that have turned to white powder. Some of the experts use G10 or FR4, which is a reinforced fiberglass material. It can be purchased from McMaster Carr here:
http://http://www.mcmaster.com/#grade-g-10-garolite-bars/=frbprj
I would use either 5/16" or 3/8" thick material.

 

[JimLor]

rhr1956 and JetStream - Thanks for your comments. I especially enjoyed the discussion on whether SS/Alum would corrode. I have a chemist PhD here and we had a pretty detailed discussion.
I think I'm at the point where I'd use either, although the overkill in me still leans towards a metal backing plate. Guess I need to take a look at a glas plate because one big requirement is that once I do this I don't want to have to go back a replace the plates due to corrosion.

Again, thanks and I'll post what I finally do and let you know how it went.

 

[shorty]

Fiberglass

I did the fiberglass from McMaster, 3/16 if I remember right. What it replaced was thinner & flimsier and had worked fine for26 years. No corrosion.

 

[Lee33-2]

Jim your leaks are most likely coming from crushed and rotted core in the area of through deck bolts. The only good solution is to remove the hardware. Gouge out a 1/4 inch section of core, fill with West System Epoxy then redrill the hole. The 1/4 inch collar of epoxy in between the upper and lower portion of the deck will provide support when you rebed the fittings. Simply rebedding your existing fitting with new backing plates will not help. I have done this on my 1986 Pearson 33-2 with amazing results. I would strongly recommend that you use 1/4 fiberglass plate for backing material, it is easy to work with, cheap, and no corrosion issues. Send a message to my user and I will give you my phone number.

 

[JimLor]

Lee - yesterday I ordered butyl tape for the bedding material and I also ordered the G-10 garolite fiberglass from McMaster, in 1/4". Thru reading articles, books, and looking at posts on this site I also decided to repot the holes with epoxy.

Epoxy apparently opens up a whole new world of different types, hardeners, additives, etc. I bought the two small bottle West system to fasten the exterior to the interior plywood before installing my new ports.
Can you tell me specifically which epoxy, filler, etc to buy?

Also you say to "gouge" out a 1/4" section of core...I assume from that you mean without enlarging the existing drilled hole, but working within that space to enlarge the space between the exterior and interior limits. Would drilling a larger hole, say 1/2", filling with epoxy and then redrilling the 1/4" hole do the same thing or would that result in a weaker base?

 

[fourports]

Using an alan wrench sharpened flat on one side of the short section allow one to use a drill, insert the short end of the wrench and spin out the bad core if that is what you are after. A strong vacuum helps get the bad stuff out

 

[JimLor]

Great idea fourports, thanks. I was hoping to find the perfect Dremel circular saw that fit into the established bolt hole but had enough play that you could use the shaft much like a jig and just move the blade around. Your idea is probably the more realistic.

 

[gundaddy]

If you're looking for an option using Dremel tools, check this page:

http://www.pbase.com/mainecruising/sealing_the_deck

The author recommends a couple of different bits depending on how you intend to seal the holes through your deck.

 

[JimLor]

gundaddy, thanks. That's the same site that had the info on using butyl tape to bed deck fittings - I ended up buying some and will see how it works soon. Another benefit is that I have an older Dremel and might use this as an excuse to get a new one!
I've got a cleat on the port side that was put in half-assed. The 2 foward bolts run thru a bump out and 1 forward bolt doesn't have a nut and the other just has the nut hanging off. This will be my first run at removing the cleat, digging out the core and potting, and using the butyl tape to rebed. Forgot, also my first work with G10 as a backing plate. And if all is sound with the universe, this will be the source of the leak(s) I'm chasing -- but I strongly doubt it.
Thanks again.
Jim