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Old 08-07-2024, 17:25   #1
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Backing Plate Recommendations

I can no longer avoid addressing my stanchion backing plates, this one gets to go first.

I have a readily available and inexpensive source for both stainless and aluminum. Although I understand the purpose to spread the load I am looking for recommendations on material thickness and overall sizing (matching the stanchion base size or bigger?)
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Old 08-07-2024, 18:07   #2
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Re: Backing Plate Recommendations

I would go with G-10 probably just replacing with same size and thickness of what is there. G-10 can be cut and drilled by you. Usually the edges of the plate are beveled / chamfered on the bottom side to reduce point stresses. How much? I usually put a 45 degree chamfer at about half the thickness of the material.
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Old 08-07-2024, 19:17   #3
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Re: Backing Plate Recommendations

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Originally Posted by djousset View Post
I am looking for recommendations on material thickness and overall sizing (matching the stanchion base size or bigger?)
General engineering guidance:

1. Thickness: at least half the diameter of the bolt(s) in thickness, for 'stiff' (metal aluminum or steel or G10) backing plates.

2. Size: at least 10 bolt diameters from each bolt to the nearest edge, with no sharp corners and tapered edges.
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Old 08-07-2024, 21:18   #4
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Re: Backing Plate Recommendations

+1 for G10.

Even if it's more expensive the value of labor makes the cost difference less important as G10 with proper installation will never give you any worries or concerns. I had my boat builder buddy help me install a windlass and reinforce bow cleats. We used G10 for backing plates. He joked that the 18,000# boat now can be picked up by a crane by its windlass.
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Old 09-07-2024, 02:02   #5
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Re: Backing Plate Recommendations

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Originally Posted by Breaking Waves View Post
General engineering guidance:

1. Thickness: at least half the diameter of the bolt(s) in thickness, for 'stiff' (metal aluminum or steel or G10) backing plates.

2. Size: at least 10 bolt diameters from each bolt to the nearest edge, with no sharp corners and tapered edges.
Good information.

A ‘chamfer’ removes the sharp corner of a part, but, a‘bevel’ is a continuous angled surface, between two parallel sides. ‘Fillets are rounded edges, on a workpiece.

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Old 09-07-2024, 03:09   #6
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Re: Backing Plate Recommendations

Good news is your stanchions are at least backed

McMaster Carr has a good selection of G3PO fiberglass sheets that are reasonably priced. Typically delivered the next business day. McMaster is an amazing resource ---- best kept secret in the US Marine Supplier universe.

https://www.mcmaster.com/products/sh...-fiberglass-1/
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Old 09-07-2024, 04:45   #7
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Re: Backing Plate Recommendations

For stanchions, I would like to suggest you use 1/4" aluminum plate. It's easy to cut, drill and its inexpensive. Many boat manufacturers embed an aluminum plate into the fiberglass for a backing plate.
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Old 09-07-2024, 04:54   #8
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Re: Backing Plate Recommendations

Thank you for the responses, good info.
I am not overly worried about costs, I have family that can provide/manufacture both stainless and aluminum, and I have a good source for G10. Apples to apples, what is the hierarchy of these three materials on the combo of strength and durability? One stanchion in particular at the starboard side gate gets a lot of activity.
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Old 09-07-2024, 05:29   #9
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Re: Backing Plate Recommendations

Quote:
Originally Posted by hlev00 View Post
I would go with G-10 probably just replacing with same size and thickness of what is there. G-10 can be cut and drilled by you. Usually the edges of the plate are beveled / chamfered on the bottom side to reduce point stresses. How much? I usually put a 45 degree chamfer at about half the thickness of the material.
Good recommendation. G10 will be superior to what I see in the photo and easy to work with. Same size means easy fit. Consider 3M 4000 as bedding and water preventative. It’s removable unlike 5200.

When I do very high load backers I bed in 48 oz Fabmat fiberglass and epoxy. Usually with aluminum or stainless but this is too heavy and expensive for your stanchions.

Metal backers should be exposed in my opinion. They always seem to corrode when buried, airless.

Are you sure the existing backers are not already fiberglass? Maybe all you need is to de-mount, clean and re-bed, new bolts. Starbright rust cleaner (buffered acid in a spray) will remove all traces of rust.

McMaster Carr.
https://www.mcmaster.com/products/~/?s=g10
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Old 09-07-2024, 08:49   #10
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Re: Backing Plate Recommendations

Start by spraying the nuts with PB Blaster. I'd have a multi-tool handy to cut the nuts. I would use G10, then aluminum then stainless. G10 won't corrode/interact with the bolts. Bed generously with butyl.
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Old 09-07-2024, 11:01   #11
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Re: Backing Plate Recommendations

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I am not overly worried about costs . . . . what is the hierarchy of these three materials on the combo of strength and durability
G10 bonded in is the king for your situation.

If it is bonded, it adds significant strength when the fitting is shear-loaded. Non-bonded backings really only help with tension.

The g10 has great properties (for use on a fiberglass hull), can be threaded, zero corrosion issues, bonds well, etc etc. It can be argued it is a bit of overkill, and there are less expensive composite plates/sheet options that will usually do the job just fine . . . . but when costs are not any real concern then you just know you are good with g10.
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Old 09-07-2024, 12:17   #12
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Re: Backing Plate Recommendations

OP, it's plain to see from the pic that the preponderance of water coming in is thru the fastener holes.
Look at the fastener holes in the stanchion bases, you'll see that the holes are at most ~3/8ths" from the edge. The flexing of the bases, (huge amounts of leverage are exerted against the bases,) leads to the water migrating from the edge of the bases to the bolt holes.
No matter what you use for an internal backing, unless you stop the water on the outside from getting to the bolts there will be leaks.
Answer: take some 3/8ths G10 and make external pads for the bases to sit on, make those pads at least 3/4" bigger all the way around than the bases.
Now with good caulk/sealant the bases will not be subject to standing water and you have a much longer pathway at deck level, (more sealant,) to resist water.
Use large washers under the bolt heads on the outside also.
It helps to have small countersinks in the deck to hold an "O-ring" of sealant.
Butyl is not a good product for this work, it lacks the resiliency/spring back for such tension loads, Butyl is good for hatches/portlights, etc. not for stanchion bases.
When you install everything don't tighten the fasteners to final torque, just take them up to a "firm" tightness, then let the sealant cure.
After curing then add that 1/4 turn or so on the inside nuts, don't turn the outside bolt heads, that'll just break the fresh sealant bond.
Done right you won't, (and don't want,) sealant all over the bolt threads/washers/nuts on the inside.
It does nothing but make any future work an unwanted hassle.
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Old 09-07-2024, 16:10   #13
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Re: Backing Plate Recommendations

My experience differs from the previous poster in that the butyl sealant I used for my stanchion bases has been leak-free for over 10 years. Same for all the other deck fittings.



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Old 09-07-2024, 16:27   #14
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Re: Backing Plate Recommendations

Bowdrie has nailed it in full,have used this system for years ,make sure all fasteners are at least 316 or greater .⚓️⛵️👍
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Old 10-07-2024, 01:03   #15
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Re: Backing Plate Recommendations

I get all my poly carbonate, Acrylic sheeting, perspex , polypropylene (you name it) out of a plastic sign jobber's recycle bin at no cost.

I would use the gray type sheeting (with the hole in it) as backing plates for stanchions. (It is not brittle)
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