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Backing plates

11K views 53 replies 20 participants last post by  therapy23 
#1 ·
My 30 foot sailboat doesn't have backing plates for any deck hardware or the backstays, just washers. :eek:
Needless to say, I will be adding them. But what material? I run a plasma cutter in a steel shop, meaning I can custom cut plates and get the steel for free, but we don't carry stainless steel. Is it ok to use some regular high strength steel (not mild steel), or is stainless the only way to go?
What would be a good thickness to use?
 
#2 ·
I wouldn't use plain steel. If you can't get 3/16-1/4" 316 stainless steel, use 1/4-5/16" aluminum instead. If you use regular steel, you'll be cleaning up a lot of rust stains... pretty much guaranteed.
 
#3 ·
My personal preference is prefab fiberglass that can be had from places like McMaster-Carr. No rust or rot, and it can be cut and drilled with regular tools (although it is tough on the blades and bits). I use anywhere from 1/4" to 1/2" depending on the situation.
 
#7 ·
As posted on a previous thread, Ipa is a cheap and excellent alternative. To call it hard wood is understating things. Your boat will be gone before this stuff rots. Seriously. Ask around at good lumber yards or flooring places, it's pronounced EE-pay. It's sold as decking (1"X4" or 1"X6") and is a lot easier to work with than metal, though it's not much easier to drill through.

The dust that comes out of this stuff is an irritant and I would suggest covering skin and eyes as much as possible if you are planing on milling a lot of this stuff.
 
#8 ·
Majorm,

SS has a modulus that is far supperior to that of regular aluminum alloys. However, on most of the race boats I have been on, aluminum has been used. And these boats are put under some serious loads. I believe aluminum is sufficient in its strength and I do not believe it would fail or plasticaly deform if the plate is oversized properly.
 
#9 ·
A very experienced and knowledgable marine machinist (he has built one-off stuff for Americas Cup boats) recommended aluminum over stainless. His reasoning was stiffness (you use thicker Al) which does a better job of spreading the load over the comparatively flexible fiberglass which is being backed.
 
#10 ·
Plasma cutter

Must be nice haveing access to plasma cutter. Surprised you don't have stainless there as thats were plasma cutters really shine. However I would prefer aluminum of appropiate size because of ease of machineing with ordinary tooling. Can't you purchase material through your employer for your own use.
 
#11 ·
I'd go with aluminum, since it will be far easier to drill the holes and such. Don't forget to coat whatever fasteners you use with TefGel or Lanocote to act as a galvanic isolator, or use Loctite, which will also serve that function but act as a thread locker as well.
 
#12 ·
Epoxy coated 1/2 inch marine plywood can also be used. It is very commonly used as backing plates on sailboats.
 
#13 · (Edited)
Aluminium is not a bad choice, but it will be coming in contact with the stainless steel bolts and nuts, and that (in dark dank out-of-sight places on your boat is a recipe for some corrossion issues).

For my money, the "best" system is a combination of compression material and rigid material. Lets face it, the inside of your fibreglass hull is not perfectly flat and even. If you just use a metal backing plate, the edges of that plate are likely to come into stress contact with the hull material because the plate is not sitting perfectly flat and only touching on its face.

So I use a 6-8mm pad of treated marine timber (I like ply, as it does not split). I THEN have my stainless steel backing plate. The plate thus compresses the timber pad and forms a perfectly flush contact witht he inside of the hull. Best of all worlds!

Using this method you can use some pretty thin stainless for backing plates. I use 16ga (1.6mm) for high torque applications and 1.2mm for most everything else. 1.2mm stainless in 316 can almsot be cut with good tin snips or average aviation snips...So do not worry about plasma cutters and such.

I mostly use a beverly shear I happen to have, but also a 4inch angle grinder with stainless cutting disks....very cheap and easy.

As much as you have some cool toys at your work, it really IS better to use stainless and not high carbon steel (High carbon will work-harden and be prone to stress failure as well as corrossion). Just buy some and use low tech tools to cut and drill it so that you get the results your boat needs.

(I might be predjudiced, the last 8 times I have set my head on fire have all been with a plasma cutter...)

Let me know if you want some pictures of the backing plate instalations we are currently doing on a friend's boat.


Oh yeah.... I would never use the pre-made fibreglass pads. They are kind of pointless and rather expensive. If you want to use a non-wood compression pad, the I would just head to the local $2 chinese shop or cheap kitchenwear supply and buy a bunch of those plastic cutting boards. They are easy to cut, compform and compress somewhat to fit the inside of hull shapes and never rot or die the way that some wood might. They even look good when framing a nice stainless plate (or just big washers).


Sasha
 
#18 ·
Using this method you can use some pretty thin stainless for backing plates. I use 16ga (1.6mm) for high torque applications and 1.2mm for most everything else. 1.2mm stainless in 316 can almsot be cut with good tin snips or average aviation snips...So do not worry about plasma cutters and such.
I am no expert but would not thin stainless end up with a compression circle about the same as a washer?
 
#14 ·
One thing I've seen used for backing boards is the new composite deck material. It is pretty rot resistant, somewhat compressible, and when used with fender washers, seems to work quite well.
 
#15 ·
For some reason I just keep coming back to the notion that it si a good idea to not use something that is held together witht he same kind of epoxy resin as is used in the hull. Not sure entirely why this seems like a good plan...but I stand by it. I want a material with different but advantageous properties.

Sasha
 
#16 ·
One thing I've seen used for backing boards is the new composite deck material. It is pretty rot resistant, somewhat compressible, and when used with fender washers, seems to work quite well.
Ipa. It's better than "trex decking". It will last longer, it's cheaper that trex, and it's not held together by glue. It's held together by some higher force... Or at least you'll think it is when you try drilling it. ;-)
 
#17 ·
It's not uncommon to have washers in use for backing support - I would not automatically replace them all with plates, unless you've run out of other boat chores...Most stanchions I've seen have only washers under the nuts, including my CS - the stanchions will bend or the bases split (been there, done that...) before you'll pull the nuts/washers through the laminate - backing plates don't fix the weak points in the installation.

Sailing gear like winches and blocks always seem to have washers, and such should be servicable and adequate. Our mooring cleats and shrouds have sturdy backing plates, and I would recommend such for that type of application.
 
#19 ·
Well....it is not so thin that it just gets stretched like tin foil... And the load being spread across the whole plate provides a ton (or two or three) of strength.
The way to think of it is that the thin stainless sheet adds a hard skin to the wooden compression pad beneath...and the pad adds a support base that keeps the plate from deforming.
You can do it with 2mm if you want...but it is harder to cut to shape (unless you just use the angle grinder) and it wreighs and costs a little more. If none of that is an issue then go for the thickness you want.
Now at about 2.5mm you are entering the realm of the ridiculous and burdensome....But between about 1.2mm and 2mm is a happiness for backing plates, in my experience.

I totally disagree about washers being as good as backing plates. Washers, unless they are very large indeed) provide a point loading and can themselves damage GRP when stressed to-and-fro. Backing plates are a broadsource support and means that instead of enlarging a bolt hole to some degree...you need to tear a big square out of the deck where there are no bolt-holes to act as starters. Much much stronger. (the plate is bigger then the footprint of the boltholes, you see)


Sasha
 
#20 ·
I did most of my deck gear (but not my stanchions...yet) on my '70s racer-cruiser with 1/4" aluminum bar stock, replacing dished fender washers. The ones for the main winches were about six inches across, and the winches themselves feel exceptionally solid now.

I did the typical drill and fill on the cored deck boltholes, but I didn't use Tufgel because I am on fresh water. I did use 231 Sikaflex as a bedding compound particularly on the parts of the bolts that passed through the holes the deck and the plates. Not a drop in five years.
 
#22 ·
If you're using a non-metal material for a backing board, please use a fender washer to protect it from the nut digging into the surface. A stainless steel nut under a high load will gouge its way into wood, plastic or fiberglass quite well.
 
#23 ·
Starboard, cutting boards and the like should not be used as backing plates! This is BAD advice! This material is very soft and even with fender washers will deform under load. On top of it's inherent lack of strength not even 5200 likes to stick to it..

While it may be fine for very lightly loaded items, like pedestal guard feet, it should NEVER be used as a backing plate material for cleats, blocks, chocks, winches or stanchions! G-10 or Garolite (available from McMaster Carr) sheet fiberglass is quite expensive and using 1/4 inch stainless steel is a far les expensive way to go. My local marine fabricator gives me scrap 1/4 inch stainless, big enough for many backing plates, and if I need them bigger he cuts me a piece for about $10.00.

When was the last time you saw a builder using "plastic" cutting boards as keel bolt backing plates or washers? C'mon the bad advice on this board is getting way out of hand!

 
#24 ·
I'm no naval architect, heck I'm a database architect...

Whichever way you go, round the corners and edges or bevel the edges of the plate, that will reduce and or spread the point loads and stress cracking by not allowing the corners and edges to dig in.
I'd go with 5/16 aluminum plate, easier to cut and shape and strong enough for the application. Prep and paint the piece with a proper primer (dip it and hang dry) for a good solid coat Then TefGel the holes and other points of contact with disimilar metals as insurance. It will last longer than you own the boat.
If the surface under the plate is not even, bed it in 4200 for that 'sleep sound at night' feeling.
Note that following the steps above will also encapsulate the free steel you get from work and eliminate the rust concern...
I've only installed one, you get the advice you pay for.
 
#26 ·
Actually, if you're going to do this, you need to bed the plates in thickened epoxy, not 4200. 4200 doesn't really work to transfer the load properly, and you'll have strangely distributed stresses, rather than the stress distributed evenly over the surface of the backing plate. While the advice to round the corners and bevel the edges is a good one-if the backing plate isn't fair to the surface it is backing, the filler between the plate and the surface needs to be one that can handle the strong compressive forces involved. 4200 doesn't meet this basic critieria.
 
#25 ·
When I bought my P36-2 late last year it came with a few stanchion bedding issues including some soft deck around one of the stanchions. Repairing the deck was a BIG job that I never want to do again. When it came time to rebed the stanchions I decided to replace the FRP backing plates with 1/8 SS. I traced the stanchion base out on a piece of paper, copied that tracing on a copy machine set to 125% enlargement. I sent this to Whitewater Marine in Port Huron, MI where the backing plates were cut, including the bolt holes (set at the 100% spacing) so I had nothing to drill myself. I had two plates made for each stanchion, one for under the deck and one for under the stanchion base. If memory serves, the price was about $10 a plate for a total of about $200. Now this arrangement my not be the best possible, but I'm a lot more confident that the stanchions won't tear up the deck if something hit's them a little too hard.
 
#28 ·
No worries... just need to keep the loads spread over as much area as possible, without hard spots, or point loading.
 
#29 ·
This discussion about backing plates is useful for sure. I am rebedding and fixing my stanchions right now on a Pearson 28. I have a few that require some deck repair and I am putting in a mix of marine 3/8" plywood and West epoxy. For backing plates I will use the same plywood together with the original fiberglass backing plate, and the original SS bolt, washer, and lock washer. What I am also doing is putting on a topping plate a little larger than the stanchion plate. I am using polyethylene curtting board. It's about 3/8" thick and I am cutting it a little larger than the existing stanchion plate so it spreads the load some on the deck and it hides the cut around the existing stanchion plate for those that required deck repair.
 
#30 ·
I'd use fiberglass for the plates above deck, and think that 3/8" marine plywood is a bit thin for backing board. Also, if you're using marine plywood, you need fender washers, at least 1.25" in diameter, not just washers. I also hope you're thickening the epoxy with colloidial silica.
 
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