Backing plates

[agprice22]

I am rebedding all my deck hardware, and where possible, I would like to use backing plates to reduce the use of washers in the cabin overhead.

What metal should I use? Can I use Aluminum? Or should I use stainless?

Thanks,

Andrew

 

[higgs]

I don't think it matters. The purpose of a backing plate is to spread the stress. Even wood will do the same.

 

[topcat0399]

what higgs said.

Stainless is stronger but how strong do you Really need? Taking on hurricanes are ya?

We used 1 inch good quality ply and kept all of the washers...

 

[centerline]

aluminum, stainless or delrin.... they will all stand up to the task.

delrin and aluminum are about equal to work with, but stainless can be a bastard if you dont have the proper tools...

i wouldnt use any less than 1/8 aluminum and 3/16 delrin on a boat that size. 1/8 stainless would be super strong for almost any size boat...

and even though plywood may work for some due to the ease of working with it, I tend to stay away from plywood as a backer as that is what I have found to be the main cause of water intrusion around the deck fittings on many boats... in time it fails and allows the fitting to be loose. I have never seen an older boat that had plywood backing plates that DIDNT need to be replaced... teak is a good material to use though.
I have had many boats and I have replaced the plywood backing with either of the three materials i listed, and have never had a failure or leak yet...

if the fitting cant move under strain, almost any sealer used under it will remain good and sealed for the lifetime of the boat...

 

[FastOlson]

I would avoid any cast resin product like Delrin. I kinda like G10 glass fiber plate. 1/4" should be just fine. Put some ss fender washers on and then some nylok (sp) nuts. Bed it with butyl rubber under the hardware piece on top, per the fine instruction and pictures on the great MaineSail site.

If you go with alum. or ss for b/u plates, it will do OK also. This is a situation with several flavors of "right" answer.

Loren

 

[centerline]

I would avoid any cast resin product like Delrin. I kinda like G10 glass fiber plate. 1/4" should be just fine. Put some ss fender washers on and then some nylok (sp) nuts. Bed it with butyl rubber under the hardware piece on top, per the fine instruction and pictures on the great MaineSail site.

If you go with alum. or ss for b/u plates, it will do OK also. This is a situation with several flavors of "right" answer.

Loren

you shouldnt discount delrin from your list.
depending on the type of delrin used, it could be nearly as strong as the glass fiber plate... without any problems, although it is easier to work with than the glass fiber plate.
and for a 22 ft boat, 3/16 delrin will be plenty strong as a backing plate for any piece of hardware mounted on the deck... and its cheaper than glass fiber plate. but glass fiber plate IS a better electrical insulator, if you ever have a need for an insulated structural connection.

if delrin is a strong enough material to be used to make deck hardware such as cleats, cam cleat jaws, rail clamps and other mounting hardware its definitley strong enough to be used as a backing plate under the hardware .... its a very strong and stable product.

but no matter what choice a person makes, its going to be better than, and outlast plywood

 

[agprice22]

Wow! Those are great suggestions! I admit I had not considered Delrin. My purpose for using the backing plates is twofold: strength and aesthetics.

New question: can I forgo washers / fender washers if I use aluminum, stainless, or delrin?

Thanks,

Andrew

 

[Ken Cross]

If you forgo washers, you may find scars where the nut turns unless the nuts have a round raised contact surface.
Ken

 

[Ron20324]

When planning things, consider placing an eye strap under two of the nuts, or use an eye nut so you have a place to hang things. I did that in the head and cabins for coat hangers in particular.

 

[jibes138]

It is really about the shear area and shear strength of the materials in the sandwich. Say you have no backing plate or washer, you can calculate the cross section area resisting pulling out the nut through the deck. Then adding a washer you add some area and the strength of the washer material to the sandwich to pull the nut through the washer and the deck. Then if the washer is sufficiently strong to resist the nut pulling through the cross section area becomes the diameter of the washer trying to shear through the deck. Now add a backing plate and that area just becomes that much larger. Good tempered aluminum can be just as strong as typical marine grade stainless steels. For instance 7075T73 aluminum has an ultimate tensile strength of 73,200 PSI. 316 stainless steel has an ultimate tensile strength of 80,000 PSI. You really have to know what you are buying though to make sure you get something with some strength. There are many types of aluminum and a huge variation in strength. You can do some easy calculations using the max strength of your bolt as the shear load on the assembly as a worst case (beyond that the bolt will break and the backing plate won't matter).

 

[Prime Time]

I would not use dissimilar metals. Asking for corrosion especially if leaks.

 

[agprice22]

I would not use dissimilar metals. Asking for corrosion especially if leaks.

Thank you. I was wondering about the corrosion properties in dissimilarity. Any idea what will happen to the stainless bolts in contact with aluminum?

Thanks,

Andrew

 

[agprice22]

Thank you. I was wondering about the corrosion properties in dissimilarity. Any idea what will happen to the stainless bolts in contact with aluminum? Thanks, Andrew

Actually, I just read the Fastenal website. 410 Stainless fasteners should not be combined with aluminum as a base metal, as it causes severe corrosion.. However, 302, 303, 304, and 305 stainless only marginally increase the corrosion of the aluminum base.





I wonder if it would change if I coated / painted the aluminum...

Looks like Delrin might be a material to consider?

Thanks,

Andrew

 

[Prime Time]

The aluminum will be degraded over time.
Star board works. I've used it for winches.

Polished 316 stainless is my first choice

 

[kloudie1]

If you have some metal working capabilities, 316 stainless is fairly inexpensive. You'd need to be able to cut and drill the plates that you'd make out of flat bar.
http://www.onlinemetals.com/merchant.cfm?pid=661&step=4&showunits=inches&id=27&top_cat=0

410 is called "stainless" but it really is not what most folks think of as stainless. It has very good high temperature properties but corrodes readily at normal temperature in water and sea water.

 

[Ward H]

Andrew,
A lot of good suggestions but maybe you should step back a moment. Why are you trying to remove the washers? Aesthetics? You still need a nut. Maybe you can recess the washer and nut? If there is a line of washers & nuts for a hand rail, can you cut a length of trim, drill part way through and mount it to hide the washers & nuts? Maybe a square or circle, with recesses for the washers & nuts routed out to hide them?
You could use teak or starboard.

 

[agprice22]

If you have some metal working capabilities, 316 stainless is fairly inexpensive. You'd need to be able to cut and drill the plates that you'd make out of flat bar. http://www.onlinemetals.com/merchant.cfm?pid=661&step=4&showunits=inches&id=27&top_cat=0 410 is called "stainless" but it really is not what most folks think of as stainless. It has very good high temperature properties but corrodes readily at normal temperature in water and sea water.

Thanks! I had actually read their excellent guide last night. I found that the flat bar 316 is the best choice as it is designated for marine use, and it doesn't cost much more than the aluminum! I am ordering some today. I have an abrasive chop saw, a grinder, jig saw, and a drill press, so I can fabricate these plates.

Thanks! I am glad for everyones' help and that I read the Fastenal page on corrosion of dissimilar metals before making my choice.

Andrew

 

[agprice22]

Andrew, A lot of good suggestions but maybe you should step back a moment. Why are you trying to remove the washers? Aesthetics? You still need a nut. Maybe you can recess the washer and nut? If there is a line of washers & nuts for a hand rail, can you cut a length of trim, drill part way through and mount it to hide the washers & nuts? Maybe a square or circle, with recesses for the washers & nuts routed out to hide them? You could use teak or starboard.

Yes, we are planning a couple of teak rails to hide those handrail nuts, but there are a bunch of others, and I would like to clean them up a bit. Yes, there will be nuts still... I will be okay with that.

Thanks!

Andrew