Aluminum Chainplate Scantlings

[knothead]

Here's the situation. My customer had rod rigging that was well past it's prime.
He just purchased the boat and had it trucked in.
We decided, based on cost and various other factors, to change the rig out to wire.
The lower shrouds were -17 rod and the closest equivalent 1 x 19 wire, without downsizing is 7/16"
The problem is that the old rod had a 5/8" turnbuckle and the smallest turnbuckle for 7/16" wire is 3/4".
The customer, on my recommendation, removed his chainplates and upon inspection, the decision was made to replace them. They were badly corroded where they passed through the deck. As I expected they would be.
The customer found his own machine shop to have the new chainplates fabricated and the machinist intimated that the lower shrouds were too large and that he didn't feel comfortable putting a 3/4" hole in the 2" x 1/2" aluminum chainplate.
I have a chart for bronze and stainless steel chainplates but can't for the life of me find anything on aluminum.
The machinist told my customer that the rule of thumb is to have at least as much material around the hole as the hole diameter. I've heard the same thing but I know that the thickness of the material has to have a direct bearing on that formula.
Does anyone have that information or can tell me where to find it?

Thanks

[tommays]

You will be able to find tensile on and alloy Like 6061 and compare it

6061 aluminum - Wikipedia, the free encyclopedia

6061-0

Annealed 6061 (6061-0 temper) has maximum tensile strength no more than 18,000 psi (125 MPa), and maximum yield strength no more than 8,000 psi (55 MPa). The material has elongation (stretch before ultimate failure) of 25-30 %.

[edit] 6061-T4

T4 temper 6061 has an ultimate tensile strength of at least 30,000 psi (207 MPa) and yield strength of at least 16,000 psi (110 MPa). It has elongation of 16%.

6061-T6

T6 temper 6061 has an ultimate tensile strength of at least 42,000 psi (290 MPa) and yield strength of at least 35,000 psi (241 MPa). In thicknesses of 0.250 inch (6.35 mm) or less, it has elongation of 8% or more; in thicker sections, it has elongation of 10%. T651 temper has similar mechanical properties. The famous Pioneer plaque was made of this particular alloy.


In my exp it is not a good material for that use without some serious Eng work as most of the off the shelf bars are just simple extrusions and there are so many different tempers

[sailingdog]

I wouldn't recommend using aluminum for chainplates. It is far more vulnerable to corrosion and fatigue than stainless steel. It is a pretty lousy chainplate material IMHO. Most chainplates are held in place with stainless steel or bronze fasteners, and most rigging hardware is either bronze or stainless steel. Both Stainless steel and Bronze are more noble than Aluminum and will generally cause serious corrosion issues with it, especially if exposed to salt water regularly, as most chain plates on a saltwater boat are.

Dave Gerr or Don Casey have the listings for chain plate dimensions in one of their books IIRC. I'd have to take a look on the boat, since I believe the book in question is there, and will do so if you want tomorrow, since I'll be down at the marina tomorrow.

[knothead]

Thank you both for your responses.
I agree that aluminum is not the best choice of material and when I suggested as much to my customer, his response was "stainless will cost 4 x as much".

I have to point out that the boat is over 20 years old and the chainplates have obviously held up OK. So I guess Charlie Morgan knew what he was doing.
Still prefer stainless though.

As has always been my custom, I encourage my customers to get their hands dirty and to take on as much as they feel comfortable doing.
In this case, I can only offer my suggestions and recommendations.
At least I was able to convince him to remove the chainplates for inspection.
After seeing them it didn't take any convincing to make him see that they needed to be replaced.

SD, If you have some information on the boat, and it's not too much trouble, I would appreciate it finding out the actual numbers.
I believe that I have come up with a solution, but for future reference, it would be nice to know.
I thought that the info would be simple to obtain. Perhaps in a machinist's handbook or something. But I had no success.

What I've decided to do is have them duplicate the chainplates and then special order toggles that will fit on a 3/4" t-bolt but are drill for a 5/8" clevis pin.
That way the customer and his machinist will be comfortable with knowing that the chainplate is as strong as the original and I will be happy that I haven't downsized the rigging.

[sailingdog]

I'll take a look on the boat tomorrow. Your compromise sounds like a good one.

[KeelHaulin]

Aluminum may not be the best choice; but as you said, 20 years is a deacent lifespan. There is a design criteria for a pin pulling aginst a clearance hole; that specifies the cross section of material around the hole. I don't have that info at my fingertips but I can look into it.

The issue of Aluminum type; I can check a bit on that too. You will want a hardened aluminum (annealed is way too soft). Basically all aluminums that are used for a structural application are heat treated (Precipitation Hardened). The type of precipitation hardening is the "T-Type" which is a specific tempering process. The temper is done at the factory and you don't weld, bend or cold work tempered aluminum with out annealing it first and re-tempering it. So for a formed chainplate you would need to anneal (heat to the transition temperature and then air cool) and then re-heat treat after bending. This is an expensive process. For a chainplate that is a flat plate (no bends) you would just buy the proper T-Type for your application. 6061-T6 is the most common aircraft grade aluminum alloy. There are other, more corrosion resistant types but they may not be as strong as 6061.

I think that you would want the chainplates to be hard anodized to protect from corrosion as much as possible; and use plastic insulating washers on the ataching bolts which would probably be stainless. Galvanic corrosion can be kept at bay if everything stays dry below the deck level; so re-bedding regularly would be the best defense. While machining stainless is 4x more; you don't have to anodize, so it might come out at 2x more than aluminum.

[catamount]

FWIW, my shroud chainplates are Aluminum. A couple of years ago the guys at Chesapeake Rigging fabricated new ones for me. The new plates are anodized instead of painted, and they pressed stainless bushings into the pin holes so they hopefully won't wear like the originals did. I don't know what alloy was used, but these plates are so substantial that I still have lots of confidence in them. (46" long by 10" wide by 3/8" thick, supporting upper, intermediate, and lower shrouds of -10 rod).

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[catamount]

OK, I give up. I don't know why the forum won't show my images -- they show up fine in the preview...

Go here if you want to see the pictures:

http://sailing.thorpeallen.net/Greyh...essReport.html

[sailingdog]

Catamount-

The fact that you have the word rig in your url to your photos is killing them. You can post them successfully if you encode the URLs using something like tinyurl.com. The autolinking engine on sailnet is stupid and tends to destroy any URLs that have keywords in them.

[sailboy21]

My masthead consists of a 1/2' aluminum plate welded vetically in the top of the spar (Spartech). The fore & backstay pins are 1/2" and I dare say there is at least 1/2" of aluminum around the outside of the hole. I can barley get toggles with legs long enough to fit. Never really liked the setup but it has been good for over 30 years... Spartech liked to do lots of welding on their spars. I have always wondered if the whole mast was hat treated after welding or not..