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Old 08-07-2024, 16:04   #1
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Aluminum chain plates

H folks,
I have a1979 Morgan. Heritage Yachts West Indies 38 ketch and am currently in Whangarei , New Zealand waiting out. For a weather window to Tonga.
I’m looking at possibly replacing my aft chain plates for my main mast. They are original aluminum 2”x1/2”x18”. I’m wondering my best options for replacing them. Stay with aluminum? Are they alloy? Go stainless…any thoughts
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Old 08-07-2024, 19:02   #2
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Re: Aluminum chain plates

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Originally Posted by akopac View Post
H folks,
I have a1979 Morgan. ...
I’m looking at possibly replacing my aft chain plates for my main mast. They are original aluminum 2”x1/2”x18”. I’m wondering my best options for replacing them. ...…any thoughts
lets take this in 3 steps:

#1 Original from 1979, means they worked out pretty damn well for almost a half a century. Thus likely fine to just replace with the same.

#2 They could be better if hard Anodized, with synthetic washers and bushings to prevent contact with the fasteners and to insulate them from potentially wet wood. We had an all aluminum boat and (oversized) anodized aluminum chainplates are the typical choice because they can be welded in and made entirely watertight.

#3 In the 'ideal world', aluminum is usually not viewed as the first or best choice for chainplates. Silicon bronze (or other bronze alloy) is often suggested as best material. Titanium is also mentioned but is relatively rare. Carbon chainplates are the 'modern' choice and can be made entirely watertight and (essentially) maintenance-free. Stainless is quite common. There are many choices, with many trade-offs, pretty much all viewed as likely better than aluminum.
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Old 08-07-2024, 19:37   #3
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Re: Aluminum chain plates

What bothers me is that the plates that surround the chainplates on deck appear to be perfect places to hold water.
And in the first pic it appears that the wood is already stained where the chainplate passes over it just under the deck.
I know they've been successful, but I never liked how Morgan placed their chainplate bolts in a nice straight line and so close together, they concentrate so much load in a small area.
It always reminds me of a zipper just waiting to un-zip.
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Old 08-07-2024, 22:08   #4
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Re: Aluminum chain plates

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Originally Posted by Breaking Waves View Post
lets take this in 3 steps:

#1 Original from 1979, means they worked out pretty damn well for almost a half a century. Thus likely fine to just replace with the same.

#2 They could be better if hard Anodized, with synthetic washers and bushings to prevent contact with the fasteners and to insulate them from potentially wet wood. We had an all aluminum boat and (oversized) anodized aluminum chainplates are the typical choice because they can be welded in and made entirely watertight.

#3 In the 'ideal world', aluminum is usually not viewed as the first or best choice for chainplates. Silicon bronze (or other bronze alloy) is often suggested as best material. Titanium is also mentioned but is relatively rare. Carbon chainplates are the 'modern' choice and can be made entirely watertight and (essentially) maintenance-free. Stainless is quite common. There are many choices, with many trade-offs, pretty much all viewed as likely better than aluminum.
Ok so here is what I have and wheat is available to replace it here.
The existing stock is :
2 inches x 1/2 inch x 18 inches
In metric
About 50.8mm x 12.7mm x 45cm

Unknown type/quality

What I have available is
50mm x 12mm x 50cm

Type T-5 aluminum

Is this comparable enough. The mast is 64 off the water, keel stepped and the back stay is split about 20 feet off the deck. So there are 2 after chainplates.

Is this aluminum stock good enough or too brittle…
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Old 09-07-2024, 11:35   #5
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Re: Aluminum chain plates

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Is this aluminum stock good enough or too brittle…
Shame we don't know what alloy the originals were - then we would have some engineering standard to meet/exceed.

is there any 5083 aluminum available (or whatever the equiv is in kiwi nomenclature)? It is probably the preference in terms of both corrosion resistance and tensile strength.

T6 would be preferred to T5 I would think. I guess either would do the job. But we are then in the zone of 'we don't really know'.
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Old 09-07-2024, 19:01   #6
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Re: Aluminum chain plates

I had a shop plasma cut some 5083 12mm stock a little over size, then machine it back to size. Hopefully the plasma cutting won’t change the hardness, strength or brittleness of the stock. I’m being a bit precautious because you don’t change out chain plates often. When I do pull them if they are bent to lineup with the shrouds is there a bending procedure for 5083 or do you just chuck it up in a vice and bend it?
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Old 09-07-2024, 19:43   #7
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Re: Aluminum chain plates

I don’t think there is a such thing as 5083 T5 (or 5xxx T-anything), it would get an H modifier for cold working.

https://en.m.wikipedia.org/wiki/Aluminium_alloy

Quote:
-H : Strain hardened (cold worked) with or without thermal treatment

-T : Heat treated to produce stable tempers
Quote:
5000 series are alloyed with magnesium, and offer superb corrosion resistance, making them suitable for marine applications. 5083 alloy has the highest strength of non-heat-treated alloys. Most 5000 series alloys include manganese as well.
If it is cold worked stock, plasma cutting in theory could anneal (soften) it. My SWAG would be it would only anneal a relatively small heat affected zone near the cut, and not have a major impact on the part as a whole. Total guess though.
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Old 09-07-2024, 19:55   #8
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Re: Aluminum chain plates

Looking at the pictures, here is a strategy you can use:

Measure the thickness and smallest cross sectional (get both sides, total area under stress) of that SS tang that interfaces with the pin. Multiply that area by 45 ksi (310 Mpa). Divide the answer by the yield strength of whatever material you are using (probably around 30 ksi), and it will give you the min section area that will provide the same strength. Then multiply that answer by anything from 1.25 to 3 depending on your risk tolerance, and that tells you how wide/thick your plate has to be.
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Old 09-07-2024, 20:12   #9
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Re: Aluminum chain plates

The time one spends in alloy this/manganese that/heat treat this or that/cold work/heat treat, etc. is chasing mental rainbows.
Have a shop cut out and drill some 316 SS and be done with it, and you'll sleep well without a bunch of second guessing.
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Old 10-07-2024, 04:31   #10
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Re: Aluminum chain plates

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Originally Posted by akopac View Post
I had a shop plasma cut some 5083 12mm stock a little over size, then machine it back to size. Hopefully the plasma cutting won’t change the hardness, strength or brittleness of the stock. I’m being a bit precautious because you don’t change out chain plates often. When I do pull them if they are bent to lineup with the shrouds is there a bending procedure for 5083 or do you just chuck it up in a vice and bend it?
You will be more than fine with what you have there. The reason 5083 is the most common marine hull alloy is its still just as incredibly strong after cutting and welding as before. Plasma hurt nothing.

Bending is best done with hydraulics, I think it VERY unlikely you will get 1/2" thick aluminum to bend in a vice without destroying things.

The shop with the plasma cutter might have a press they can use. There are many forms of DIY press dies. The picture is one idea, but remember that you can substitute just about anything you have laying around for the angle iron: round bar works great, old bolts, even hot roll "square" tube works with its rounded corners.. But you material will require a machine to bend.
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Old 10-07-2024, 06:41   #11
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Re: Aluminum chain plates

I would bet they are stainless steel. Regardless of what's there, that's what you should use for replacement.
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Old 10-07-2024, 16:26   #12
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Re: Aluminum chain plates

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I would bet they are stainless steel. Regardless of what's there, that's what you should use for replacement.
Could you please explain your logic for this statement? Why should aluminium not be used?

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Old 15-07-2024, 08:46   #13
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Re: Aluminum chain plates

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Originally Posted by akopac View Post
H folks,
I have a1979 Morgan. Heritage Yachts West Indies 38 ketch and am currently in Whangarei , New Zealand waiting out. For a weather window to Tonga.
I’m looking at possibly replacing my aft chain plates for my main mast. They are original aluminum 2”x1/2”x18”. I’m wondering my best options for replacing them. Stay with aluminum? Are they alloy? Go stainless…any thoughts
I have the same setup for my 83 Dickerson Ketch. I replaced about 8 over the years. Each time I cleaned out the slot and used a oscillating tool to cut back the top decking a bit, filled with epoxy, installed the new chain plate and then used butyl tape to seal the chain plate slot and under the cover plate. No leaks not for over 10 yrs. I do pull the cover plates off every now and then to confirm the tape is still flexible, and it is.
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Old 15-07-2024, 08:48   #14
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Re: Aluminum chain plates

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I have the same setup for my 83 Dickerson Ketch. I replaced about 8 over the years. Each time I cleaned out the slot and used a oscillating tool to cut back the top decking a bit, filled with epoxy, installed the new chain plate and then used butyl tape to seal the chain plate slot and under the cover plate. No leaks not for over 10 yrs. I do pull the cover plates off every now and then to confirm the tape is still flexible, and it is.
Oh a local metal fabricator made up the new aluminum ones, wasn't very pricey
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