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Old 01-08-2012, 17:20   #1
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Aluminium Chainplates

Does anyone have any experience with aluminium chainplates. As my hull is aluminium, my first choice is to weld suitable layers of aluminium on in order to spread the load and bring it all up to an eye at a convenient point. I'm pretty sure I can calculate the breaking strain required ( most seem to suggest 1.5 times the rigging) and thus the cross section, but I'm more worried about fatigue in general and wear on the eye in particular (I was thinking of a urethane bush). Any ideas welcome.
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Old 01-08-2012, 19:31   #2
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re: Aluminium Chainplates

Nick,

Aluminium is frankly a terrible material for chainplates for a number of reasons.

1) it doesn't have a critical fatigue limit. Which means it will fail at some number of cycles, regardless of the load imposed
2) it is relatively soft, so the eye is a problem
3) it is very weak relative to other materials. Just a quick chart to demonstrate, with more common materials...

...................yield strength.....tensile strength
6063-T4 Al........13,000psi........25,000psi
304..................31,200psi........73,200psi
316..................34,800psi........74,700psi
Grade 5 Ti.......128,000psi......138,000psi

Most engineers design chainplates based on yield strength, so if you go to aluminium you would need to roughly tripple the size, and increase the size by 10 times compared to titanium. So your chainplates just got massive in size. This could effect deck cut outs significantly, local stiffness, and of course you will likely need to weld them to the hull since aluminium bolts are not strong enough in most cases.

4) Trying to penetration weld these massive chainplates will be difficult at best.
5) why? Didn't the boat designer already spec chainplates in the design? If so why would you change them. If you want to try them, I would go back to the designer to get his blessing.
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Old 01-08-2012, 20:45   #3
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re: Aluminium Chainplates

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Nick,
1) it doesn't have a critical fatigue limit. Which means it will fail at some number of cycles, regardless of the load imposed
I think the usual practice is to calculate this for a large number of cycles, sometimes so large that there is no conceivable limit (as done with airframes). This is the part that I am not familiar with. One of the reasons that I am an interested if anyone already has something similar. Much more reasuring to look at something that has lasted 20 years and copy that.

Quote:
2) it is relatively soft, so the eye is a problem
That is why I am considering a urethane bush.

Quote:
3)
Most engineers design chainplates based on yield strength, so if you go to aluminium you would need to roughly tripple the size, and increase the size by 10 times compared to titanium. So your chainplates just got massive in size. This could effect deck cut outs significantly, local stiffness, and of course you will likely need to weld them to the hull since aluminium bolts are not strong enough in most cases.
Size would not be a problem and I would weld them to the hull on the outside so they don't pass through the deck.

Quote:
4) Trying to penetration weld these massive chainplates will be difficult at best.
This is one I need to check up on, luckily I know someone who builds ali powerboats.

Quote:
5) why? Didn't the boat designer already spec chainplates in the design? If so why would you change them. If you want to try them, I would go back to the designer to get his blessing.
Yes, the specs say 40mmx8mm stainless, welded to the mild steel hull so I'm out of luck there. So if I went 10mmx100mm in ali, that would be 3 times the cross sectional area, and not really that big. I've seen ones like that in bronze on wooden boats of a similar size.

I don't like the idea of bolting stainless to the hull, then I have holes, crevice corrosion etc. I don't think they do this for plate ali workboats, but then they don't have chainplates which may or may not undergo different cycles than some other deck fittings (cranes etc)

As to the blessing of the designer.... I think that would open a whole can of philosophical worms However, I do value opinion and experience. If it was a matter of assembling a boat to an exact plan, then I probably would just go and buy a used one for less.
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Old 01-08-2012, 21:03   #4
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Go with stainless, silicon bronze, or titanium. Paint epoxy the hull to keep the different metals apart, my two cents. Angle grinder, cobalt bit, a file, and some stainless bar stock.
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Old 01-08-2012, 21:11   #5
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re: Aluminium Chainplates

I think it depends on the scantlings. It will need to be much larger than a comparable stainless steel chain plate. I wouldn't rule out the idea if you make it large enough. Consult with a naval architect on what scantlings you would need.

Of course if you use stainless then you have a potential electrolysis problem if you attach it improperly.

The point at which a stainless steel pin touches the aluminum eye is not much of an issue if the eye is big enough and strong enough. That's an important "if".

If you do want electrical isolation you won't find that epoxy will take that kind of compression in pounds per square inch load. You will want something like a Delrin bushing.

Welding aluminum to aluminum is not an issue here if the welding is done properly. I have seen aluminum pad eyes welded to aluminum hulls for the purpose of lifting the entire boat out of the water with a crane
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Old 01-08-2012, 21:19   #6
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re: Aluminium Chainplates

There is a metal that is aluminum on one side and steel on the other. They are explosively fused. It is used to join aluminum and steel, so you could use it to attach a steel chain plate or a steel loop on the toe rail for the rigging. I can't remember what it is called though.
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Old 01-08-2012, 21:36   #7
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re: Aluminium Chainplates

Garcias, Ovnis and a friend of mine with 38 Kechito have aluminium chainplates, in the Kechito the plate actually is a long flat bar welded to a interior frame and trough the deck , the opening in the deck is welded to the chainplate, is probably 1 1/2 inch thick and with a stainless steel bushing.
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Old 01-08-2012, 21:39   #8
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re: Aluminium Chainplates

Friends don't let friends have aluminum chainplates. Is it possible? Doesn't seem so.
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Old 01-08-2012, 21:40   #9
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re: Aluminium Chainplates

I do not know much about aluminum boats but Ovni, Garcia, Boreal all use aluminum chain plates and some of these companies have for years. Never have heard of a problem after many years of use, they last longer than SS chain plates do on wooden or glass boats. Just make sure your aluminum is all the same or compatible. From what I understand Aussy has some really good aluminum welders. Don't use SS, Just aluminum.
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Old 01-08-2012, 21:49   #10
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re: Aluminium Chainplates

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Originally Posted by stevewrye View Post
I do not know much about aluminum boats but Ovni, Garcia, Boreal all use aluminum chain plates and some of these companies have for years. Never have heard of a problem after many years of use, they last longer than SS chain plates do on wooden or glass boats. Just make sure your aluminum is all the same or compatible. From what I understand Aussy has some really good aluminum welders. Don't use SS, Just aluminum.
Yes i agree, dont use any stainles steel in a aluminium , a well designed and welded alu chainplate can last forever....
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Old 01-08-2012, 22:42   #11
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re: Aluminium Chainplates

Have a friend who is currently on the way to Alaska from Hawaii in his 25 year old 50' plus Aluminum Boat. Boat has aluminum chainplates about 1/2" thick, IIRC. They've worked fine for him. He had the boat built in Canada and has been cruising it ever since. Aluminum isn't the best material for chainplates but on an aluminum boat it's the only material I'd use. Just upsize the scantlings to compensate for wear and the lower strength of the metal. Not a problem, if you do that. Any naval architect should be able to size them for you. No crevice corrosion in Aluminum.
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Old 01-08-2012, 23:18   #12
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re: Aluminium Chainplates

On an aluminium boat use aluminium chain plates. I would not consider anything else. I have never seen any aluminium boat with anything else.

I have never heard of an aluminium chain plate failing on an aluminium boat. They are welded so will never leak and therefore don't suffer from the crevice corrosion problems seen in stainless steel chain plates on fiberlass boats.

They do need to engineered correctly and I would get professional advice on this. Typically they are about a foot long 12-20mm thick aluminium and welded to a strong bulkhead and where they penetrate through the deck. The weld makes them integral with the bulkhead distributing the rigging loads.

Bushings are not normally used.
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Old 01-08-2012, 23:30   #13
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re: Aluminium Chainplates

<<Aluminum isn't the best material for chainplates but on an aluminum boat it's the only material I'd use.>>

Likewise. The fussiest and most knowledgeable people on materials you'd ever find who have alu boats, like Peter Smith, Evans S and Beth L, Steve Dashew et al ... they all use aluminium chainplates on aluminium hulls. There is no alternative option even worth considering.

You must ensure the thickness is more than adequate for the bearing loads of the pin. If necessary, weld on discs either side, but if the proportions are correct this should not generally be necessary.

Often, perhaps usually, they're not sleeved, but personally I think they should be on any serious offshore vessel. FWIW, the relevant ISO document (which does not have the force of an official standard) suggests sleeving alu chainplates with stainless for pin sizes over 10mm.

It also lays out the thickness and pin diameter for 5083 alu alloy chainplates. I think it's probably online by now; ISO/WD 12215-9

I personally think the gold standard is to sleeve the holes in such a way that the sleeve cannot move axially, while still ensuring there is no electrical contact between sleeve and chainplate.
I don't think Duralac or other insulating paste is quite good enough in this instance, although I'm almost sure most alu boatbuilders would disagree.
The platinised gold standard would be to turn up a stainless sleeve with a flange at one end like a headed bush, fit a thinwalled (maybe 1/6 pin diameter MAX) Tufnol or Micarta headed bush around it, insert in chainplate, fit a T or M washer at the opposite end to the heads, and outside that either screw on a slim stainless, fine threaded nut with loctite, or fit a plain stainless (close fitting) washer with the internal bore countersunk towards the outside, and rivet the end of the slightly overlength sleeve to capture everything. The neatest way to rivet, (assuming you turn the end of the sleeve to a suitable fineness) is to insert a ball bearing ball maybe 1.5 times the pin diameter and use a powerful G clamp or a machine vice to swage the end of the sleeve to a flare, so it captures everything tightly, axially. If you get the fineness right (by trial on short offcuts), this flare will not split.

You hardly ever see this - or anything like it -done in practice, but you do earn points towards your karma balance, arguably.

You could make up a trial sleeve out of mild steel, with enough clearance for Duralac (and rivet it both ends as suggested, but without washers) through an offcut of alu plate, and do an electrical continuity check. I'm pretty certain it would conduct. Then drill it out and try one with washers, ditto.... I guess you could additionally try a bit of teflon tape around the ends, inside the washers, before rivetting?
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Old 02-08-2012, 00:25   #14
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Missed the part in the original post where the aluminum hull was mentioned. Getting old.

What are you using now, and why are you replacing them? Aircraft grade aluminum is amazing stuff. Ask a proper certified aluminum welder what types of aluminum can be welded to your hull.
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Old 02-08-2012, 01:41   #15
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re: Aluminium Chainplates

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... Aircraft grade aluminum is amazing stuff. Ask a proper certified aluminum welder what types of aluminum can be welded to your hull.
Aircraft grade (eg 2024, 7075 et al) alu is indeed amazing.
Phenomenally strong: gets used as a substitute for mild steel when doing 'soft tooling' for trialing moulded items, for instance

However another of the amazing things about it is the way it disappears in seawater.

Admittedly not quite as fizzy as soluble aspirin, but.....

ON EDIT: note to GaryMayo: this is a new, self-build

Note to Nick: I had not connected the name with the project: for a boat your size and presumed intentions, forget my 'gold standard', which I think would be almost akin to polevaulting over a mouse turd.

I don't think you need to bush them. There's just not going to be that much inertia in the rig on a boat that size to give the holes a hard time. You could wrap the toggle pin in teflon tape and then slather with Duralac or Tef-gel as a sop to your conscience.
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