CAS or "Chainplate avoidance syndrome"

[Rumrace]

I’ve seen folks measuring bridges and using blue line to show cracks up. I’ve seen the same method on cast iron heads to check for cracks. Alarm bells light up in blue. I imagine blue line would test your hardware quickly.

[skipperpete]

Originally Posted by Rumrace

I’ve seen folks measuring bridges and using blue line to show cracks up. I’ve seen the same method on cast iron heads to check for cracks. Alarm bells light up in blue. I imagine blue line would test your hardware quickly.

What’s “Blue line”? Are you referring to the FAD?

[Gone Gypsy]

Every now then something comes up on CF I actually know something about. The answer to the question about whether the ‘hidden’ side shown with red arrows would have shown a crack: YES! Quite likely visible by naked eye but certainly with die penetrant. As discussed in the linked article the crack initiated by corrosion cracking but extended through most of the chain plate by progressive stepwise cracking from cyclic loading - a fatigue crack. A pretty common mode is that 90% of the part life was initial crack formation and the fatigue time was the last 10% of life. The chain plate survived until cracked almost all the way through. The goal is to see it before the rig comes down. It was definitely observable.
Fair winds.

[markxengineerin]

all of the specifications for the alloy's mechanical properties were readily available

This statement is nonsensical considering the nearly infinite mechanical properties that one could think of. In the case of chainplates, I wanted to have a fatigue curve for the material, after having been bent 30 degrees, in my specific thickness, with my specific radius of bend. I wanted to know what temperature to use when bending. And I wanted to know that it's never going to be affected by corrosion. If someone else can find this information for another material, or doesn't care, that's great.

[LifesBetterWhenYoureBeating]

Originally Posted by SJ2581

Every now then something comes up on CF I actually know something about. The answer to the question about whether the ‘hidden’ side shown with red arrows would have shown a crack: YES! Quite likely visible by naked eye but certainly with die penetrant. As discussed in the linked article the crack initiated by corrosion cracking but extended through most of the chain plate by progressive stepwise cracking from cyclic loading - a fatigue crack. A pretty common mode is that 90% of the part life was initial crack formation and the fatigue time was the last 10% of life. The chain plate survived until cracked almost all the way through. The goal is to see it before the rig comes down. It was definitely observable.
Fair winds.

I'm about to remove the chainplates on my new (to me) boat for a closer inspection. Some are almost spotless, but others have some signs of corrosion. Any suggestions on how to clean and what to look for would be great. Specially what die penetrant to use

Cheers

[Cyrus Safdari]

Originally Posted by gsalgado

I'm about to remove the chainplates on my new (to me) boat for a closer inspection. Some are almost spotless, but others have some signs of corrosion. Any suggestions on how to clean and what to look for would be great. Specially what die penetrant to use

Cheers

Small pits and cracks are giveaways but I'd say forget dye penetrants etc, if it doesn't shine up with a light sanding, replacing it could be easier to & more certain too for another few years.
Chainplates are typically just a bar of metal with some holes drilled in them, maybe a slight bend. Any machine shop can duplicate, the hardest part is drilling the SS

[Cyrus Safdari]

Originally Posted by kmacdonald

Can you size bronze chainplates the same size as 316 stainless steel? What is the best alloy of bronze to use?

There are two "bronzes" available online (and there's a CF huge debate over what "bronze" means and whether it includes for example naval brass -- which is not relevant!)

Aluminum Bronze would be better than Silicon Bronze (which is best for things like bearings) however aluminum bronze does not bend very much without cracking so it may not be useful for chainplates that have a sharp bend

Bronze in general is slightly softer than SS but I'd consider increasing the size (Width & thickness) of chainplates if replacing them anyway regardless of metal used

I'm curious if anyone has used G10?

[Gone Gypsy]

Corroded surfaces confuse die penetrant tests. Clean, smooth surfaces make penetrant results an easy yes-no result. I would replace any pitted plates and only inspect those with no corrosion or those merely showing staining but no pits. It’s a pit if it catches your fingernail. Intertec and Magna-Flux are reliable die products. Be sure to perform all three steps. Spray die, wait, use cleaner and then developer (chalk). This final step of spraying on a dry chalk ‘pulls’ die out from any crack and highlights it as bright red against the white chalk. There are easy to follow YouTube videos. This is not a high tech process. Even small cracks, just a few thousandths deep, will show up. If your surface appears smooth and clean to the naked eye and comes back checkered after die test you have fine cracks. Replace. A long straight line penetrant indication says the independent fine cracks have coalesced and are propagating as fatigue with additional load cycles. Go to church and then replace. If all you see at the end is white chalk, dust them off and put them back in with confidence.

[svpatience]

Thanks Rumrace, SJ2581 and Cyrus Safdari. Given your comments, I'm going to bring my loupe/magnifier down and take a good look at the side I can see. Mine look good on the surface (i.e. shiny, not obviously corroded) on the side I can see. I think it's the side I cant see that worries me a bit more! I think I need to remove them, clean them up, and do a proper inspection on a workbench. My shrouds are set up in such a way that they shouldn't be too difficult to remove. It honestly might make sense just to replace them as Cyrus Safdari suggests... they are 35 years old. The chainplate that attaches to my bobstay is a different story as it is embedded in the hull beneath the anchor locker. Getting that one out will be a substantial project that can only be done when the boat is on the hard.

[ItDepends]

Originally Posted by svpatience

ItDepends, this was a really helpful article that helped me understand crevice corrosion a bit better - thank you! I have 35 year old stainless chainplates which worry me a bit. I inspect the rig, but struggle to know exactly what I am looking for. The article you linked to states that the corrosion was invisible on the side that could be inspected (green arrows in picture below), but I'm curious what would have been visible on the other side that couldn't be inspected (red arrows). I'm guessing pre-failure the crevice corrosion would have manifested itself as a very thin hairline crack on the hidden side?
Attachment 265426
Tim

svpatience,

Thanks for the kind review. The situation was such a classic case, and the photos showed the situation so clearly, it was pretty easy to write. I worked in pharmaceutical manufacture for many years, and we handled a lot of very salty solutions, and corrosion was always a concern and was watched VERY carefully.

I am sure the cracking would have been visible to a careful visual inspection, if it had been possible to see the originating side of the part. For this part, it was not at all possible to disassemble in such a way as to observe the location. If there was a significant amount of surface "wrinkling" from the bending of the part, that could have obscured the issue.

However, many things can obscure a simple visual test. Surface roughness and superficial corrosion can make a quick and easy inspection impractical. If you can carefully polish the part, the crack will be visible, to a loupe always, and usually to the naked eye. Even if the part is a bit grungy, a bit of "rust" can frequently be seen leaching from the crack. It gives you a good idea where to look.

Some parts are almost impossible to inspect. Bolts, for example. There is no simple way to inspect the roots of the threads for cracking.

[ItDepends]

Originally Posted by markxengineerin

This statement is nonsensical considering the nearly infinite mechanical properties that one could think of. In the case of chainplates, I wanted to have a fatigue curve for the material, after having been bent 30 degrees, in my specific thickness, with my specific radius of bend. I wanted to know what temperature to use when bending. And I wanted to know that it's never going to be affected by corrosion. If someone else can find this information for another material, or doesn't care, that's great.

Ah, come on.

People have been building boats with bronze chainplates for hundreds of years. The idea that it is somehow risky and the knowledge is not available to do it safely is just plain silly. Just because you (apparently) don't know how to do it, does not mean it can't be done and the knowledge is not available.

In my case, I had two plates that needed bending, about 20 degrees. I had a guy working for me who was taking machining classes at the local tech collage, they took it as a project. With the alloy specs, they had the right temperature profile to do the bend. The most important advice was "bend it all at once, don't stop." They took the new part out of the furnace, put it in a vice next to the original, and it bent perfectly. Worked a charm. I do not recall if there was additional annealing needed afterwards, but it isn't important, I had experts doing that part that I needed help with.

Tensile strength of these bronzes is almost identical to the 300 series stainless steels, so the same scantlings can be used. We ran the numbers anyway just to be sure. I don't remember exactly the result, but it was pretty straightforward to calculate the amount of metal that needed to be on pull side of the clevis pins based on the strength of the wire in the shrouds.

[Cyrus Safdari]

Originally Posted by ItDepends

They took the new part out of the furnace, put it in a vice next to the original, and it bent perfectly. Worked a charm.

Just curious: as I understand it aluminum bronze cannot be heat much as it disintegrates so you must have been using silicon bronze, right?

[markxengineerin]

Just because you (apparently) don't know how to do it, does not mean it can't be done and the knowledge is not available.

I did not say or imply it can't be done, or that the knowledge is not available. I explained why I made titanium chainplates.

The most important advice was "bend it all at once, don't stop."

I've never heard of anything like this. What is the explanation?

[dlj]

All materials have advantages and disadvantages, and their respective failure modes. Stainless steels and bronzes have a long history of use in boats in different applications within the marine environment, specifically chain plates. Titanium alloys do not.

None of these alloys are or will be the "magic bullet" to solve all problems. Chain plates are a specific application where I am quite leery to use Ti6Al4V.

I'm happy that some people will use the titanium alloys for chain plates. It is not a well know application for this material. I'd like to see a few decades of use of Ti6Al4V for chain plates in sailboats across a wide range of boats and installations to see how it works. But I will not be alive long enough to wait for that "test of time".

I personally will never use Ti6Al4V (Grade 5 as you have called it - that would specifically be ASTM B348 Grade5) as chain plates for a number of reasons. I will use it for other applications on my boat, in fact I am using it in applications where I personally feel it is more appropriate.

Why do I not feel comfortable using this material in chain plates? 1) This alloy is a very notch sensitive alloy. 2) It performs badly in wear applications.

The notch sensitive aspect means if you nick or scratch your chain plate and a fatigue crack starts - that chain plate will break much faster than your stainless steels will through stress corrosion cracking.

The wear problem produces a concern for me at the connection points. You connect your standing rigging to the chain plates through pins. At the pin to hole interface, you have minor articulation from vibrations and loading under sail. Now, you could eliminate this by sleeving the hole in the chain plate. That could be a good solution but makes for a notably more complex installation process.

So with these limitations, I will not use this alloy for chain plates. I don't think we have enough data and usage of this alloy system over time in this specific application for any of us to really know how well this alloy will work over 10 plus years as chain plates.

My sailboat has 316 stainless steel chain plates. It is almost 40 years old at this point. Those chain plates are in excellent condition. And yes, I have examined them thoroughly. I honestly expect another 40 years of trouble free use short of some kind of disaster.

dj

[Dockhead]

Originally Posted by Manateeman

. . .

The chain plates that can last the life of the boat. We know exactly the two types.

Solid aluminum bar welded in an aluminum boat...and solid bronze exterior chainplates bolted to a glass hull.

We’ll stop the fear mongering and get back to the pizza.

The manatees.

Well, there are two other types of lifetime chainplates:

3. Titanium Chainplates. See: https://www.colligomarine.com/campai...YNdAb4BlkN3rCl

and

4. Grossly Overbuilt Moody Chainplates. These are made of twin triangular pieces of 17mm thick high tension steel plate (not stainless) throughbolted on either side of a specially enlarged stringer, using 9x 15mm diameter grade 8 bolts. Easily accessible, nothing encapsulated, simply unbolt to inspect. Each assembly weighs I guess 20 kg.


I broke a chainplate on our previous boat (a Pearson 365); it was terrifying. Miraculously, the mast stayed up. I vowed then - never again. I'm betting that something similar happened to whatever engineer in Bill Dixon's office designed the ridiculous overkill Moody chainplates.