Anchor chain stretch?

[Jim Cate]

I always thought the chain pawl was very strong, but that was ridiculous. It's a simple piece of stainless steel, a quarter to maybe three eighths of an inch thick (come to think of it, I should measure it, sometime!), with a slot for the chain link. It hinges down over a similarly thick plate that lies against a thick area of fiberglass, through which an SS tube for the chain passes. Very robust! Not a scratch or stress crack.

Sounds like a good design... is it a "store bought" thing or a custom fabrication? I'd love to see a sketch of it if it is custom built.

Thanks,

Jim

[contrail]

Originally Posted by Jim Cate

Sounds like a good design... is it a "store bought" thing or a custom fabrication? I'd love to see a sketch of it if it is custom built.

Thanks,

Jim

It's very standard on Leopard's, at least the older ones. I think it still is. I remember when I first saw it, I thought, "well done", but I never realized just how good it is. So I guess it's a custom fabrication for Leopards, as opposed to store bought, but it is far from unique to my boat. I suppose the key is that there are no bolts or backing plates to let loose. Under load, there is nothing in shear or tension; everything is in compression against something else equally heavy duty.

[leftbrainstuff]

Originally Posted by BjarneK

I am wondering if any of all you smart people know how much anchor chain stretches under load? I am doing some calculations on dynamic loads on anchoring gear and chain stretch may play an important part in this, even if it stretches much less than the snubber being used.

Any input would be most welcome. I did e-mail a couple of chain manufacturers but have not heard back yet.

If you're stretching your chain then you are overloading it. A load greater than the yield strength and less than the utimate tensile strength will result in permanent deformation.

Measuring link length and any lateral deformation or link necking will indicate stretching. If it's stretched then it should be considered to have failed.

If you are interested in anchoring dynamics then it's not stretch but catenary shape you should focus on.

[a64pilot]

There is deformation and then there is stretch, deformation may not be the name, but I’m certain he is speaking to the elasticity of steel that allows some deformation under load that is 100% reversed when the load is removed.
Think of the millions of times a valve spring is compressed for example

[BjarneK]

Originally Posted by a64pilot

There is deformation and then there is stretch, deformation may not be the name, but I’m certain he is speaking to the elasticity of steel that allows some deformation under load that is 100% reversed when the load is removed.
Think of the millions of times a valve spring is compressed for example

This is exactly what I meant. It turns out that this temporary deformation is called "elastic deformation". And as you can see from my other thread (Effect of anchor chain elasticity - Cruisers & Sailing Forums), it does play a role along with catenary and elasticity of the snubber.

[Auspicious]

Young's modulus.

[Dockhead]

Originally Posted by leftbrainstuff

If you're stretching your chain then you are overloading it. A load greater than the yield strength and less than the utimate tensile strength will result in permanent deformation.

Measuring link length and any lateral deformation or link necking will indicate stretching. If it's stretched then it should be considered to have failed.

If you are interested in anchoring dynamics then it's not stretch but catenary shape you should focus on.

I think you're confusing elastic with plastic deformation.



https://www.differencebetween.com/di...c-deformation/


Steel makes an excellent spring.

[jamhass]

Originally Posted by Jim Cate

Sounds like a good design... is it a "store bought" thing or a custom fabrication? I'd love to see a sketch of it if it is custom built.

Thanks,

Jim

Probably something like this (this is a cheap one on the web, I've seen much more robust ones.)

[a64pilot]

A picture of it would let us know

[thinwater]

Originally Posted by Dockhead

I think you're confusing elastic with plastic deformation.


https://www.differencebetween.com/di...c-deformation/

Steel makes an excellent spring.

This can be confusing, since there are three kinds of chain stretch.

• Elastic, which is what you see below the WLL. Like a spring.
• Plastic, which you see somewhere between the WLL and the BS. You've ruined the chain. The chain is going to fail from fatigue, well below the BS, if you keep this up.
• Wear. Chain "stretch," on a bicycle or conveyor, for example, is the result of wear between the links and has nothing to do with the first two.

[a64pilot]

There should be no wear in an anchor chain.
A bicycle chain stretches due to friction wear between the pins and I guess the barrels if that’s what they are called.
But with an anchor chain if there were enough movement to wear the chain the zinc galvanizing at that wear point would be very short lived and you would get rusting very early.

[Shrew]

I feel like a lot of this is academic without a lot of real-world applicability. While possible, it's doubtful how probable. The forces required to achieve this would more than likely cause a failure in the anchors ability to hold the seabed before a chain parted. I'd be surprised if a chain parted before a cleat pulled.

It would be far more likely for a chain to part due to either being undersized or significant corrosion. If neither of these are a factor, then you're already in the middle of a $#1t show to begin with and this is your least problems.

[Compass790]

Originally Posted by Shrew

I feel like a lot of this is academic without a lot of real-world applicability. While possible, it's doubtful how probable. The forces required to achieve this would more than likely cause a failure in the anchors ability to hold the seabed before a chain parted. I'd be surprised if a chain parted before a cleat pulled.

It would be far more likely for a chain to part due to either being undersized or significant corrosion. If neither of these are a factor, then you're already in the middle of a $#1t show to begin with and this is your least problems.

Well I can tell you, from real world experience, this is not just academic.See post #7.
However to describe the situation further my anchor was jammed in rocks the wind was over 90knts & the boat was getting flung back in the waves. The nylon rode was wrapped around the windlass ( which was inoperable we didn't use it) & cleated as well.
I couldnt believe the windlass wouldnt get torn out of the deck at the time. The storm only lasted about 15 hrs & the next day the 10 metres of chain was noticeably stretched & deformed to the point where it looked thinner & I have no doubt it would have parted had the storm lasted another day.
The anchor shank was bent but it was a home made ss. one
This was only a 24ft keeler too so not that high a shockload on the chain but I can't remember what size chain it was. It may have only been 6mm but probably 8mm. I should have taken a picture of it but it was pre CF days to me anyway.
It was a lucky escape but the lesson I took from it was make sure your windlass has a big backing plate & your chain is big enough.

[contrail]

Originally Posted by jamhass

Probably something like this (this is a cheap one on the web, I've seen much more robust ones.)

No, not at all. If you read my description, you will see that I explained that, unlike the fitting in your picture (which is a decent design), what is on my boat does not depend upon bolts which can be ripped from the deck.



I will try again. The boat is a 1999 45 foot Leopard. The anchor roller is in a housing that protrudes from the forward part of the bridgedeck. From there, the chain passes aft, on a gentle upward slope, through a channel between two locker lids, and finally through a short pipe, into the windlass locker, which opens to expose everything. That pipe passes through thick fiberglass, sort of a partial bulkhead, that is also the forward wall of the windlass locker. The aforementioned channel, also forms something of a fore and aft U section brace, adding to the strength. On the windlass side of the wall is a thick stainless plate that cannot be pulled through that wall and into the pipe....it's much to big for that. The pipe ends at that plate, and on the aft side of that plate is a heavy stainless pawl that cannot be pulled through the plate or the pipe. It is on a hinge and has a slot for the chain. The actual pawl is similar to the one in your picture, but, in the case of the picture, the whole fitting can be ripped from the deck where it is bolted down. On the Leopard, the pawl cannot go through the plate and the plate cannot go through the tube or the partial bulkhead. Everything would have had to be ripped out of that part of the boat, including the bulkhead and fore and aft channel, sort of like ripping the bow of a monohull off. I don't doubt that it could be done, with sufficient force, but it was up to Irma. Nothing got bent or distorted. However, the two lockers, one on each side of the anchor chain channel, had their lids ripped off, never to be seen again. Each lid was fiberglass, flush to the surrounding deck, and about two feet across, and maybe four feet, fore and aft. Each lid was secured by thee hinges, aft, and a robust latch forward. I have no idea what happened to the latches or the hatches. The hinges were a mangled twisted mess. It was blowing pretty hard!!! Hope this clarifies things, along with my original post.



If you read my post, you will note that, in addition to the chain, Jet Stream was secured with 35 different lines, so I am not saying that the load was not also carried elsewhere. But, I noted that the bolts on two cleats were sheared off, and a couple of bollards on land were destroyed. The boat also went aft enough to hit the dock behind, stretching the chain in the process. The bow also pitched high in the air, and was probably kept from further disaster by the chain.


All of which demonstrates that a fitting can, in fact, be stronger than the chain, which, in this case, was 3/8 schedule 40 HT, not rusty, and in good condition. SWL is 5400lbs, but since it was stretched, I assume the load was between that and the breaking load.

[a64pilot]

Can you take a picture of it? I’m interested but can’t get what it is in my mind, sorry