Anchor chain stretch?

[Fore and Aft]

I wonder how much your chain would stretch before your cleat or bollard gave way?
Cheers

[Dockhead]

Originally Posted by Fore and Aft

I wonder how much your chain would stretch before your cleat or bollard gave way?
Cheers

Very good question, reminding us that any chain is only as strong as its weakest link. So if your cleat or sampson post or chain lock is less strong than your chain, your whole system is only that strong. Or your method of attaching your chain to the cleat or sampson post.

[BjarneK]

Originally Posted by Dsanduril

For your science you might find this DnV standard useful.

They specify asking the manufacturer for effective modulus values but specify defaults from 54 to 60 GPa for studless chain depending on type of steel.

Note that one reason these values are so low is that they intend the use of one wire diameter/area in subsequent equations.

Thanks a lot. "Det Norske Veritas" have really put a lot of thought into this. I guess it is not too surprising given the big oil industry in Norway.

[BjarneK]

Originally Posted by AndyEss

How are you planning to model soil deformation around the anchor? Soil
moduli are many orders of magnitude lower than your chain steel. The soil is also highly likely to be deforming plastically. Finally, it isn’t hard to cause shear failure in seabed soils, so anchor drag/creep, even if small and slow, will be a very large portion of deformations,

Thanks for this comment. This is something I had not considered. I may not be able to model this in any good way, but at least it is a factor that will reduce forces, so calculations without this effect will represent worst case scenarios.

[acem]

I have never stretched chain with a boat but I have stretched chain at my farm and pulled cleats out of my boat. I am also an engineer but chain stretch is not my specialty
.

I have overloaded chain many times and sometimes have stretched it. Most quality chain will stretch quite noticeably before it breaks. The quality of chain is quite variable depending on grade and manufacture (country of origin). Using a good snubber will help alot. Multiple snubbers would be even better.

My catalina 27 was tied up to a marina and a tornado rolled the marina up into a mess. My sailboat broke two cleats off the sailboat, one steel cleat off the marina (along with part of the marina) and one line.

Snubbers would not have helped my sailboat because it was best for it to be separated from the marina. However if anchored, I strongly recommend snubbers.

Thx-Ace
.

[a64pilot]

I don’t think he is speaking of permanent stretch but temporary deformation and return to original dimensions once the load is removed, remember steel is more elastic than rubber and does return to original size very well. Just usually it’s a very small amount of stretch to do that, and of course this is exactly how you get fatigue failure.

[a64pilot]

Originally Posted by contrail

My anchor stretched noticeably in Irma.

Anchor? Which kind, size and how much (estimate) if you don’t mind.
I assume shank?

[contrail]

Originally Posted by a64pilot

Anchor? Which kind, size and how much (estimate) if you don’t mind.
I assume shank?

Oops! The chain, not the anchor...typo there. The chain was 3/8 HT (G40). The boat, a 45 foot catamaran, was in her usual and previously considered "well protected" slip that has withstood numerous hurricanes, over the years. I secured her with 35 lines from various attachment points to various things and places ashore. Additionally, I ran the anchor chain across the fairway and around a piling about 100 - 110 feet away. In our worst winds the bow was actually lifted up to a reported 30-45 degrees, in a lengthwise direction, and observers were sure she would go over, but she sat back down in her slip (!), thank goodness. I had moved her about eight to ten feet forward from the dock that was astern, for a safety margin but had gotten the chain as tight as I could with the windlass and then loosened it about two feet to accommodate surge, which we had. However, she was blown far enough aft to damage both sugar scoops. The part of the chain in use was visibly, but very slightly elongated, and whilst it could be raised with the windlass, would occasionally slip a link, which was not the normal behaviour. I would guess the elongation to be a bit more than the distance from the dock astern minus any tightening effect around the piling, minus the fact that the wind would have tightened the catenary more than the windlass so lets say maybe five feet? The bolts on two cleats sheared off (no evidence of corrosion of any sort), one further cleat had a horn broken off, and two bollards ashore were torn off. Notably, the backing as originally built, held on all the cleats. The chain pawl, a very strong construction crossing a very strong point in the fiberglass, had no damage. Neither did the roller. I was impressed! Irma was "officially" measured at 180-185 kts, with gusts of 220, but the Department of Disaster Management lost their anemometer at 230, the airport lost its at 270, and I know of a report of 300. Tornadoes, of which there were many, probably accounted for these readings. At least one went through the marina. One other boat at my dock survived, and a couple of others were declared constructive losses by the insurance company. The rest were sunk, flipped, or up on docks and lost. From the position of the boats, and the general experience on Tortola, the worst winds were in the second eyewall. Those would have been the winds that stretched the chain. It took me three days to get down to the marina from where I had sheltered, and I was amazed to see Jet Stream afloat and upright and in her slip. I think the anchor chain was what kept her from flipping, and I am glad it stretched and didn't break.

[Dsanduril]

The OP is proposing an interesting exercise. I would propose we change the word "stretch" to "elastic strain". Elastic strain is stretching of a material at loads less than the deformation load. When the load is removed from a chain with elastic strain the length of the chain will be identical to its length before the load was applied.

The standards Gord linked above about removing stretched chain from service refer to "plastic strain". Plastic strain occurs when a material is loaded past its elastic limit. The deformation in this case is permanent. The unloaded chain will be longer (stretched) than it was before the load was applied. Plastic strain can provide shock absorption, but only once. The chain will not return to its original length (it could help absorb another shock with more plastic strain, but it will fail pretty quickly).

The OP's proposal is to look at the shock absorption in the elastic region. We all expect snubbers to provide shock absorption through elastic strain and think of nylon as being "stretchy". Will be interesting to see an analysis of steel chain in this regard. (I think I already know the answer, but very interested to see the math - open mind).

[evm1024]

Here is a graph of steel under tension. The "linear" portion of the graph is where the elastic stretch (recoverable) happens. Above that and the steel deforms permanently.

Also is a link to a hapfway good site to learn more about steel. Not my field but it is interesting.

https://www.cliftonsteel.com/knowled...yield-strength

[Cheechako]

Originally Posted by Nicholson58

The stretch due to elastic force is orders of magnitude less than the catenary sag and even the elasticity of your hull. Engineers toolbox is a good source for calculating raw numbers but chain makers can probably give you empirical data. BTW, the modulus for steel doesn’t care what steel you use. They vary only a tiny bit.

This ^^. It's all about the load and easy to predict for steel. It's elastic stretch (comes back) up until you go above the yield strength.

[Dsanduril]

Originally Posted by Nicholson58

The stretch due to elastic force is orders of magnitude less than the catenary sag and even the elasticity of your hull. Engineers toolbox is a good source for calculating raw numbers but chain makers can probably give you empirical data. BTW, the modulus for steel doesn’t care what steel you use. They vary only a tiny bit.

Actually, I'm not sure that's the case. In the G4/G7 thread we looked at some 10mm chain in 10m of water (with catenary it's all about the length vs depth vs weight as opposed to simply length and load for elastic strain). The end result was that from 200lbs load to 2500 lbs there was about a foot of effective "stretch" from catenary shape changes.

That's why I'm curious to see the OP's results. Intuitively I do think there are orders of magnitude difference, but based on the initial calcs it seems it may be less than "orders" and may not even be a single order. Over that same load range we're already looking at inches of elastic strain in the chain itself, and compared with the effective stretch from catenary it, at first glance, looks to be an actual participant in absorption of shock loads.

The math(s) will tell, looking forward to seeing them.

[BjarneK]

Originally Posted by Dsanduril

The math(s) will tell, looking forward to seeing them.

The wait is over. I started a new thread based on the modulus values I found: Effect of anchor chain elasticity - Cruisers & Sailing Forums

[a64pilot]

Originally Posted by contrail

Oops! The chain, not the anchor...typo there. The chain was 3/8 HT (G40). The boat, a 45 foot catamaran, was in her usual and previously considered "well protected" slip that has withstood numerous hurricanes, over the years. I secured her with 35 lines from various attachment points to various things and places ashore. Additionally, I ran the anchor chain across the fairway and around a piling about 100 - 110 feet away. In our worst winds the bow was actually lifted up to a reported 30-45 degrees, in a lengthwise direction, and observers were sure she would go over, but she sat back down in her slip (!), thank goodness. I had moved her about eight to ten feet forward from the dock that was astern, for a safety margin but had gotten the chain as tight as I could with the windlass and then loosened it about two feet to accommodate surge, which we had. However, she was blown far enough aft to damage both sugar scoops. The part of the chain in use was visibly, but very slightly elongated, and whilst it could be raised with the windlass, would occasionally slip a link, which was not the normal behaviour. I would guess the elongation to be a bit more than the distance from the dock astern minus any tightening effect around the piling, minus the fact that the wind would have tightened the catenary more than the windlass so lets say maybe five feet? The bolts on two cleats sheared off (no evidence of corrosion of any sort), one further cleat had a horn broken off, and two bollards ashore were torn off. Notably, the backing as originally built, held on all the cleats. The chain pawl, a very strong construction crossing a very strong point in the fiberglass, had no damage. Neither did the roller. I was impressed! Irma was "officially" measured at 180-185 kts, with gusts of 220, but the Department of Disaster Management lost their anemometer at 230, the airport lost its at 270, and I know of a report of 300. Tornadoes, of which there were many, probably accounted for these readings. At least one went through the marina. One other boat at my dock survived, and a couple of others were declared constructive losses by the insurance company. The rest were sunk, flipped, or up on docks and lost. From the position of the boats, and the general experience on Tortola, the worst winds were in the second eyewall. Those would have been the winds that stretched the chain. It took me three days to get down to the marina from where I had sheltered, and I was amazed to see Jet Stream afloat and upright and in her slip. I think the anchor chain was what kept her from flipping, and I am glad it stretched and didn't break.

Good Lord, yes it sounds like you loaded it to an unbelievable amount.
I assume when she tried to fly was when it was stretched, somewhere about 45 degrees is going to I believe be the max amount of lift possible, and it must have been huge numbers, I’m very surprised the part of the boat the chain was attached to was stronger than the chain, I’d think that is unusual.
Very strongly build obviously

[contrail]

Originally Posted by a64pilot

Good Lord, yes it sounds like you loaded it to an unbelievable amount.
I assume when she tried to fly was when it was stretched, somewhere about 45 degrees is going to I believe be the max amount of lift possible, and it must have been huge numbers, I’m very surprised the part of the boat the chain was attached to was stronger than the chain, I’d think that is unusual.
Very strongly build obviously

I assume the same. The old Leopards are known for their toughness and this was a pretty severe test. These were the winds that inflicted Dorian-like damage, over a shorter period of time, thankfully. 60 foot cats were flying 50 odd feet high in the air, for considerable distances, and landing on top of two story buildings and I personally know of a container that flew over five miles.



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.