Possibly Original Thought About Chain Catenary - or - The Myth of the Bar Tight Chain

[wsmurdoch]

Originally Posted by Dockhead

I've been trying to calculate the sag of a 100m piece of 12mm chain, weighing 3.3kg/m or 330kg, at the chain's breaking strength, or 8480 kgf, stretched between two points 30 meters different in height, and the resulting angle of the chain at the lower point.

Is it "bar tight" at the limit of its breaking strength, or is it still sagging, and therefore creating effective catenary effect? That is the $64 question, so to speak.

Unfortunately I have not been able to find a calculator for this....

I put your numbers into Alain Fraysse's static case Excel spreadsheet
Tuning an Anchor Rode and got these results.



In the last graph huge increases in the force give little increase in swinging radius, so little energy can be stored (not absorbed) in the chain at high loadings.

I can not check his math or method. Perhaps some others might be able to.

Bill

[Dockhead]

Originally Posted by wsmurdoch

I put your numbers into Alain Fraysse's static case Excel spreadsheet
Tuning an Anchor Rode and got these results.



In the last graph huge increases in the force give little increase in swinging radius, so little energy can be stored (not absorbed) in the chain at high loadings.

I can not check his math or method. Perhaps some others might be able to.

Bill

Alain's brilliant spreadsheet does finally answer all my questions.


My theory does work by the time you get to 150 meters of 12mm chain -- at the breaking force of the chain, you have less than 10 degrees of angulation, which means that the catenary is sufficient to maintain holding all the way to breaking strength of the chain, at 3:1 and in 30 meters of water.


With the given 100 meters of chain, an angulation of less than 10 degrees is maintained up to 1042 daN of force, which corresponds to about 50 knots of wind on my boat. Up to the WLL of the chain, the catenary is able to keep angulation below 14 degrees, which should be below the critical angle for a good anchor, and which is significantly less than the 19.47% of angulation which would exist at 3:1 without catenary.


What this means is that with 100 meters of 12mm chain in 30 meters of water, catenary is still working very well indeed right up to the WLL of the chain -- that means -- "no bar tight effect", as far as angulation is concerned. That is about the same as ABYC worst case wind loads for my boat at 50 knots of wind.



Furthermore, we see that the last link of chain will not be lifted until 445 daN of force, corresponding to more normal loads (say 25% of ABYC worst case loads) at 50 knots of wind.

[s/v Jedi]

Except ofcourse, at 3:1 scope in 30 meters of water, you don't have anything near 150 meters of chain out.

This math needs to be done in common setups, like 35 meters of chain out in 10 meters of water. Also, 1042 daN is about a metric ton, which sounds low as the wind load in 50 kts wind... where do we get that data? Is it for a stripped hull or with sails on the booms, roller furlers etc?

[Juho]

I hope the wind loads include also the wave loads.

[Dockhead]

Originally Posted by s/v Jedi

Except ofcourse, at 3:1 scope in 30 meters of water, you don't have anything near 150 meters of chain out.

This math needs to be done in common setups, like 35 meters of chain out in 10 meters of water. Also, 1042 daN is about a metric ton, which sounds low as the wind load in 50 kts wind... where do we get that data? Is it for a stripped hull or with sails on the booms, roller furlers etc?

Yes, 150 meters was just, as a thought experiment, the length of 12mm chain required to have good catenary still at the breaking load of the chain. The original idea of the thread was that maybe that would be true at 100 meters -- it is not.


Wind loads come from ABYC standards, which are respected based on various testing, as kind of worst case loads for real cruising boats in cruising condition. A design standard, if you like.


According to testing, normal loads will be 3 or 4 times less than that.


The loads are complicated of course -- and will be much higher if a boat is yawing, something essential to eliminate if anchoring in a storm.


There is a calculator for wind loads in Alain's spreadsheet.

[jhulmer]

Thanks for posting Alain's website. This is an explanation on steroids! And a whole lot to study!

My take-a-way at first glance is:

1. Adjust scope to maintain the angle to anchor at or below 10deg based on conditions.
2. Adjust the weight of the rode close the anchor (combination rope and chain)
3. Using combination chain and rope rode or long rope snubbers to reduce dynamic shock loads from waves.
4. Adding a Kellet as close to the anchor as possible will benefit all the above.

All pretty much what sailors have understood for decades but is very nice to see mathematical proof.

Jim

[s/v Jedi]

Originally Posted by Dockhead

Yes, 150 meters was just, as a thought experiment, the length of 12mm chain required to have good catenary still at the breaking load of the chain. The original idea of the thread was that maybe that would be true at 100 meters -- it is not.


Wind loads come from ABYC standards, which are respected based on various testing, as kind of worst case loads for real cruising boats in cruising condition. A design standard, if you like.


According to testing, normal loads will be 3 or 4 times less than that.


The loads are complicated of course -- and will be much higher if a boat is yawing, something essential to eliminate if anchoring in a storm.


There is a calculator for wind loads in Alain's spreadsheet.

I’m not sure, I see different numbers... using Alain’s data:

[Cruising Man]

I am late to the party - but this page may be of interest.

Catenary-Length

Sorry for the muck up of variable names in one of the images, but it is easy enough to work out.

Mike

[FranklinGray]

Put it like this:

There was a superyacht here that didn't use a snubber, just chain and a CQR. Due to the CQR, he did have difficulty getting it set but after a couple tries he got it. Due to the lack of snubber, he pulled his anchor out several times and had to reset.

Later comes a large Oyster, almost as big, with chain, CQR and a snubber. Like the other fella, he had a hard time setting in the same area but got it after a couple tries. He though didn't break free and drag at all. Winds about the same and anchoring in the same spot, same scope, same anchor with same type of hull, deck and rigging. Only thing different is one used a snubber and the other didn't.

So yes, snubbers do work to help keep that anchor set.

[FranklinGray]

Also want to point out, my boat has 3/8th chain (a little light for this boat) and with just 20 knots of wind, the chain pulls higher (I am sure not bar tight) but the snubber bridle which is 1 1/8th 12 strand of 6' will stretch about 3" when the wind gusts from 14 knots to 20 knots. In 14' of water with about 80' of chain out with a Mantus anchor.

So the snubber is at work even when the chain isn't bar tight and it helps keep the anchor set.

So conclusion after these two posts: Snubbers always help but not required if your chain is way oversized like ships. Another point about ships is they always have somebody on watch in the bridge and he is able to turn the motor on to take pressure off if needed. A ship is never left unattended. Large fishing boats are left unattended in many places in the world and yes, they drag and cause all kinds of damage. So don't compare apples to oranges.

Boat is Whitby 55'.

[Pelagic]

Originally Posted by FranklinGray

Put it like this:

There was a superyacht here that didn't use a snubber, just chain and a CQR. Due to the CQR, he did have difficulty getting it set but after a couple tries he got it. Due to the lack of snubber, he pulled his anchor out several times and had to reset.

Later comes a large Oyster, almost as big, with chain, CQR and a snubber. Like the other fella, he had a hard time setting in the same area but got it after a couple tries. He though didn't break free and drag at all. Winds about the same and anchoring in the same spot, same scope, same anchor with same type of hull, deck and rigging. Only thing different is one used a snubber and the other didn't.

So yes, snubbers do work to help keep that anchor set.

You are assuming a lot from one observation.

Super yachts (+50m) vary a lot in ground tackle , windage and behavior at anchor.

If not properly specified to be one over Lloyd's Number , the ground tackle can be inadequate for high windage
Super yachts.

Whereas a large Oyster sailboat with lower windage is fine.

Lloyd's are constantly revising for ships but lag behind on the smaller Superyacht requirements given the different locations.
Often in crowded anchorages where we anchor with less than optimum space and scope.
http://info.lr.org/l/12702/2018-06-28/51vkzg

FYI, most proper Super yachts do use snubbers at windy anchorages, but primarily to lower the angle and the cosmetic risk of a taught chain
wrapping around the bow when you get wind against tide.

Then the deeper draught yacht reacts more to tide and over rides the chain and swings around when you get a lull in the wind.

Now a newer trend in these conditions are to install dynamic positioning so you don't have to drop the anchor.[emoji4]

[FranklinGray]

Forgive me for using the term superyacht incorrectly. An 82 foot Oyster and approximately 100 foot slope sailboat. As I said, very similar boats.

Snubbers help. Use them.

[Jasmynonblue]

This sounds more like rocket science than anchoring. Why bother with a dopey subject like this. It's not that complicated really.

[charliehows]

hmmm - 100m of 12mm chain weighs 330 kg and has a breaking strain of 8480kg - my boat weighs somewhere around 7000kg - which means whatever force was applied to the boat to break the chain would have to lift the boat out of the water first...not losing any sleep over chain strength

[RaymondR]

Many years ago I supervised a ship shaped floating drilling rig off the east coast of Peninsular Malaysia. From memory it had eight each 2 3/4" anchor chains and used to break one occasionally.

I had just bought my small son a TI 84 computer and whilst lying about in a hotel decided to do some catenary calculations. One of the things I discovered about Asian countries with aspirations to improve their economies was that they had excellent technical bookshops. So I slipped down and bought an appropriate Sloans on mechanical engineering applied maths and wrote a basic program.

One of the things we monitor on floating drilling rigs is the relationship between the hole position on the rig and it's position on the seabed so I was very familiar with the amount of horizontal translation which occurred in a seaway.

After mucking about with the program for a while and pondering on the outputs I was surprised to find that the excessive loadings which were causing the chain breakages were not caused by the horizontal translations but by the vertical motions at the winches because of the vessels pitching.

This confirmed a conversation I had had with the superintendent of an Offshore Company rig in Brazil which had the anchor winches mounted on a rotating turret in the middle of the rig so that they could rotate the rig head to weather at all times. At the time we were having anchor cable breakages from the action of a bacteria in the mud producing hydrogen whilst converting the zinc in the galvanize, this resulted in hydrogen embritlement of the steel the wire was formed from. He was not having the same problem because his same sized wires were very lightly loaded the rig end of the wire being situated at the centre of pitch of the vessel with little consequent vertical movement.

Where's this extended narrative leading?

I have often wondered whether we would see much lower loads if we took a bridle back to either side of midships and if so could it be rigged so that it did not fall foul of the bow when the vessel sheared about or if this was even going to be a problem?