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

[Kelkara]

Originally Posted by s/v Jedi

Was the weight reduction for being immersed in water being used for the weight of the chain?

Good point ... I did not do that ... makes results a bit less favourable, but not by too much.


eg. for Dockhead's 100m chain and 2000Kgf gives an angle of 13.48 at 99.92% straight.


But absolute precision isn't really too useful when looking at the static system, and I'm not going to touch calculations for the dynamic loads.

[groundtackle]

Attempting to interpret my own elongation post..
I like to use 5/16" G43 just because it's a popular size.
To determine the actual chain length "not greater than 10% proof" = 585lbs.
If linear elongation (I could very well be wrong about that) When approaching WLL 5% = 5' per 100', and that's a minimum. G43 3-1 WLL vs UBS, another subject but relative to;
G80-100 at 20% 4-1 WLL vs UBS also = 5%.
Then consider G30 at 4-1 is 15%.

Chris

[Pete7]

Originally Posted by Dockhead

Very helpful and very interesting!! Great work!!

Time for some experiments I think:

https://www.ebay.co.uk/itm/SALTERS-I...4AAOxyx-BScCmD

[Dockhead]

Originally Posted by thinwater

ABYC table 1 does NOT related to wind load, and only to rode tension in very specific circumstances (shallow water, long fetch, all chain, and no snubber). It is worst case.



Your measured tension will be 2-5 times less, depending on the set-up and situation. Many testers have reported this.

Very useful information.

[conachair]

Originally Posted by Pete7

Time for some experiments I think:

https://www.ebay.co.uk/itm/SALTERS-I...4AAOxyx-BScCmD

Nah,
https://www.ebay.com/itm/YZC-133-Min...LNtxRwVw0GIhEA

fraction of the cost & loads of times more accurate

Just add an ESP8266 to send the data over wifi to a raspberry pi running openplotter then plot the results against windspeed, sog, cog , heading...

Would be like going to data heaven

[wrwakefield]

I'm coming to this party a little late...

Lots of good info so far.

In case it fits in with your needs, here is an online mooring catenary calculator I have found useful for helping to verify what I thought I understood...

Cheers! Bill

[thinwater]

Originally Posted by conachair

Nah,
https://www.ebay.com/itm/YZC-133-Min...LNtxRwVw0GIhEA

fraction of the cost & loads of times more accurate

Just add an ESP8266 to send the data over wifi to a raspberry pi running openplotter then plot the results against windspeed, sog, cog , heading...

Would be like going to data heaven

I belive that is a beam load cell and very low capacity. You need a shear load cell.

[conachair]

Originally Posted by thinwater

I belive that is a beam load cell and very low capacity. You need a shear load cell.

Not if you have it measuring the compression load from a small deflection in a snubber run along the deck. 50mm deflection on a 8m span should give about 10Kg for a 400Kg load down the snubber, just need to sort out some sort of low friction pulley system where it goes over the load cell, not too hard. And those things are crazy accurate, down to below 1g though they tend to drift a few grammes over time. Still not exactly rocket science to sort out something waterproof and cost less than a beer & a posh burger
Google rope tension meter for similar concept with lots more money involved .

[S/V Alchemy]

Originally Posted by Panope

Dockhead, another consideration about a long, heavy chain's catenary is that not only must the chains WIEGHT be overcome, but also it's INERTIA and DRAG through the water as the forces (wind gusts and waves) cycle. Both of these improve the chain's energy absorbing ability.

Steve

True. Also consider all that "new gen anchor" marketing-speak that claims "get the holding power at 5:1 scope you get with 7:1 scope with a CQR!"

Well, why would you want to do that? Isn't the better statement "make 7:1 scope as good as 10:1 with WonderHook (TM)!"

Less chance of getting to "bar-tight" then, isn't there, as Panope (a very useful fellow and one of the reasons I now own a 30 kg. SPADE) helpfully points out, simply by letting out more chain.

I realize how tight some anchorages are, but to me, cutting corners on scope for reasons related to this is like hiking without knowing how to double-knot your boots.

[S/V Alchemy]

Fun thread, also!

[wsmurdoch]

Originally Posted by thinwater

My first one was just like that, and rigged much like that. Very useful. In truth, there is so much fluctuation you don't need to fret over 5-digit accuracy!

The other thing that is interesting to test, is the effect of sailing-at-anchor on the load. With some boats, it can double when the boat gets somewhat broadside.

Yes, the two things that I learned with the dynamometer really fast were that sailing about at anchor was bad and waves in the anchorage were bad. The maximums that the tell-tale needle records are the jerks, and as you said, they run as much as double the more normal loads. It is the jerks that you need the elasticity and dampening for. (At least so it seems to me.)

Bill

[jhulmer]

Originally Posted by thinwater

a. Ships are different, because they are barely affected by gusts or chop. Primarily just the steady load.


b. Drag through the water is trivial and can be effectively neglected. I've done the math. Look at the actual speed (1-2 knots) and this is obvious. A red herring.



c. At 30 knots or so the chain lifts off the bottom, depending on scope, chain weight and water depth. In shallow water it happens early, in deep water, hardly at all. Another way to think about this is that it is not so much the scope, as the to total amount of chain out; once you reach 250+, it's a lot more weight and better leverage.


d. Then both the angle AND the amount of energy absorbed declines, gradually, until at 40-55 knots (again, depends on the depth and scope, so don't try to pin me down) the angle at the bottom is between that off 100% rope and flat, and the energy dissipation is effectively zero. Though not straight, the difference between straight and the curve you have is only inches.


The math is complex, particularly when you consider the dynamic effects. I've measured and I've dived when it was blowing. The true is in the middle and hard to nail down. But something the math will quickly show you is that with G70 chain you can fit twice as much in the locker, and that will ALWAYS hold better. I think that is unarguable.


There is a common area, with medium depth water and moderate weather, where heavy chain allows less scope and might be helpful. Thus the popularity of heavy chain; it is a good answer in 25-30 knots, in 15 feet of water, in a crowded anchorage. In 60 knots and enough room, not so much.


The breaking strength argument is off the point for three reasons:
*The rode better not operate above the WLL, or it won't last long.
*No, it does not take nearly that much force for the difference to become academic.

* Using a proper snubber you should be several times below the WLL. If not, the anchor will drag.


I suggest you take a length of chain and a come-along (chain hoist), and stretch a chain between two trees. Do you really believe it takes 4 tons to get the chain pretty straight? I don't think you believe that, but try it. You will also learn the stretching that last bit is so hard there is not shock absorption; tje stretch/stress relationship is not linear, like rope.



Seriously, stretch a chain at the boatyard. You will find it enlightening.

This is true. But, take it further. stretch a chain and a rope rode side by side. Put a tension meter between the come-along and rope and chain.

Measure the force required to stretch both straight. It obviously, will require more force to remove the catenary from the chain.

The benefits of the catenary will prevent the anchor from dragging much longer then for the rope road and in much stronger conditions.

It simply requires more force with chain then with rope to lose the benefits of catenary.

I'm with you Dockhead. I don't buy it!

[Juho]

Thanks for a good thread. My learnings so far have been simple and traditional. In addition to a good anchor I need a good snubber and sufficient scope.

The heavy vs. light chain question resembles the question if you need a kellet, or if you should rather improve the scope. If the scope is long, there may be a need to limit the swing space. A second anchor could be a better option than a kellet or a heavy chain at the seabed.

[Pelagic]

Originally Posted by conachair

I reckon keeping the boat speed down must help a lot to reduce the force on the hook, and obviously a good snubber to help increase the distance it takes to stop the boat when if shoots off.

As an adage to this.... In marine college we were taught in ship anchoring that the "Break Out" condition for a vessel was when the chain pinched across the bow as the ship sheared off to one side and that momentum of the ship heading at about 45°s from anchor could cause a break out condition.

We were taught to always deploy the anchor opposite from the closest danger so that if a series of "break outs" occurred...that would take you away from the closest danger.

[techmans]

Originally Posted by Dockhead

Seems like a solid method, and a good approximation would be gold at this stage, but I do disagree about the depth of the water -- the depth of the water, or vertical displacement between the two ends of the chain, has a big effect on the force vectors on the chain so should change the catenary quite a bit.


So by 99.995% straight do you mean that there is 0.005% of 5mm (!) of sag along a 100m length of chain? Really? I have not figured out the math, so can't argue with you, but that does not seem intuitively possible. If it is really true, then my theory is way wrong.

What everybody seems to forget is that under (sea)water you don't have 3.3kg per 100 m but rather something like 2.8 kg/100m.Somebody by the name of Archimedes told us so after jumping in his bath.So if you're going to do some serious math with differentials and limit theory and all that delightful stuff you should take that into account.