Chain & Catenary: Challenging a Maritime Myth


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Dick
Dick
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Hi Andrew,
Thanks: your comment is a good addition to the article.
3-strand does, indeed, cause the chain to wrap around the rolling hitch when loaded. This, however, I have never experienced as a problem as the wrap “un-twirls” itself when you transfer the load from the snubber to the chain when upping-anchor. When the rolling hitch gets to the bow, the wrap “nest” is gone, the load solely on the chain, and the knot is easy to get to and untie.
You cite problems dealing with the twisting to get the snubber released when under load. I am not sure why one would try to do so. Unload the snubber and the twist takes care of itself. I also suspect that undoing a rolling hitch under load is actually not possible, certainly not easy, and may be dangerous to fingers (same with other snubber-chain connections such as a chain hook).
That said, the twisting of chain and 3 strand snubber connection when loaded is unsightly and I suspect there is little functional difference between my 3 strand and your 8 plait.
What diameter 8 plait do you use?
My best, Dick Stevenson, s/v Alchemy
freedomandadventure
freedomandadventure
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hi Dick,
The twisting i have experienced was on my boat but with old chain that was worn and quite happy to stay twisted and it had a good hold on the chain hook as it got 'hockled' around the hook and wouldnt unwrap even when the chain was loaded. the chain hook might have contributed as something for the chain to grab onto. I have seen the problem on other boats nearby too.
We use 14mm 8 plait but 12 would probably do. I do like to use a long snubber. i keep the chainhook out of the water when calm but as soon as the boat starts sailing about in more wind a longer snubber sure softens the ride.
cheers
Andrew
neilm
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I agree totally that the myth about chain providing a "spring" should be bust.
No realistic chain provides useful shock absorbing in over 40 knots wind.  Two of our experiences support this.
We anchored in the Alligator River in the Carolinas in our former boat, a 40 foot 14 ton vessel   We set a 60 lb CQR on 180 feet of 7/16" chain in 10 feet water.  After allowing for bow roller height, that is 15:1 scope.​
The wind was reported at over 60 knots by a nearby airport.  We had some shelter from waves but little from wind. Waves were only about 3 ft high
The chain was secured to a large cleat (secured by two 1/2" bolts through the deck to a solid backing plate) by a couple of feet of 1/2" nylon. 
The chain was bar taught, and after a couple of hours the rope snapped, half way along its length, with no chafe involved.
Breaking load of the rope was about 8,000 ​lbs, the chain about 20,000 lbs.
The boat was delivering sledgehammer blows to the anchor, which dragged about 100 yards through the thick mud.  When the​ wind dropped we had a heck of a job getting the CQR up, and found it and the first 10 feet or so of chain nicely polished.

​"Oceanography and Seamanship" by William Van Dorn has an excellent analysis of the mathematics of anchor rodes that shows that the ideal rods has half its length chain and half rope.

​A few years later we sat through  hurricane David, 1979 in St Martin.  Cat 4 as I recall. 
Fortunately, I had read Van Dorn's book, and applied his mathematics  to our boat to select the anchor rode.​
They eye of David went over us, so we had wind from two directions.  Nobody local could measure the wind speed.
​​We had same CQR but with 90 feet chain and 90 feet 3/4" nylon on the inboard end.  We barely moved, and the boat did not bang and clang.

These anecdotes support Dick's myth busting.

Of course chain is necessary on the bottom to avoid chafe on rocks, but there is no advantage in heavy chain.​​

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Dick
Dick
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neilm - 3/26/2020
I agree totally that the myth about chain providing a "spring" should be bust.
No realistic chain provides useful shock absorbing in over 40 knots wind.  Two of our experiences support this.
We anchored in the Alligator River in the Carolinas in our former boat, a 40 foot 14 ton vessel   We set a 60 lb CQR on 180 feet of 7/16" chain in 10 feet water.  After allowing for bow roller height, that is 15:1 scope.
The wind was reported at over 60 knots by a nearby airport.  We had some shelter from waves but little from wind. Waves were only about 3 ft high
The chain was secured to a large cleat (secured by two 1/2" bolts through the deck to a solid backing plate) by a couple of feet of 1/2" nylon. 
The chain was bar taught, and after a couple of hours the rope snapped, half way along its length, with no chafe involved.
Breaking load of the rope was about 8,000 lbs, the chain about 20,000 lbs.
The boat was delivering sledgehammer blows to the anchor, which dragged about 100 yards through the thick mud.  When the wind dropped we had a heck of a job getting the CQR up, and found it and the first 10 feet or so of chain nicely polished.

"Oceanography and Seamanship" by William Van Dorn has an excellent analysis of the mathematics of anchor rodes that shows that the ideal rods has half its length chain and half rope.

A few years later we sat through  hurricane David, 1979 in St Martin.  Cat 4 as I recall. 
Fortunately, I had read Van Dorn's book, and applied his mathematics  to our boat to select the anchor rode.
They eye of David went over us, so we had wind from two directions.  Nobody local could measure the wind speed.
We had same CQR but with 90 feet chain and 90 feet 3/4" nylon on the inboard end.  We barely moved, and the boat did not bang and clang.

These anecdotes support Dick's myth busting.

Of course chain is necessary on the bottom to avoid chafe on rocks, but there is no advantage in heavy chain.



Hi Neil,
Good to hear from you.
Thanks for the field report: always the best data in our marine world. And your support that chain should be bought with regard to strength and not to weight.
And yet, at least once a year or more, in the popular magazines, one sees the usual diagram of a boat at anchor with a nice arc from bow to anchor lauding the benefits of chain catenary.
I wonder whether the author has ever been at anchor in a gale: sort of like going below a boat in a boat show, finding no handholds, and wondering whether the naval architect has ever been off-shore in a sailboat.
I am surprised that your ½ inch snubber gave way: ½ inch nylon should not have parted in the way you described. It is usually incredible strong tough stuff.
I would suggest that I would have veered more than a couple of feet of the ½ inch nylon snubber. A couple of feet gives some stretch and would protect chain and windlass, but so short a snubber would be subject to continual impressive shock loads in the conditions you describe. I am no expert, but I read about loaded up nylon getting hot and weakened, but I have always supposed that to be where it gets compressed going around a chock or something. There are also some nylon manufacturers who use dis-continuous strands (something to check at purchase and a good reason to buy from well-respected manufacturers who can answer the question). Where one strand ends and a new strand begins is a weak spot which can appear when the line gets stretched (little puffs of nylon revealing a break). A longer snubber eases the shock loads considerably and the boat might lie better. (I write this for others to read as I realize that you may not need     -generally- a snubber with your present ground tackle set up.)
And I think the hybrid system you describe is one that is under-appreciated and should be a viable consideration for many boats with the caveats you mention. It is an especially decent solution for boats that anchor shallow most of the time (they will usually be on all chain with the nylon as snubber). And for those where weight is very important (catamarans). And for those that like chain but are sometimes required to anchor deep, 150 feet of chain followed by 300 feet of nylon allows anchoring safely 100+ feet and is far less weighty and easier to handle than 300 feet of chain or more.
BTW, are you still using a CQR?
After Hurricane Bob, weathered in Onset, Mass, we needed 6 hours to get our 3 anchors up.
My best, Dick Stevenson, s/v Alchemy

neilm
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Hi Dick
Agreed we should have had much longer nylon in the Alligator River.
The nylon I used was just to take load off the windlass. Until that experience I believed the myth about chain catenary

tharris
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Dick - 10/12/2017
Hi all,I write periodically on ground tackle effectiveness and contributing factors. The attached article is one of those. As always, I invite questions and comments.Dick Stevenson, s/v Alchemy
Dick, I believe your photo of a presumed straight chain used in article is unlikely to show a chain with no catenary. Although it may look straight over the short distance from bw to water the chain will undoubtably have a catenary unless the anchor is stuck in seabed and chain is vertical so that gravity cannot act on it.
 If the scope was that short the anchor would have pulled out.
Having anchored in a fair few locations over the last 30 years including deploying and recovering up to 40 ton anchors, I can say with certainty
that there is no substitute for a good heavy chain and plenty of scope.
If a sailing vessel does not want all the chain weight in the bow they can consider having a portion of the chain pulled aft to around the mast area and secured there except for when needed.
A reasonable amount of chain to have available is dependent on the draft of vessel and the depths where you are anchoring,
Most vessels will carry 7 to 8 shackles of chain on either bow anchor as a minimum.  (Shackle is 27.5 meters)  This hard won experience form the merchant marine world ( and required by classification societies) should be replicated in the yachting world.
Your point to have all loads removed from the anchor winch are very well made and boats must have a snubber or a proper full load rated chain stopper.
To better see the effect of catenary, -  a simple demonstation stretch out a 200 meter of chain and attach one end  to a strong point on land at same hieght as your hitch, attach other end to your utility vehicle and stretch the chain out. Gravity will be your friend.


Dick
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tharris - 3/28/2020
Dick - 10/12/2017
Hi all,I write periodically on ground tackle effectiveness and contributing factors. The attached article is one of those. As always, I invite questions and comments.Dick Stevenson, s/v Alchemy
Dick, I believe your photo of a presumed straight chain used in article is unlikely to show a chain with no catenary. Although it may look straight over the short distance from bw to water the chain will undoubtably have a catenary unless the anchor is stuck in seabed and chain is vertical so that gravity cannot act on it.
 If the scope was that short the anchor would have pulled out.
Having anchored in a fair few locations over the last 30 years including deploying and recovering up to 40 ton anchors, I can say with certainty
that there is no substitute for a good heavy chain and plenty of scope.
If a sailing vessel does not want all the chain weight in the bow they can consider having a portion of the chain pulled aft to around the mast area and secured there except for when needed.
A reasonable amount of chain to have available is dependent on the draft of vessel and the depths where you are anchoring,
Most vessels will carry 7 to 8 shackles of chain on either bow anchor as a minimum.  (Shackle is 27.5 meters)  This hard won experience form the merchant marine world ( and required by classification societies) should be replicated in the yachting world.
Your point to have all loads removed from the anchor winch are very well made and boats must have a snubber or a proper full load rated chain stopper.
To better see the effect of catenary, -  a simple demonstation stretch out a 200 meter of chain and attach one end  to a strong point on land at same hieght as your hitch, attach other end to your utility vehicle and stretch the chain out. Gravity will be your friend.


Hi THarris and all,
In the following, I will assume that all have read the article and I do not need to repeat the over-all thesis.
Tharris, thanks for your comments
You are correct, of course, That there will always be catenary: even nylon or wire cannot be pulled straight no matter how much load is used. There will always be some “droop” and gravity will always be unable to be ignored.
And I also agree that scope is a remedy for many anchoring challenges: when there is room.
I do not have my paper in front of me, so I do not know how clear I was in that discussion, but what I hoped to convey, but perhaps I should have gotten across more successfully, that the effective result in gale conditions (certainly 40kn and above) was not that catenary actually disappears (as it never actually does), but that your chain rode reaches a point where the catenary/droop is effectively bar tight.
Bar tight means that shock loads will be transferred pretty much unmitigated from bow to anchor. There just is not enough catenary to absorb and average out the shock loads (shock loads come from primarily 3 sources: surging, fore and aft movement which allows the boat to gather speed; yawing, which allows wind and wave to strike to side of the boat and exert far greater pressure; and waves/swell, which causes the bow the rear up and pull hard on the rode. The greatest danger is when two, or worst case all three, of these shock loads occur simultaneously.
When the statistically likely event of these shock loads coincide, without effective shock averaging in your ground tackle, the danger is that forces will just snatch the anchor out of the bottom. Shock load averaging can certainly be accomplished by catenary in higher winds, but often at scope lengths that unreasonable or are hard to come by. Weight can also contribute, but the weight that would be necessary to keep catenary effectively in the picture at say 5 or 7-1 scope is more than most of us are willing to carry.
In high wind conditions, one’s snubber is the most effective shock load averaging element of one’s ground tackle: catenary plays a part in the lulls, but effectively disappears in the predictable high load moments. The snubber, in addition to averaging loads in all wind conditions, and thereby making for a more comfortable boat all around, is attached to the rode in order to stretch and absorb the shock loads and therefore averages out the forces over a longer period, protecting the anchor from being snatched out of the bed.
My goals in this writing were two-fold; to challenge the catenary myth when anchored heavy winds (when you want catenary the most), and to suggest, when buying chain, to buy with strength being the more important criteria rather than weight. If you have weight “to spare”, I suggest that ground tackle effectiveness will be best served by putting weight in the anchor, a new generation anchor, rather than in the chain. Once chain strength is established, put the weight the next size anchor (or even a 2-size jump) in a new generation bower. Weight in the anchor pays far greater dividends than weight in the chain in ground tackle effectiveness.
I think it is an excellent suggestion for getting chain out of the bow by routing excess down to the bilge near the mast step. I keep 150 of my 280 feet in my bilge amidships, ready to run out easily in the few occasions a season where more than the 130 feet in the bow is necessary. Not all boats can accomplish this, but if you can, it makes a big difference in trim. (see Alchemy’s method of accomplishing this elsewhere in the Forum)
I do disagree about simulating Merchant Marine ground tackle practices on an average cruising sailboat. Some practices do not scale. In essence, in casual observation, I see that they substitute weight for design. Their anchors are primitive, and they make up for that by veering lots of their very heavy chain attached to their very heavy, but inefficient, anchor when in their off-shore anchorages where there is a lot of room for great scope. They are able to handle long lengths of heavy chain and a big anchor with massive windlasses and a skilled crew and are 24/7 “manned”: someone awake on watch. Moreover, their vessels appear “better behaved” at anchor in a blow than our far lighter vessels which “dance” around a good deal.
Finally, you say: “boats must have a snubber or a proper full load rated chain stopper”. I would suggest that vessels are wise to have both*: the chain stopper will protect the windlass if things go pear shaped but will do nothing to average out loads: that is a snubber’s job and, in moderate winds, catenary will accomplish averaging also. The danger of no snubber is that shock loads can get transferred directly to the chain stopper and/or the windlass. If they survive, it should be noted that chain has great strength, but that shock loading forces can and does break chain. Consider the not-unheard-of event of having your chain wrap around a coral head or a boulder, thereby becoming short-scoped. And then some larger swells come through and the bow is thrown up. In this event, a snubber might survive, but there has to be: in the chain, chain stopper or windlass, the likelihood of damage.
My best, Dick Stevenson, s/v Alchemy

•    Those vessels without a chain stopper are wise to leave their windlass set to run free (or with just a bit of friction) to save the windlass from damage if the snubber breaks as Neil just reported his did once upon a time. Ensure the bitter end is adequately secured.

GO

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