The Offshore Voyaging Reference Site

Anchor Chain Catenary, When it Matters and When it Doesn’t

The quickest way to start a brawl is to walk into a bar full of sailors and yell:

Who believes that chain catenary improves anchor holding?

The fight usually breaks into two opposing gangs: those who believe that having a lot of chain on the bottom increases holding, and those who have actually observed an all chain rode being pulled bar straight in strong winds who cry “bullshit”.

But the reality is much more nuanced, and understanding that can help us anchor more safely…and avoid bloody noses in bar brawls.

Let’s turn our attention to the former: how to use catenary to help us anchor, and when not to rely on it.

The Governing Theory

First off, a bit of theory. I’m sure most of us know this, but it’s worth revisiting, because it’s the basis of everything else I’m going to write about:

With almost all anchor types, both ultimate holding, and speed and reliability of setting, increase as the angle of pull, when measured against a horizontal line, decreases.

(By the way, Danforth-type anchors, including the Fortress, are, as far as I know, the only exception to the rule. They set better if the stock is lifted off the bottom a bit.)

Let’s leave ultimate holding out of it for a bit (we will come back to it later) and focus on setting.

There are three ways we can decrease the pull angle and thereby help our anchor to set:

  1. Increase the scope.
  2. Increase the catenary.
  3. A combination of both.

I’m going to dive into all three in more detail, but first a story.

A Lesson Learned

About 20 years ago, we changed from a CQR to a SPADE anchor, both set on 7/16″ (~ 11mm) G40 (high test) chain. The new anchor worked so well that we got a little slap happy about our anchoring technique:

  1. Drop anchor.
  2. Let the chain run to 5:1 scope before putting any load on it.
  3. Wait for the breeze to straighten the boat out.
  4. Back down hard to set.
  5. Have tea.

In thousands of sets, over some 15 years, from the Bahamas to Greenland, our SPADE only failed to set a few times, and we never dragged it once set.

Then, six years ago, we changed to 3/8″ (~ 10 mm) G70 (heat treated) chain, both to reduce the weight in the bow and to increase the amount of chain we could carry from 340 to 400 ft (103 to 122 m)—in the high latitudes, where anchorages are often deep, there is no such thing as too much rode.

Suddenly we were experiencing more failures to set, particularly in soupy soft holding. At first we were totally perplexed. What the heck? Had we forgotten how to anchor?

And then the light went on:


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More Articles From Online Book: Anchoring Made Easy:

  1. Introduction
  2. 4 Vital Anchor Selection Criteria and a Review of SPADE
  3. SARCA Excel Anchor—A Real World Test
  4. SPADE, SARCA Excel, or Some Other Anchor?
  5. Rocna Resetting Failures and Evaluation of Vulcan and Mantus
  6. Some Thoughts On The Ultra Anchor, Roll Bars and Swivels
  7. Specifying Primary Anchor Size
  8. Kedge (Secondary Anchor)—Recommended Type and Size
  9. Third Anchors, Storm Anchors and Spare Anchors
  10. Anchor Tests—The Good, The Bad, and The Downright Silly
  11. Making Anchor Tests More Meaningful
  12. We Love The Way Our Anchor Drags 
  13. Things to Know About Anchor Chain
  14. Selecting a Chain Grade
  15. Anchor Chain Catenary, When it Matters and When it Doesn’t
  16. Anchoring—Snubbers
  17. Anchor Rode Questions and Answers
  18. Q&A: Hybrid Rope And Chain Anchor Rodes
  19. Anchor Swivels, Just Say No
  20. A Windlass That Makes The Grade
  21. The Perfect Anchor Roller
  22. Install A Wash-down Pump—And Save Money!
  23. Anchoring—Kellets
  24. Anchoring—Chain: Stoppers, Termination and Marking
  25. 20 Tips To Get Anchored and Stay Anchored
  26. Choosing an Anchorage
  27. Choosing a Spot
  28. 15 Steps To Getting Securely Anchored
  29. One Anchor or Two?
  30. Two Anchors Done Right
  31. It’s Often Better to Anchor Than Pick Up a Mooring
  32. Yawing at Anchor, The Theory and The Solution
  33. Yawing at The Anchor, an Alternative Cure
  34. How To Use An Anchor Trip Line
  35. ShoreFasts—Part 1, When to Use Them
  36. ShoreFasts—Part 2, Example Setups Plus Tips and Tricks
  37. ShoreFasts—Part 3, The Gear
  38. Gale And Storm Preparation, At Anchor Or On A Mooring
  39. Storm Preparation, All Chain On Deck
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Stein Varjord

Hi John.
Since I’ve been participating on AAC a few years now, and learned a lot from it, the conclusions in this article come as no surprise, and they all fit with my understanding and experiences. That, however, hasn’t always been the case.

Some years ago I was a firm believer in the importance of the catenary curve to get good holding power. I also frequently gave advice accordingly. When I had to do some math to explain why a steel wire wasn’t as good as chain, (specific question), I found, to my frustration, that I was completely wrong. Then, I started looking around on the internet to find more opinions and thinking about what I’d been observing a lot of times at anchor in heavy weather; a chain with no real curve.

Then I realised that I’d been filtering what I observed. My beliefs made me overlook the proof I was looking straight at. Yet another illustration of the animal species Homo sapiens, whose “pure logic” is often lacking so many elements in its foundations it’s anything but logic. This realization led to a somewhat embarrassing complete change of “opinion”. I’m happy that I’m still not too mentally rigid to do that exercise.

Eric Klem

You guys are not alone, I did it as well. Growing up it was always mixed rodes and then I went to boats with large chain and no snubbers anchoring in deep water and with very different motions. When I switched to cruising size boats with all chain, I was a big believer in the chain weight until that first real storm when I realized how wrong I was and started looking at the numbers. Looking back, I think that in certain conditions, even the boats with large chain may have benefited from snubbers. The piece of the puzzle that is still missing to me is the damper in the system, we have figured out springs but to have a stable system you need damping and pulling the boat through the water seems inadequate to me (the people who tail drogues off the stern may be onto something here).

Eric

Eric Klem

Hi John,

A perfect spring is basically an energy storage device. If you were to attach a weight to the spring and drop the weight, it will oscillate back and forth each time returning to the original height of the weight. Where people may be aware of this is when they have a car with worn out shocks, when you hit a bump, it keeps bouncing for quite a while. On a boat with a perfect spring for a snubber, think of driving over your anchor on a calm day at 2 knots, assuming the anchor didn’t drag at all and you cut the engine power right before the rode took up load, you would bounce back and end up going backwards at 2 knots (this ignores resistance from the water on the hull).

Since a spring is simply an energy storage device, sometimes you want to actually take energy out of the system, usually in the form of heat and this is called a damper. The damper takes energy out so that you don’t bounce back doing exactly the opposite of what you had been doing going in. In a car, we have shocks which keep the car from bouncing for a while after a bump, they allow some bounce to absorb the impact but then quickly dampen out further oscillations. On an anchored boat, if you are underdamped the boat will come up towards the anchor too much when it rebounds. This allows the bow to fall off and it allows the boat to get reverse speed before it is met with resistance from the anchor rode. Snubbers and chain are not perfect springs but on their own, they are underdamped and for most boats, I don’t think that the water resistance is sufficient damping. How much damping is required is very dependent on the boat design (boats that sail around at anchor would require more), the rode design and the conditions. Some of the better behaved boats actually probably would not benefit from a damper in a measurable way but a significant population of boats appears underdamped when I watch them.

For most conditions, a damper would simply serve to keep the boat better behaved and lessen shock loading. In gusty winds from different angles, a damper that is independent of the anchor rode (like a stern drogue) could actually keep the boat from getting significant speed up before fetching up at the end of the rode. The drogue setup that colin uses is a form of a damper but because it is outboard of the snubber, it is only damping the spring of the chain (catenary and stretch). A drogue attached tight to the bow but held in the water would be a decent form of a damper but I am not sure how you would implement it in practice.

Eric

Rob Gill

Very interesting thread thank you guys,
Two years ago we were sheltering at anchor on the tip of the Coromandel Peninsula in NZ, in a beautiful bay called Port Jackson. A 30 knot SW wind was alternating from coming straight over the lower part of the headland, to hugging the bluff at the Western end and gate crashing the anchorage from seawards. This resulted in alternating gusts hitting us from the port bow and then the starboard bow and we were skating around accordingly. Dusk was coming and we were looking at a disturbed night with anchor watches. I had anchored as close into the shore as was seamanlike in about 4.5m of water and was using only about 6 to one scope in case the wind veered in the night and drove us further towards the sandy beach. If I had anchored any further offshore, we would have been in the wave action from the wind coming around the bluff.
So we had all chain rode (10mm) and a 7 metre nylon snubber in place. But how to stop the boat from sailing around to her anchor so avidly? Without knowing it, I needed a dampener.
My experience as navigator on commercial ships was that catenary does act as a great dampener and the first response to yawing was always to let out more chain. The catenary takes up the slack in the chain, then eases it out again. But in the shallow water chain induced catenary was not going to happen for us. I wanted to be instantly ready to retrieve the anchor and head out to sea, so adding any weight to the chain was not an option (having John’s sage words on kellets ringing in my ears).

What we ended up doing was setting our snubber and then paying out a further 20 metres of chain from the bow roller creating a ~ 5m bight lying on the flat sandy bottom. It worked rather well in two ways, the weight restoring a moderate catenary between gusts acted as a dampener – have I got that right Eric? Secondly, the drag on the sea bed slowed the bow from yawing to the extremes of the rode. So much so, we set the anchor alarm, went to bed and slept soundly. This will obviously work only for a shallow anchorage with a clear seabed and chain wear may not be good over longer stays.
I like that this requires no special setting/retrieval method – just the windlass and snubber. Anyone else experimented with this dampening “hack”? We have used it quite a few times since but never in full gale conditions and would be keen for some feedback.
Rob

Dick Stevenson

Hi Rob, I love your “out of the box” thinking. Thanks for sharing.
Dick Stevenson, s/v Alchemy

Eric Klem

Hi Rob,

You had indeed introduced a damper in a clever way that had never occurred to me. Your experience also highlights that the forces of the damper are necessarily lower than the forces of the spring and it appears that in your case, the relatively low forces of the chain dragging on the bottom and being pulled through the water were sufficient. By the way, catenary acts as a spring and the only damper associated with it is the resistance of the water on the chain.

There is a technique known as a hammerlock (thanks to Drew for teaching me the name) that goes a little further and uses an anchor dragging around on the bottom at very short scope instead of your lazy loop of chain. This similarly acts as a damper and people report good results with it, I have only used the technique once but it did work in that case.

I often do simplified calculations of stuff associated with sailing and find it to be very helpful to getting my thinking in the right ballpark. Unfortunately, dampers are a tricky subject and the math involves differential equations so it is not something that I have ever taken the time to try to play with

Eric

Dick Stevenson

Hi Rob,
As said, I like your “out of the box” thinking, although what John said about having a drooped chain looking for trouble on the seabed concerned me also.
I was wondering whether a weight dropped straight to the seabed with a bit of slack might have the same results: say a small diameter nylon line with a weight attached. At first, I was thinking of one of those mushroom anchors the chandleries sell for dinghies, but then I thought of the kellet I carry anyway which also has more weight.
The kellet would be quite unlikely to foul on the bottom and were it to do so, would not be a huge loss the way fouling the bower rode would be.
I am also thinking that hanging it off the stern might have even more of a dampening effect than off the bow.
Random thoughts.
My best, Dick Stevenson, s/v Alchemy

Eric Klem

Hi Dick,

The sole time I can remember trying the hammerlock, we used a small bruce on a nylon rode equivalent in diameter to the snubber. For us, this worked well, I believe Drew may have given more thought to actually what the “anchor” should be. With the snubber sized nylon, it wasn’t stretching much at all at the low load so we didn’t add another spring but it would have provided give had it snagged. One thing to keep in mind with this is that you will be forced to constantly adjust the rode length in an area with any real tide as otherwise your scope will vary a lot and it really need to be short scope. While the hammerlock is probably the best damper that I am aware of, it is far from an elegant solution and I wish there was a good way to implement one without the complication. It feels to me like a damper reacting to the water would be ideal much like Colin’s drogue except not on the far end of the snubber. I wonder if anyone with a waterline fitting at the bow has ever tried a drogue?

Regarding stern versus bow placement, I believe that bow should be superior for a damper. The reason is that I don’t like the spring force and the damper force being separated by something that can become a lever. If we think about the relatively simplistic example of a boat surging forwards due to the snubber recoiling, with a small stern damper, the speed is kept low which is good. However, once you start going backwards again, the damper is now providing a forward damping force to the stern so that as soon as the bow has fallen off and is not straight anymore, it is a torque that wants to spin the boat 180. If you have the damper attached at the same place as the snubber, it is just as effective as you go backwards but when you rebound forwards, it is not trying to spin the boat around. Also, it provides much needed damping in the yaw direction.

Eric

Ernest

Hi Dick,
I was intrigued by the idea of having the chain looped down behind the snubber, and Johns cautionary comments had me as well. I especially like your idea of using either a small secondary anchor that wouldn’t set, or a kellet if you happen to have one on board.
However I personally wouldn’t advocate mounting the “damper” at the stern as this might have the bow go to one side which then is leeward, and the stern doesn’t completely follow, creating a windward side. One effect would be that the boat stops sailing around, which is good, but then it would always present a broadside to the wind – which doesn’t matter a lot in light winds but might be counterproductive during a blow. Having such a “damper” at the bow would effectively slow the lateral bow movement while allowing the hull to align itself with the wind.

Dick Stevenson

Hi Eric and Ernest,
My my, I do like having these random thoughts responded to by thoughtful people. Thanks for your comments and I agree that the stern deployment has that possibility/likelihood that I had not considered: I was only thinking of the yawing, not the surging.
My best, Dick

Rob Gill

Hi everyone,
Thanks for the responses. Firstly to Eric, haha yes it’s obvious now – the catenary acts as a spring in the forwards direction. What I meant was my observation was that catenary acted to calm yaw action noticeably, reducing an otherwise violent pull from a bar tight chain that precipitated a counter movement of the bow with ever more extreme gyrations until the anchor broke out. But this must be the action of a spring, not a dampener, to the speed and direction of the yaw.

And a wonderful example of constructive paranoia from John, exactly what makes this site so valuable. I thought I had been careful to add there should be no obstructions on the seabed and to be fair, mostly these can be spotted over sand in shallow water. But I guess we could pick up an unseen abandoned anchor or such like with serious consequences – I will need to give that some more thought.
However, I’m equally not keen on shore-fasts for lying off a beach, no matter how golden and inviting the sand. Especially one that could quickly become a surf beach with a wind change – too hard to recover quickly.
Similarly, I am just not keen on any kind of weight, line or drogue attached to the chain rode (other than the snubber) that needs to be remembered and dealt with trying to exit an anchorage at 02:00hrs.
Finally, I am not keen on any kind of drogue attached on the bow, no matter how effective that would likely be. I just don’t fancy having to remember to remove it in a hurry, even if I could rig some kind of easy release.
And nor do I like anything in the water attached at the stern. What Eric explained so well scientifically, bears out my experience that to do so delays the stern in coming back into line with the wind. This exacerbates “sailing” at anchor rather than reduces it – I know, I tried it.
We don’t seem to be any closer to a fool-proof dampening solution that reduces yaw for a monohull. I should mention in normal conditions Bonnie Lass lies beautifully at anchor, but in storm force gusts from a violent cold front we need all the dampening we can get. Look forward to more on this valuable topic.
Rob

Rob Gill

Hi Dick, Ernest,
I just saw the comment about using a mushroom type anchor or kellet, which I think you intend would be set directly from the bow itself?
I am not sure this would have the same effect – having a longish snubber (7 metres) means that the chain bight was stretched out ahead of the vessel and acted to dampen the movement of the bow and the chain itself – in much the same way Colin’s drogue set from the snubber hook did here: https://www.morganscloud.com/2015/02/23/stop-swinging-around/
Keen to hear any results if you try it (in a strong blow say 30+ knots). Still not sure about having another thing to remember in the middle of the night.
Rob

Bill Balme

Rob,
We’ve been doing similar things routinely. If anchored in 15ft of water, we’d typically put out about 5:1 scope plus a further 25ft snubber and then drop a deep loop of chain – probably another 15 – 20ft so that when all is said and done we’ve probably got about 125 – 150ft of chain involved.
I don’t typically look for the chain to touch bottom as you did/do, though it might occasionally, but the deep loop certainly reduces the sailing we would otherwise do. (We still do too much in my mind.)
I can see the danger in snagging the loop on the seabed – but then the main rode can also get snagged during a lull and while that would be protected by the snubber, the loop would also be half protected by a long snubber – wouldn’t it? If something major caught on the side without snubber, one could always let out even more rode and another snubber if needs be! (My wife already thinks I’m crazy with the amount of chain I use to anchor!)

Rob Gill

Thanks Bill,
Nice to have validation that the technique is working for others.
In deeper water, I have set the anchor with a bight well off the bottom, and find it especially useful in a constrained anchorages where you may only be able to anchor on 3:1 or 5:1 scope. We have dropped 20 metres of chain with the bight off the bottom and it really does calm the boat down (we have quite high clipper bows susceptible to wind gusts). It suggest to me that the dual action of the spring and dampener (chain moving through the water column as Eric suggests) is doubly effective.
But I also think John is correct and the main snubber would provide you no protection in the event of a snag, but you are most certainly correct in your assertion that you could veer more chain and attach a secondary snubber using a rolling hitch – sorting the whole mess out in the morning.
Having wrapped a chain figure of 8 around a Pacific coral bomb in 18 metres of water, in my first anchorage attempt in the Pacific Islands, I consider myself somewhat qualified in such matters. Since that event, we have religiously trolled over deeper or murky anchorage grounds using the fish finder in our Raymarine MPS (who knew such things had a use on a yacht), to provide a continuous image of the seabed before dropping our hook. Once I learned that hack, I haven’t snagged the bottom again, even with laying out a large bight of chain as we have on numerous occasions.
Cheers, Rob

Drew Frye

Very well written! Let me add a few more variables:

Depth. I am particularly sensitive to this, since as a multihull sailor I think 4-5 feet is plenty (if no large waves). Really, catenary is as much about the amount of chain out as the scope. In other words, 5-1 scope in 4 feet is about 35 feet of chain. It straightens out even earlier than you mentioned and is basically useless by 15 knots. On the other hand, in 40 feet of water that would be 215 feet of chain, which may work to 25 knots at least. But at 60 knots they are both pretty straight. But what about 400 feet of chain? It is really quite hard to straighten that unless there is considerable wave exposure.

Straight vs. “still on the bottom.” An interesting exercise is to calculate the difference in length between a chain that is just lifting off the bottom and one that is straight. Generally it is less than you think, only a foot or so. Many sailors believe that because they can see some curve that means it’s still working. In fact, the difference in length and the force required to straighten it may have rendered it ineffective. The graph above is a nice illustration of how abruptly the catenary cushion ends.

Rode angle vs. anchor holding. I did a bunch of tests relating to holding capacity vs. up-lift angle. It is not the same for all anchors. Fortress holds well with considerable up-lift, once deeply set. Anyone who has tried to break out a Fortress after a blow knows this! Anchors with more shallow angles–Claw, Mantus, and Spade–lose holding capacity fast at short scope. Others–Manson and Rocna–are just a little better, but nothing like Fortress. The setting behavior of plow types is so dependent on the bottom general rules were tough, but generally not so good.

Scope vs. Setting. The other thing to remember is that although an anchor is set and holding at short scope, that short scope may prevent it from digging deeper as the storm builds. Put another way, an anchor at long scope will dig until the rode prevents it from going deeper, but an anchor at short scope will be robbed of that chance to go deeper. when in doubt, set at VERY long scope before shortening up. A correlarly is to add scope as soon as the wind starts. Once the wind is up, all boats will swing the same way. In this way the anchor gets the best chance to dig. Adding scope later, once the anchor starts t move, won’t do as much.

Soil vs. liquefaction. Some soils will consolidate around the anchor if allowed to set quietly, after which deeper setting is possible. Some times a series of increasing pulls, 20 minutes apart makes a huge difference. This is typical of soft Chesapeake mud. Other soils can be very prone to liquefying if the anchor is twitched by the rode. Coral sand and silt are bad this way. One more reason to stop yawing!

It’s really complicated. Every anchor, every rode, every boat, and every bottom combine to change the math. I can change the rank-ordering in anchor anchor test program by changing the expereiment. Isn’t that depressing.

Drew Frye

Yeah, the engineer in me loves to make things complicated. As you say, 98% of the time, good fundamentals are all you need. Really, anchoring is dead simple… once you get the right set-up.

There is a reason, however, for my obsessive view. I think I read once that one of the very few places your Spade ever dragged was in the Chesapeake Bay, in Solomons Island (please correct me if it was someone else). The bottom is pudding, or as the locals say, “the water is just thicker at the bottom.” I recall that short scope was part of the problem. In fact, some of my favorite places are even worse; you can barely feel the bottom with a paddle. In testing, I only ranked the bottom as “very soft” if I could easily push a paddle in 3 feet with one extended arm (no fair leaning a shoulder over it). The other problem is plant and shell debris that clogs anchors and reduces holding. Often sticks under the rode prevent the anchor from settling into thicker layers. In recent testing, 45-pounds anchors by Spade, Rocna, Manson and others generally held 500-600 pounds near Solomons, which just aint’ much. Were they well-set? Probably not long enough. But there are worse places. I could move along, but some of the world’s finest saltwater marsh paddling is in such areas. So that experience has influenced my practice.

Curiously, if you have a Fortress in through a major thunderstorm, getting it back up is a project. They can bury 4-6 feet in minutes. Better than dragging… just barely.

You don’t break stuff here, you just ooze away….

Bob Pfifferling

Agree with points made, but from “helicopter” viewpoint surprised there wasn’t more discussion of shock load reduction. As you say, a good snubber is important – we always set a long snubber back to the midship cleat. Always because you never know for sure what’s going to happen with the weather. We also always set our V shaped fin delta riding sail. I suspect that jerks on the anchor from sailing back and forth are often more problematic that straight line pull holding issues. One time in Vliho Bay, Lefkas, Greece an unforecast afternoon storm with multi-directional gusts over 40 knots had many boats dragging their anchors. We were fine until another boat with no crew on it dragged down onto us. The other boat got caught alongside us, basically a raft up but we held both ourselves and the other boat until the other boat crew got back. So we were very happy with our combination of spade anchor, long snubber and riding sail. Who knows how much each element contributed to the good outcome, but I suspect in that situation all the elements were important.

Dan Manchester

Hi John,

This is really interesting and I don’t necessarily share your conclusions, though it is a very complex field. We design moorings for buoys, and though these use much larger chain (42mm) the calculations around chain length for a given depth are highly complex, and if inadequate can result in a 3.5t dump dragging in storm conditions. We use modelling software to determine the correct ratios, it is called Orcaflex. I’d like to give this more thought before commenting further.

Michael

Hi John,
I continually get confused about G40 (G4) and G43. I can find them both listed, yet some describe them as the same. Above you refer to G40, but the first reference you give on chain grades in Further Reading only refers to G43. And there is potentially a gypsy issue if not identical.
Thanks