Effect of chain on anchoring

[foolishsailor]

Originally Posted by Kettlewell

In my experience those conditions never exist in the real world. There is always some wave action when the wind gets high, and the wind always has ups and downs in a sheltered harbor so chain does not stay bar tight for long. Even if a nearby buoy or your masthead anemometer is measuring a steady 40 there will be some gustiness at deck and hull level, and unless you are a fool you will have out a long snubber or a significant length of nylon rode between the boat and anchor. In the lulls the chain will sink down, in the gusts your boat will move back, hopefully cushioning the blow with your snubber. Meanwhile, your boat is going up and down. Anchoring in a storm is a dynamic situation so these static models don't really tell us an awful lot.

I lived in these conditions in south brazil for 6 months with a good bottom. They do exist.

[foolishsailor]

Originally Posted by Seaworthy Lass

Having two in series is not much different to having a larger anchor. As far as weight of chain goes, it makes a big difference to the holding ability in up to 20 knots, a moderate difference at 30 knots, and very little difference at 40 knots and above when it is "bar tight" (very rough figures).
The length always makes a big difference as it determines the angle of pull at the anchor.

Hugely different to having single anchor. In series there is twice the opportunity to grab, it is like the difference betwwen kinetic and static friction

[Seaworthy Lass]

Originally Posted by foolishsailor

Hugely different to having single anchor. In series there is twice the opportunity to grab, it is like the difference betwwen kinetic and static friction

If anchor x or y (I dont want to get into any anchor wars) is not setting, why should two of anchor x both set well? If they are not set, their holding is abysmal. If they are set, their holding is only the same as a similar surface area on a larger anchor.

A larger anchor has the additional benefit that it can penetrate more difficult substrates like weed, that two smaller anchors may struggle with.

Tandem anchoring should be a last resort when the primary anchor is inadequate.

[micah719]

Some day soon someone will invent the anchoring equivalent of the series drogue, and make a mint. I'd love to get into experimenting with the idea of an energy buffer on the rode, using the boat's energy to move water rather than most of the energy going to the hook and the bottom. The "waterwalking" physiotherapy system uses paddles attached to arms & legs...I'd go for some kind of bellows or drogues or turbine, or combination thereof, with a shock absorber, all in one handy package on the snubber. And a decent anchor & rode, of course.

[Kettlewell]

I'd love to get into experimenting with the idea of an energy buffer on the rode, using the boat's energy to move water rather than most of the energy going to the hook and the bottom.

Some folks put a big float part way down the anchor rode between the boat and the anchor. As the wind picks up the boat surges back and before the rode can go straight it has to pull the float below the water--and you know how hard that is to do. The float keeps a big bend in the anchor rode. The same principal helps to give permanent moorings with a mooring ball extra holding power. However, one can imagine that the wind could get strong enough to overpower the float and than the anchor rode would become almost straight. Still, it would be a way to introduce a lot of elasticity into a mostly chain rode.

[foolishsailor]

My two year old climbed on me and said "get off your phone dada", so i secede from ths debate

[lancelot9898]

I agree with Evans that the chain makes a difference even under storm conditions when the rode is bar tight. It's the dynamic loading that the weight of the chain helps reduce as the boat surges back and forth.

One other observation that I had when staying anchored during a cat 1 hurricane and that is the sling shot effect. I was anchored with 80 ft of 3/8 BBB and then spliced on to 5/8 inch 3 strand nylon line(probably another 100 feet of line). We were getting gusts in the 90 knot range and when the gust let up that is when the nylon line would unload and a "sailing we would go". Hard to describe.

[estarzinger]

Originally Posted by micah719

I'd love to get into experimenting with the idea of an energy buffer on the rode, using the boat's energy to move water rather than most of the energy going to the hook and the bottom.

Originally Posted by Kettlewell

Some folks put a big float part way down the anchor rode between the boat and the anchor. . . . would be a way to introduce a lot of elasticity into a mostly chain rode.

Beyond weight, snubbers and floats are the most obvious ways to dissipate energy. Rather than purely dissipating it, it would be interesting to try to combine some of these ideas into electrical energy generation.

Back on thread . . . I guess I am not quite sure what question the past few pages of debate are trying answer? I actually think we all essentially agree on most all the relevant points.

I think we can all agree (with some debate about the specific priority order) that the important aspects are #1 picking a protected anchorage, #2 with a good bottom, and #3 laying out proper scope and #4 then correctly setting a #5 good size #6 decently designed anchor . . . and that the weight and composition of the rode come in after all that (so relatively low on the importance list of keeping your anchor stuck)

. . . just to support that contention (that the composition of the rode is not so important) . . . the almost perfectly 'bad' rode would be a 100% dyneema single braid with almost no weight AND almost no elasticity. But I know some people who quite successfully anchor in quite bad conditions with exactly that sort of rode by doing #1 - #6 well. But that said, one can design a rode system (with weight and elasticity) that will help marginally improve the setting and holding power (but at the cost of additional weight vs that dyneema rode).

For example, I think we can all agree that an 'all chain' rode (or also the 'all dyneema' rode) should have some decent length of snubber in it - both to help protect the windless and as a very simple way to create additional elasticity.

I think we can all agree that in considering the weight and composition of the rode, that loads (and catenary) is only one factor, and that chafe resistance and windless usability and weight carrying ability of your specific boat (bow design) are equally (or I believe more) important in the decision. So, the catenary question should not be considered in isolation of these other factors.

I think we can all agree that at some point with some amount of wind the chain will be pulled almost straight and the catenary will be almost all gone, and that exactly where that happens depends a bit on the boat (how much windage) and how heavy the chain is, but that for most typical boats and chain it's somewhere in the strong gale area.

I think we can all agree that in most strong gales and above, with most boats, the load on the rode is not steady but dramatically oscillates due to gusts and waves and sailing back and forth. And that in these conditions chain does help to damp the shock loads (in a number of ways) and to allowed the anchor an opportunity to reset. And that there are also other ways to increase damping (like the float idea).

I think we can all agree that the models (at least all that I have seen so far) do not at all accurately model the complexities of the dynamic behavior in 'the real world'. So, making any important decision based on the output of these simple models is very suspect (note all the nice pretty graphs on Peter Smith's web site come from a such a suspect simple model). Really these models are a red herring - interesting in a simple theoretical sort of way but not complete or accurate enough to provide input on system design.

So, what substantive topic here don't we all pretty much agree on? What is it exactly that we are debating?

[Kettlewell]

So, what substantive topic here don't we all pretty much agree on? What is it exactly that we are debating?

I agree with that!

[Dave Chain]

Interesting responses.

If you get a green screen it is likely because your browser has Java Script turned off. Microsoft refers to this as Active Scripting, I believe. You can turn it back on if you wish to use this web page. Sorry, I should have mentioned that.

Everyone is correct that the dynamics are very important and that this model only considers static conditions. Still, if you don't understand the statics is unlikely that you will be able to understand the more complex dynamic conditions.

Our boat is has a fin keel and is an enthusiastic sailer while at anchor. I have been surprised that the anchor can hold at all when the boat is tacking back and forth. It's very dynamic.

Some have indicated that when the chain is lifted off the bottom it no longer has any affect on the holding power of your anchor. Others, I believe understate the value of the catenary shape of the chain. These two issues are closely linked.

My web page a generic sketch of an anchor which attempts to explain why you can pull at small upwards angles on the stock of the anchor without large decreases in holding power. It's makes sense that if you pull at an upward angle of 1 degree, the anchor will still hold just fine. If you pull upwards at an angle of 30 degrees, it will start to rotate, and the hold power will be much less. The shape of the catenary has the effect of decreasing this angle at the anchor. Even when the chain is lifted of the bottom the angle may be quite small.

I have never seen data on the effect of the applied angle on the holding power of anchors. If anyone has a small anchor, a load cell, a sandy beach, a pickup and a free afternoon, they should study this.

Although I think that understanding the statics involved is a good first step toward wisdom, I would never suggest ignoring 100's of years of experience. If you are sitting up in the cockpit in the middle of the night worried that you might drag, it's a much better idea to warm up the engine than to get out your calculator.

Dave

[Kettlewell]

Robert Danforth Ogg, co-developer of the Danforth anchor, did a lot of research in developing anchors for the Navy and it is worth it to find and read some of the old brochures and papers published by the Danforth company. With regard to the weight of chain helping with the catenary, which it does, but he also concludes, "...we see the value of using a buoy and pennant where possible instead of connecting the boat and anchor directly." He believed, based on his testing, that keeping the weight off the bow of the boat was very important in order to allow the boat "freedom" to respond to wave action. "Allowance for this freedom is a major consideration in the proper design of a mooring or selection of anchor rode. Failure to provide this freedom may result in tremendous and destructive surge loads." In another spot he wrote: "NOTE: For safest mooring or anchoring, a buoy should be used to carry the bulk of the vertical load, leaving only a horizontal load on the bow of the boat as well as on the anchor."

Interesting stuff.

[klem]

Coming to this kind of late but I largely agree with estarzinger's last post.

One thing that I have always wanted to see is a plot of load versus time on the anchor rode in different conditions. I know that at least 2 of you have used load cells on your anchors, has anyone with a digital load cell taken data at a speed greater than 1Hz or so in a situation where dynamic loading was a factor? I think that it would be really interesting data and would be very useful if someone ever tried to come up with a dynamic model. This is something that I would love to do myself but I don't have a load cell and it isn't high on my priority list.

[Andrew Troup]

Thank (h)evans for a more nuanced and all-encompassing viewpoint than the uber-simplistic opinion-driven rationalisation which has been disseminated over the net in the last few years, much of it from a relatively small number of people.

I had a similar reaction to the claim that if the elasticity of steel were ignored, the load peaks with an all-chain rode would be infinite ... in fact at that point I lost faith in the analysis as a whole.

It seemed to me that anyone capable of making that statement was disqualifying themselves from any claim to having a grasp of the system's behaviour.

It's completely misleading to focus on the residual distance* or sag of chain (as on the Rocna website, inter alia) : what needs to be analysed is the profile of the energy required to straighten the chain, and the peak load and direction presented to the anchor as a function of that energy during a 'straightening' episode.

The purpose of the chain is to decelerate the vessel as smoothly as possible and over the longest possible distance prior to the chain pulling straight: By the time the chain is straight, the job of deceleration is done.

The damaging loads are almost by definition dynamic, arising from the boat having the opportunity to store up kinetic energy because of the 'runway' offered by a lull. It will accelerate to a peak speed along that runway during the ensuing gust and feed that energy into the rode/anchor system.

Looking at a graph and saying "the chain's virtually straight at such and such a load" and claiming that it's therefore incapable of doing its job is like saying the sherriff's not doing his job 'cos he's got his feet on the desk, when in fact the perpetrator has already been locked up.


It's a strange contention that because too-light chain doesn't do a very good job, one should choose an even lighter one. For anchoring in deep water, I don't think any other system approaches the performance of a heavy chain, when it comes to presenting the loads as gently as possible to the anchor, both in strength and in 3D direction.

If the windlass (and chain locker) is aft by the mast, the first section of chain from bow roller to windlass with the anchor stowed, can be really heavy, say 16mm studlink for a 44 footer. This will make a big difference to how uniformly the load direction is presented to the anchor. Naturally the anchor manufacturer would prefer you to put that weight into the anchor, which means paying them considerably more. (Heavy chain is available for scrap prices at any commercial port: it gets condemned when it still has massive scantlings for our purposes) The new-gen anchor manufacturers don't have to carry your anchor about, put it in a dinghy, etc etc.

Shallow water does present challenges which heavy chain does little to help. I've dragged a 66lb genuine Bruce on 5/8" chain with a 23' sail boat. It took 24 hours to drag it one boat length, but it was dragging, and the water depth was 3' at low tide. (Greasy mud bottom)

For hurricane conditions in shallow harbours, especially where the boat is unattended or shorthanded (hence difficult to keep up with snubber degradation) I have cut up some large stud-link chain into short sections to use as periodic kellets, alternating with floats. Not spaced too evenly, to avoid harmonic resonance problems.

* ie the diff between the actual end-to-end distance of the chain at max load and the end-to-end distance when completely straight

[Dennis.G]

Originally Posted by foolishsailor

... My understanding is that matematically there is zero difference once the average pull level exceeds the weight of the line. ...

This is incorrect. It takes a lot more force than just equivalent to weight of chain to come close to straightening chain. (In fact chain will always have some catenary under any tension).

In air, a 100 feet of 3/8" chain weighs about 150 pounds. If under a tension force of 150 pounds a horizontal chain would sag over 12 feet in the middle. If under a ton of tension, would still sag almost a foot. (values would be less under water for steel chain, I think around 85%?)

[Kettlewell]

Did anyone see the PS article Anchor Testing and Rode Loads, May 2012 issue? The test confirmed several things. First, the loading on the rode was very dynamic--loads came and went, mainly due to the boat swinging at anchor, though it was protected from much of a sea. Second, the maximum load on a 38-foot cat in winds of about 23 knots was about 1400 lbs, but that was at 3: 1 scope. At 7:1 the load never got over 700 lbs. They said the maximum loads generated were snatch loads when the boat swung, and usually the load was much less. At 10:1 the snatch loads were just about gone in winds up to 30 knots, and I believe the load never got above 400 lbs or so. The boat used a bridle made of Dyneema and an all-chain rode (size not specified), so there wasn't much elasticity in the system. Anchor Right was involved in the testing. Has there been a thread on this test before?