Effect of chain on anchoring

[Cheechako]

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

Originally Posted by Kettlewell

I agree with that!

OH! I disagree strongly!

[Palarran]

This is a good discussion and since there is so much experience here, would you mind telling me what was wrong with my anchoring and observations this summer in the Mediterranean. I never had a problem in the Caribbean with my anchor holding but several times it dragged in the Med.

When we anchored in sheltered bays with moderate waves and strong gusts it seemed that too much scope became a detrement instead of an asset. What would happen is the boat would surge back until the anchor chain was bar tight, the boat would stop, then surge back forward maybe 10 to 15 feet. Everytime this cycle would happen the anchor would drag about 4 inches. If I shortened the scope, the surge would stop and the anchor also stopped dragging. This was in about 30' of water with 3:1 and upto 7:1 scope ratio. I noticed that most of the boats had about 3:1 scope and didn't surge or drag. The wind was blowing a solid 20 with gusts into the 30's.

We spent hours snorkeling over the anchor and adjusting the scope and found it was better to have less scope and less boat surge then more scope and more surge.

[Kettlewell]

What type of anchor and size of chain? Did you have a snubber? Are you using a bridle? Any decent anchor in decent holding ground wouldn't drag under those conditions, and I have never experienced an anchor that is dragging to stop dragging when the scope was shortened, unless it became snagged on something. I would generally have around 5:1 out for those conditions. Cats do have more windage, and tend to react quicker to wind gusts.

[Palarran]

Hello Kettlewell,
It is a Delta anchor and is definatly undersized given the bottom conditions we encountered. I use a very heavy bridle which would have little elasticity.

The reason for me asking in this thread is the "effect of chain on anchoring". I would have thought that the weight of the chain as it started to go tight would have slowed and stopped the boat before the shock load of the boat hit the anchor but it didn't. Again, you could watch the chain hang straight down from the bow to the bottom then draw up and tight, moving the anchor a little at a time. If you shortened the distance, the surge wasn't as bad and the anchor stopped dragging. Lengthening only allowed the boat to accelerate more backwards when a gust hit.

Regardless I'm getting a new Rocna next year that's two sizes larger then my current Delta so hopefully it will stop.

[kefroeschner]

Re the PS article on anchor loads: I recall that they gave up on the short scope all chain tests because the shock loads as the boat surged and the chain came taught were so bad that they feared for their safety.

[Kettlewell]

Palarran: That's rather an odd situation. Cats have a lot of windage relative to their weight, and therefore they accelerate faster creating a greater shock load when they reach the end of the line. However, I have anchored in a cat in those conditions and never experienced anything like that. First, you probably do need an anchor with greater holding power. I might also suggest a springier bridle or some nylon rode in the system in order to reduce the shock loads on the anchor. On my cat I usually used about 50 feet of chain and the rest nylon, but you probably want more chain. A second anchor could also be used off the bow on mostly nylon and it could stop the oscillating by staying taught in the lulls.

To give you an idea of what a well dug in anchor should be able to take, I have been anchored peacefully when a tremendous squall came in from the opposite direction. The wind was 180 degrees opposed to the original wind and it caught us on the transom, sending our boat whistling downwind until we reached the end of the anchor rode, when the boat came to a sudden halt with a tremendous jerk, whipping the bow around into the new wind. And still the anchor did not budge.

kefroeschner is right about the PS article. The short scope 3:1 tests were ended because of the tremendous strain being put on the boat. Usually, longer scope means less stress on everything.

[klem]

It seems that we all agree that the biggest loads on your anchoring system are dynamic loads due to the bow moving around from wind and waves. If you take a given anchorage, conditions and boat, the question is then, how do you lower the loads. The two best ways to do this are to keep the boat from moving around so much (removing windage forward, trim, bridles, etc) and to add a shock absorber to the system. Unfortunately, adding a shock absorber can have the effect of allowing the boat to move around a lot more so care must be taken. If the wind and waves were perfectly cyclicle, then you could do a dynamic analysis and figure out how much shock absorption is optimal. My observation is that boats with all chain rodes very rarely have too much shock absorption and boats with combination rodes often have too much and the boat "sails" on the hook.

An easy way to define shock absorption is to look at the force versus deflection curve. A curve with a lower slope has better shock absorption and a curve with a higher slope has less shock absorption. Thinking about it from an energy standpoint, you apply a force over a distance which is work, for a given amount of energy, you can either apply more force over a short distance or less force over a longer distance. To try to do a real simplistic comparison of where the shock absorption comes from, I have set up a simplistic scenario of a boat with 200' of 3/8" anchor chain out at infinite slope (the two ends are at the same vertical height). To compare the shock absorption capabilities, I am comparing the loads of 50,500 and 5000 lbs on the rode.

One area of shock absorption will be chain catenary. At 50 lbs of load, the distance from the bow to the anchor will be 120'. At 500 lbs of load, the distance will be 197'. At 5000 lbs of load, the distance will be 199.9'. Just to add another data point, at 2000 lbs of load, the distance will be 199.8'. What this means is that chain has a relatively shallow curve at low loads and an incredibly steep curve at high loads. From 2000 to 5000 lbs, only 0.1' of give is due to catenary, this is the definition of bar tight. In practice, as your bow blows off downwind in an extreme loads situation, there will be very little resistance which will slowly build until there is suddenly a huge amount of resistance. I would argue that normal sized chain has good shock absorption from catenary at low loads and poor absorption at high loads.

As pointed out earlier in this thread, chain actually deforms under load. I built a very simplistic finite element analysis model to look at this. I did this model quickly and it is not the way I would model this if I really needed an exact number but it should be very close and good enough for this discussion. A single link of 3/8" chain stretches approximately by 0.00006" at 50 lbs of load, 0.0006" at 500 lbs of load and 0.006" at 5000 lbs of load. Because the pitch of this size chain is 1.23", this equates to 0.01', 0.1' and 1'. This analysis ignored the fact that as the chain is tightened, the links actually move very slightly relative to each other causing it to give slightly more. Compared to catenary, chain stretch is very low at low loads and a better shock absorber at very high loads although it still is not much if your bow is jumping up and down in 6-8' waves.

There are also friction losses present which I have not tried to estimate. These come from the chain moving through the water and the links moving on each other. Both of these factors will be less as the chain gets closer to bar tight as there is less movement. These losses are also present in line but the magnitude will be different.

Now, assume you add a 30', 5/8" nylon, 3 strand snubber. At 50 lbs, it stretches <1", at 500 lbs it stretches 1.2' and at 5000 lbs, it stretches 6'. Nylon line has a relatively linear, low slope for the force deflection curve which is the reason why it makes a good shock absorber.

Obviously, this is a slightly simplistic way of looking at everything but it helps in getting a handle on what is at play in different situations. In low load situations, the best shock absorber is undoubtedly your chain because of catenary. Once the loads get really high, you will get the most give from your snubber or nylon piece of your combination rode although the chain will be giving some as well. There are other small factors at work including the deflections of the bow of your boat and seabed but I strongly believe that a nylon snubber provides the best give in truly extreme conditions. The good news is that people with 3/8" chain rarely see loads over 2000 lbs and hopefully never see loads of 5000 lbs.

In practice, I believe that an all chain rode has many advantages, primarily in chafe. In most conditions, a chain rode with a rigid connection to the bow provides plenty of shock absorption. In really extreme situations where large boat movements must be dampened, either extremely heavy chain must be used or something else like nylon, rubber, etc. The great thing about a nylon snubber is that you can vary the length and therefore the deflections. Overall though, protection, bottom type, the anchor and other factors are what should really be concentrated on.

[Bash]

Originally Posted by estarzinger

I have argued with respect to anchors that setting (especially in less than idea bottoms) should be more important and more the focus of testing, than the holding power that the anchors guys like to so focus. For me, if the anchor has trouble setting/resetting (in less than ideal sand bottoms) it does not really matter what it's holding power is.

Evans, do you change your thinking in terms of storm anchors? Right now my storm anchor has enormous holding power but is not as good setting/resetting as my working anchor. (Likewise, even though my working anchor uses an all-chain rode, my storm anchor is chain to nylon, my thinking here being that once the catenary is gone there's no real reason to keep the chain, since the nylon will shock-load better if I'm careful about chafe protection.)

I don't anticipate this setup for dealing with hurricanes where I need to deal with radical wind shifts. Just for the "normal" 40-50 knot blows we might get in my home waters.

So, do the requirements for storm anchors make holding power trump resetting ability?

[Kettlewell]

So, do the requirements for storm anchors make holding power trump resetting ability?

I can't answer for Evans, but they both are critical. As Evans said, it doesn't matter how high the theoretical holding power if you can't get the darn thing set in the first place. Here on the East Coast we deal with hurricanes on a fairly regular basis, and we are also blessed with many well-protected and relatively shallow harbors with excellent holding. Almost anywhere between Florida and Maine you basically don't worry much about setting as the bottom is mud or sand, or sometimes a combination. No need for much chain as chafe is not an issue. Something like a Fortress or a Danforth of almost any decent size will dig in so hard you will have a tough time getting it out after a strong blow. They make ideal storm anchors with minimal chain. Set them so that the direction of pull won't change much. However, for day to day use, something that can pivot and reset makes a better main anchor, as the wind switches around all the time, and that is where Evans' setting issue comes in. In other parts of the world, you deal with kelp, hard bottoms, rocks, or other tricky situations where getting the anchor hooked is difficult, and therefore you need something that is heavy, can penetrate, and maybe has some hooking ability. I've used a Herreshoff type Luke anchor on a rocky bottom where a Danforth would just skip across the rocks, yet the Herreshoff was able to find a cranny and hook in, despite having much less ultimate holding power.

[Andrew Troup]

Originally Posted by Palarran

Hello Kettlewell,
It is a Delta anchor and is definatly undersized given the bottom conditions we encountered. I use a very heavy bridle which would have little elasticity.

The reason for me asking in this thread is the "effect of chain on anchoring". I would have thought that the weight of the chain as it started to go tight would have slowed and stopped the boat before the shock load of the boat hit the anchor but it didn't. Again, you could watch the chain hang straight down from the bow to the bottom then draw up and tight, moving the anchor a little at a time. If you shortened the distance, the surge wasn't as bad and the anchor stopped dragging. Lengthening only allowed the boat to accelerate more backwards when a gust hit.

Regardless I'm getting a new Rocna next year that's two sizes larger then my current Delta so hopefully it will stop.

It's pretty hard to make any useful comment or draw any useful conclusions when you won't tell us how small your chain is. (or your anchor, for that matter, but in relation to the question, it's the chain which matters)

For a boat as big and heavy as yours, it's not going to provide appreciable shock absorbing from catenary sag if it's only, say, 3/8" (10mm)

[estarzinger]

Originally Posted by Bash

So, do the requirements for storm anchors make holding power trump resetting ability?

We anchor every day with a 'storm anchor' with is big enough to have loads of both. So, in that regard, no, what I said applies directly to our primary storm anchor.

We rarely put out more than the one anchor, but if we do there will be two possible scenarios:

(1) we have anticipated hurricane force winds, and have had time to find just the right spot in just the right anchorage. In that case, yes, holding power will be primary both because I will have chosen a bottom that sets well and because I have time to make sure the anchor is set. I will probably use our huge fortress in that situation.

(2) we have not anticipated hurricane force winds and they catch us by surprise and we feel we need to get a second anchor out right here and right now. In that case, no, what I said still applies setting first and holding second. We would use our Manson Ray in that situation, which again is big enough to have loads of both, but the design is skewed toward setting and short scoping rather than holding.

That's what we do. There are certainly other good approaches. But this way seems both the easiest (mostly setting just one anchor) and most secure (We sleep well and don't worry unless we are surprised by truly severe winds).

[noelex 77]

Originally Posted by estarzinger

We anchor every day with a 'storm anchor' with is big enough to have loads of both. So, in that regard, no, what I said applies directly to our primary storm anchor.

I think this is the best policy for a boat with an electric windlass and is exactly what we do.
Even with modern forecasts there is still some unpredictability. The strongest winds I have experienced at anchor was when only a Force 7 was predicted.

As a cruising boat we often spend a week at anchor, if the forecast changes who wants to pull up the anchor and change a well set anchor if its not necessary.

There is a saying in aviation that there is nothing as useless as the runway behind you. The same could be said for sailing. There is nothing as useless as an anchor stored below decks.
If your anchor winch can handle the storm anchor (and if cannot do this in normal conditions what is going to happen in extreme conditions) why not use it? It has the added advantage that in milder conditions you can often anchor to a short scope, or in poor holding grounds.

[JonJo]

Originally Posted by 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?

It was 8mm chain. The bridle was Dyneema simply to remove any elasticity and a means of connecting the load cell to the chain (via the Dyneema). The load cell could not be used forward of the bow roller (because at times it might have been heavily soaked - did not know initially) The use of the bridle was to keep the cat in the same oreintation as its normal (bridle) would ie reduce swinging. The short scope tests were discontinued as we thought we might pull something out of the yacht (not sure it was built for what we call snatch loads, and I think here are called dynamic loads). Anchor Right had no involvement in the tests at all, other than the anchor was an Excel, but it could have been a Spade or Fortress.

We did some subsequent tests on lifting 8mm chain. I think 30m of 8mm chain weighs around 45kg and at 5:1 it takes a load of about 80kg to lift it all off the ground, still has a lot of sag. I'm working from memory here. Loads would be a bit less in the sea, SG effects. Another function not mentioned is that when a yacht veers from side to side the friction of the chain on the seabed dampens the movement. If you do not believe this, go down to your local beach, layout 50m of chain and pull it across the seabed in a straight line and then try to pull it at an angle. Its only when the chain is lifted, that 75kg, that friction disappears (other than friction of chain in water?!). But a 75kg dynamic load is basically, nothing.

Evans has produced a nice summary - cannot fault it. A monor point he missed is that holding capacity tests are a steady increase in load, winch, tug or whatever. In real life loads are snatch or dynamic.

Consider, worst case scenario? well set anchor, enough chain, how much is enough?, good holding bottom, but then a violent thunderstorm passes over and the wind changes suddenly through 90 degrees. We now have the anchor set at 90 degrees to sudden and fierce wind gusts. What happens next?

J

[Seaworthy Lass]

Originally Posted by JonJo

.....
Consider, worst case scenario? well set anchor, enough chain, how much is enough?, good holding bottom, but then a violent thunderstorm passes over and the wind changes suddenly through 90 degrees. We now have the anchor set at 90 degrees to sudden and fierce wind gusts. What happens next?
J

We anchor 24/7/365 and have done so for five years, so regularly experience thunderstorms with violent changes in wind direction. This is where the design of anchor is critical. Our Rocna had not once broken out in these conditions, it simply rotates below the surface. For eight months of the year I snorkel to check (both ours and anyone else's close by out of curiosity) and in the early days we used a bathoscope to check when it was too cold to swim, so I am reporting extensive first hand experience

[Maine Sail]

Originally Posted by JonJo

The load cell could not be used forward of the bow roller (because at times it might have been heavily soaked - did not know initially)

J

This is one thing that really bugged me about that article. The friction over the chock reduces the peaks the load cell sees and gives quite different readings then when ahead of the chocks..

I have done this both ways on my own boat. I simply "water proof" as best I can my load cell and leave it ahead of the chocks for the most accurate readings. Never had a single corrosion issue.. I used No-Ox-Id electrical grease to coat the cell and wrap the wire with self amalgamating tape.

I have measured peaks, using "peak hold" on our 36 footer in just 17 - 20 knots, and some associated chop, at 218 pounds. When I place the load cell aft of the chocks I do not see the same data. There are many boats who's cleats are right on the bow with no chock to pass through and I want to see the loads that type of cleat would see thus I place my load cell ahead of the chocks..