Using your new anchor with kinetic rope

[noelex 77]

Originally Posted by Lodesman

Do you have (a) link(s) to the formal tests that prove this? Spade claims that their alloy anchors have the same holding power as their equal-sized (and heavier) steel anchors - are they lying?

As one of few anchor manufacturers that produce an aluminium anchor, they obviously need to be careful what they say in their advertising material, but Spade stress the aluminium version is not recommended as a primary or bower anchor.

The aluminium Spade is still a good anchor, but in my view it does not have the brilliance of the steel version. MaineSail had a video of an aluminium Spade performing poorly and Spade swapped this for a steel version. (My internet is terrible at this anchorage so sorry I cannot find a link) Spade increased the ballast in the alloy Spade many years ago to try and compensate for the difference, but this seems to have only made a slight difference.

There have also been some formal anchor tests that have included the steel and aluminium versions of the Spade and these show the maximium holding power of the aluminium version is much lower than the steel version. For example, in the large independent 2009 multi magazine test, the holding in hard sand for the steel Spade was 1705 kg. This dropped to 1052 kg for the aluminium version. In soft substrates the difference in performance seems less marked.

The test can be read here: (Unfortunately this link is for the French version)

https://pdf.nauticexpo.fr/pdf/sea-te...130-15224.html

[noelex 77]

Originally Posted by CassidyNZ

So if I had a modern anchor design (prefer not to be specific ) made of platinum (approx 3 times mass of stainless steel, ignore strength for this argument) and made it to the exact dimensions of the original anchor, it would hold in the same ground three times as well as one made from stainless. Personally I doubt that.

An anchor constructed from platinum would not be be “ the same design and material” as a steel anchor. Therefore the holding power could not be expected to be predicted from the weight alone.

The linear relationship between anchor weight and holding capacity holds true only for anchors of the same design and material (as I indicated). Eg a 20kg steel Spade could be expected to have roughly double the holding capacity of a 10kg steel Spade. This formula cannot be used to compare a steel and aluminium (or platinum for that matter) Spade.

[FAST FRED]

"chain is simply easier to handle. Windlasses are more common than they used to be."

Perhaps chain is easier to bring aboard , but it still needs every bit of mud scrubbed off , or you will have that low tide dead fish stench on board 24/7.

[thinwater]

Fun stuff. I've done a lot of research, a lot of instrumented testing, and there are a few things I'm pretty sure of, plus some things that might need explanation.

• All materials get thinner when they stretch. They get longer, but the volume remains relativity constant, and in the case of rope, decreases slightly. Obvious. In fact, this makes it practically impossible to make a seized eye in nylon that will not stretch at high load (polyester and hemp ropes can be seized).
• Why snatch straps? Specifically, if the tow vehicle is also in the mud, you just can't generate any static pull, but you can store energy by getting a running start with slack. But this is NOT the same case as a boat on a rope rode, because the rode is never slack. If you try to use snatch straps without slack, guess what. They don't work. This is not to say a boat with a rope road does not yaw more, for several reasons. But the motion is complex (I've measured it with a load cell) and the behavior is very different from chain (which CAN generate slack in the form of excess catenary in the lulls). Put simply, a boat on rope tugs at the anchor from a greater range of angles, but a boat on chain pulls MUCH harder (unless there is a long snubber). Then it becomes a continuum of answers.
• Nylon ropes are not ruined after a few impacts. Climbing ropes routinely take thousands of good falls before retirement. Watch any sport climber at work or go to a gym. Like any material it has a fatigue life, and you need to know where you are on the scale. If you stay withing the WLL (about 5:1 for steel and Dyneema, and about 8:1 for nylon) it will last a good long time. Go outside that range, not so much. This is engineering design.

Weight vs. holding. For the same design, there are MANY studies, including Bruce, and US Navy, Neeves, and me who have studied this. Hold ~ mass^x, where x is bout 0.88-1.1, depending on who you ask. As a conversational approximation x=1, so hold ~ mass. This is true for set anchors in uniform bottoms and does not require debate. Personally, I've tested same-manufacture anchors from 2-35 pounds and the data fit a straight line with x=0.9. It's just true. People have asked, "but fluke size is not linear" and "big anchors go deeper." All true, and that fits withing the x=1 calculation.


Alloy vs. Steel. It's all about fluke area if the design is the same. I've modified alloy anchors to match the area and angles of a steel anchor. Once set, it makes no difference. Argue that if you want. Go build models and play.


Initial set vs. Weight. I'm not going to argue this. On good sand or mud a good anchor will set every time. On cobbles and in weed, performance is all over the place. We all have our experiences.


Bigger is much better. Yes and no. Bigger is better in proportion to mass, except in bad bottoms where it is a little better than that (can bite better). But the main thing people observe is the difference between success and failure. If can pick up a 100-pound weight but not a 110-pound weight, the 110-pound weight seems much heavier. A 35-pound anchor is not that much bigger than a 25-pound anchor, but it may be the difference between never dragging and dragging frequently, just because the 25-pound anchors is not quite enough.


But think on this. You read and anchor test, and for each anchor, the hold may range from 1000 pounds to 5000 pounds. All of these are more than you could tell apart by power setting. Does the anchor hold 1000 pounds or does it hold 5000 pounds. Why did it only hold 1000 pounds that one time? Answer that, and you will know why you drag or why you might drag.


Catenary. If anyone really wants to know, they will get a load cell and measure rode tension, and then look-up a caternary calculator. You can also dive, noting the wind speed and recognizing that you're not going to dive during a storm, when the wind may be over 60 knots (if it isn't gusting over 60, it's really breezy, not a storm). There so many variables (water depth, scope, waves, chain size, yawing) that making broad statements is pointless. In shallows, at 5:1 scope, with long fetch, I promise the chain is effectively straight in 15-20 knots. I've seen it. The impacts were frightening, on the order of a ton (measured). In 20 feet with some protection there will be a nice curve and no jolts with no snubber. It all depends on the math.

[CassidyNZ]

Originally Posted by noelex 77

An anchor constructed from platinum would not be be “ the same design and material” as a steel anchor. Therefore the holding power could not be expected to be predicted from the weight alone.

The linear relationship between anchor weight and holding capacity holds true only for anchors of the same design and material (as I indicated). Eg a 20kg steel Spade could be expected to have roughly double the holding capacity of a 10kg steel Spade. This formula cannot be used to compare a steel and aluminium (or platinum for that matter) Spade.

Geez I must be dumb. Please help me understand how the material from which an anchor is made influences the holding power.

A 20kg steel spade anchor could only be expected to have double the holding capacity of a 10kg steel spade anchor only because to weigh twice as much it needs to be roughly twice as big. My point exactly.

If the material is twice as heavy and the anchors are the same size/design, they will have the same holding power. The heavier one may dig better and perhaps will dig marginally deeper but better holding? I don’t think so.

[thinwater]

Originally Posted by CassidyNZ

Geez I must be dumb. Please help me understand how the material from which an anchor is made influences the holding power.

A 20kg steel spade anchor could only be expected to have double the holding capacity of a 10kg steel spade anchor only because to weigh twice as much it needs to be roughly twice as big. My point exactly.

If the material is twice as heavy and the anchors are the same size/design, they will have the same holding power. The heavier one may dig better and perhaps will dig marginally deeper but better holding? I don’t think so.

If an anchor is made of a weaker material it will need to be thicker and therefore have less fluke area for a given weight. True? For example, an anchor made of low strength steel will hold less than one made of high strength steel, because the high strength steel can be thinner for a given strength. Platinum (or gold) is rather weak.


A 10kg anchor is not nearly twice as big as a 20kg anchor because it is thicker. But it sets deeper (because of the holding capacity, not the weight) and solid density and strength increase with depth. Thus, even though the area is not double, it holds twice as much.

[Lodesman]

Originally Posted by Dockhead

Alain Fraisse gives exhaustive data and engineering calculations on catenary; check it out if you don't know this brilliant site: Tuning an Anchor Rode

Yes I'm aware of this site.

You'll see from this that the "bar taught chain" is not a myth; in fact there is a point, within the range of conditions which a normal cruising boat might encounter, where an anchor chain might get less energy absorption from catenary than from elasticity of the chain. That is "bar tight" indeed, just about literally speaking.

You'll also see that lifting the last link is something which WILL happen in some conditions with any realistic scope and weight of chain.

Agreed, but you will agree that this will not happen until conditions are extreme for most cruisers with substantial amounts of chain. I like how Alain's diagram below describes these states - the orange line being the point where the rode is away from the seabed, but it has not yet lifted the shank. To me planned scope should have accounted for the expected conditions, plus a "what if" factor, to not exceed this state. Once the shank is above horizontal, the hold of the anchor decreases. The graph on shank angle to hold give some idea of the percentage drop-off in anchor power but should not be considered to be exact - the actual results depend on the type of anchor, how deeply it was buried and the bottom composition. The point is that even after the chain is completely off the seabed and the anchor shank has started to come up, there is still a fair amount of catenary. But to someone on the bow the chain is "bar taut" long before they get to this condition. If we had enough chain out to require 4000 lbs of pull to completely lift it off the bottom, but looking at the chain between the fairlead and the chain stopper - if there was 500 lbs of pull against the anchor, it still would appear bar taut, and you wouldn't be able to deflect it measurably by stepping on it (unless of course your weight approaches 500 lbs ).





I don't understand this:

Quote:
Originally Posted by Lodesman
However, heavier chain would imply a shorter scope requirement.

Why? What difference would that make? Chain weight and scope requirement are totally unrelated in any way I can see.

If you look at Alain's formula:


(1.1)

This solves for the minimum rode to keep the anchor shank horizontal - w is the weight of chain per unit of length. You can see that as w increases the L goes down, and vice versa.


This is all incorrect. A certain force at a certain scope will always pull the chain completely off the seabed -- you can run the numbers yourself in Alain's calculators. It is true that there is always a slight curve, but the energy-absorbing capacity of a suspended chain disappears on an exponential curve, as the chain approaches straightness. So actually Dashew and Smith are correct, that this quality of anchor chain does disappear just when you need it most. And they are correct that no feasible chain weight and no feasible scope on a yacht-sized vessel can ensure that there is always chain on the bottom in heavy conditions.

You contradict yourself here. A certain force will pull a certain scope completely off the seabed. Given enough scope, there would not likely be enough force generated to lift the entire length of it off the seabed. Yes there is a limit to how much anchor chain you can carry - but for most of us fair-weather seeking sailors, we have a reasonable chance of ensuring the chain never gets to that point, except in truly exceptional circumstances. For those who cruise high latitudes, and anchor in much deeper anchorages, then there probably is a practical limit in their ability to ensure this safety factor. For them, perhaps heavier chain, longer chain or kellets might need to be considered.

But the good news is that modern anchors will tolerate a fair amount of "angulation" -- upward pull on the shank. According to Alain's calculations, a good modern anchor will still have about half of its theoretical holding power at 20 degrees of angulation. If the scope is at least 3:1, then angulation cannot exceed 20 degrees. So if your anchor is sized to have at least double the required holding power in the particular sea bed you are in, then you are good even at 3:1.

Yet another reason to oversize the anchor is to have the flexibility to work at short scopes if you need to.

Scope to lose half the holding power of your anchor, thus requiring an anchor twice the size - or increase the scope to maintain 100% of the anchor's power; two ways to achieve the same thing, so I suppose that comes down to the individual's anchoring philosophy.



P.S. If you have in mind that catenary never disappears enough to eliminate elasticity in the rode that there is always at least a little curve in it which will absorb energy -- I had the same thought, and posted on it here: https://www.cruisersforum.com/forums...in-215250.html. And was conclusively proven wrong.

I didn't take a fine-toothed comb to a 10-page thread looking for this conclusive proof, but I didn't see it in my cursory glance. Again I think this comes down to a way of thinking about it. The pull at the fairlead can be substantial, while there is still a significant amount of catenary. For example if the steady wind/sea is putting 3000 lbs of pull at the fairlead and the gusts spike that to 3500 or even 4000 lbs, then it's going to seem very abrupt and appear there's no spring left, but in fact it is still absorbing and releasing those loads.

[Lodesman]

Originally Posted by noelex 77

There have also been some formal anchor tests that have included the steel and aluminium versions of the Spade and these show the maximium holding power of the aluminium version is much lower than the steel version. For example, in the large independent 2009 multi magazine test, the holding in hard sand for the steel Spade was 1705 kg. This dropped to 1052 kg for the aluminium version. In soft substrates the difference in performance seems less marked.

The test can be read here: (Unfortunately this link is for the French version)

https://pdf.nauticexpo.fr/pdf/sea-te...130-15224.html

I have seen that article, and my impression was that they did not do a very good job of trying to explain that situation, perhaps because they seemed to have a bias against aluminum anchors in general. I was not left with the impression that multiple tests were done on each anchor for averaged results, but rather that it was a one-pull and done. That said, I buy the argument that weight can increase the dig-ability of the anchor, and also see why soft substrates would tend to eliminate that advantage.

[noelex 77]

Originally Posted by CassidyNZ

Geez I must be dumb. Please help me understand how the material from which an anchor is made influences the holding power.

A 20kg steel spade anchor could only be expected to have double the holding capacity of a 10kg steel spade anchor only because to weigh twice as much it needs to be roughly twice as big. My point exactly.

If the material is twice as heavy and the anchors are the same size/design, they will have the same holding power. The heavier one may dig better and perhaps will dig marginally deeper but better holding? I don’t think so.

I think you are misunderstanding the formula. It relates the holding power of different sized anchors that are made to the same design and material.

I assume from your post that you think it is obvious that a 20kg anchor would have double the holding of an otherwise identical (same design and material) 10kg anchor. Good, this is exactly what the formula indicates. The formula does not say anything about anchors that are the same physical size but different weights (because they are constructed from different materials).

If I understand correctly, you are suggesting a different rule that relates to anchors made of the same design and dimensions but different materials and therefore different weights. You feel these anchors should have the same holding power (perhaps because they have the same fluke size and shape). Personally I don’t think it is quite as simple as you are suggesting.

Certainly the experimental evidence does not always agree with your suggestion. Look at the results I referenced that tested a steel Spade (S80) with an identically sized (but different material and weight) aluminium Spade (A80). The holding of the steel Spade was much greater than the identically sized aluminium Spade. My own underwater observations as well as user reports on the steel and aluminium Spade anchors agree with this finding.

However, I think it is very dependent on the design of the anchor. Lightweight materials such aluminium and titanium can have some distinct advantages in certain parts of anchor construction. For example keeping the shank as light as possible helps most modern designs perform better. Fortress have shown it possible to use aluminium effectively and I hope we will see better use of this material in future. As well as producing lighter weight anchors, there is possibility of high performance designs with an emphasis on using lightweight materials in parts of the anchor with the goal of increasing the performance rather than the primary purpose of reducing the weight.

So lightweight materials can be useful. I think high performance, general purpose aluminium anchors will come along in the future perhaps with a design that was specifically developed for aluminium rather than just a steel anchor design that has been adapted. However, to assume that significantly changing the material and therefore the weight of the anchor as well as the thickness of components (affecting penetration) and balance will have no effect on performance or holding power is too simplistic. Tests and my underwater observations show the assumption is not accurate.

[Izikalvo]

Few reasons in having the right size and weight anchor,
On top of being easy to handle and having a smaller statue on your bow roller, assuming that the anchor has the right design, by not oversizing it, it will dig deeper, and by doing that its shank will be more protected against sideways forces, it will not stick out tripping its own chain when half set and it will have better chances of revolving in the same spot in case of pull direction change except 180 degrees sudden change.

Making the same good design and size anchor to be used with the same boat, not oversized, that will be heavier by using less tensiled steel and heaving thatת making the anchor thicker and therefore heavier to hold the stress of the holding will compromise the anchor ability to penetrate (thicker fluke, thicker shank) and would not give any other benefits except the price.

So oversizing is bad and making the same size of anchor thicker is bad, therefore heavy weighted anchor is bad.

For old designs its a different story.

[rslifkin]

In theory it's possible to oversize enough that you won't be able to set it properly. In reality, for most of us, that would require an anchor so big it wouldn't fit on the boat. There might be some exceptions with extremely underpowered boats though.

[Dockhead]

Originally Posted by Izikalvo

Few reasons in having the right size and weight anchor,
On top of being easy to handle and having a smaller statue on your bow roller, assuming that the anchor has the right design, by not oversizing it, it will dig deeper, and by doing that its shank will be more protected against sideways forces, it will not stick out tripping its own chain when half set and it will have better chances of revolving in the same spot in case of pull direction change except 180 degrees sudden change.

. . .

So oversizing is bad and making the same size of anchor thicker is bad, therefore heavy weighted anchor is bad.

For old designs its a different story.

There are a lot of assertions here:


". . .assuming that the anchor has the right design, by not oversizing it, it will dig deeper. . . " leading to this conclusion:


". . . So oversizing is bad"


There are a lot of problems with this syllogism:

Why would a smaller anchor "dig deeper" than a larger one? Never mind any actual evidence, what even theory is there to support this idea?



If this were true, then people would have difficulty anchoring with larger and so "shallower digging" anchors, and would see better performance with smaller, "deeper digging" ones. But they don't -- not a single case I ever heard of.

[a64pilot]

It does take more force to set my 40 KG Rocna over my 25KG Rocna, as in normal anchoring one could argue the bigger anchor isn’t fully set as in you may can see one edge of the fluke when the wind changes and pulls the anchor around, when the anchor gets pulled around it rolls a little, just as say a boat does in a turn, but let the wind get up and of course it gets pulled in deeper or better set if you will.
However even when lightly set as in I’m only stopping for awhile waiting on the tide to get through a pass the shank end is always in the bottom. So a chain can’t trip on it or get under it.
The only thing a chain might get caught on is the roll bar, but it’s such a round profile I doubt that it would.

However spend a day of two in 40kt gusts and even in heavy clay type of sand it will bury out of sight, roll bar under the sand completely. That’s when I sleep well, when you look and the chain seems to end with no anchor in sight.
Last night today and tonight, wind 25 to 35 with occasional gusts exceeding 50 kts. I have 7 to 1 out and it’s buried deep from two days worth of wind.

I’m just a 38” boat with an 88lb anchor, been so for a few years and never have I thought the anchor is too big. It’s bane though is grass, it will set well in grass and you can pull it as deep as the throttle can, but in a current reversal often times the anchor won’t drag around it comes completely out carrying a wheelbarrow load of grass and sand and will not reset, it just drags across the bottom with very little friction.
Normal sand is sort of like a very thick fluid, it moves around the anchor as it gets dug in deeper or repositions, the grass will hold this type of fluid into a solid mass, it won’t “flow” as the anchor resets, it will hold the anchor until enough force is applied then the anchor comes out with a big chunk of sand/grass.

[thinwater]

Originally Posted by Dockhead

...Why would a smaller anchor "dig deeper" than a larger one? Never mind any actual evidence, what even theory is there to support this idea?...

I suggest you try this in firm sand with, say a 2-pound Guardian and a 15-pound Guardian. At the same force, the large anchor will barely submerge the shank, while the small anchor may be 2-3 feet under ground and very difficult to recover. I've done just this in testing, many times. Didn't always enjoy it.



I parked the boat using a large (oversize) Fortress off the bow. I then pull smaller anchors to failure off the stern. The large Fortress stays on top, while some of the smaller anchors go quite deep.



That does not mean smaller anchors hold better in all ways. I'm not making that argument. I'm saying only that at the same setting force it is obvious that a smaller anchor will go deeper. That is what it must do to hold the same force. There is no other way.

[Dockhead]

Originally Posted by Lodesman

You contradict yourself here. A certain force will pull a certain scope completely off the seabed. Given enough scope, there would not likely be enough force generated to lift the entire length of it off the seabed. Yes there is a limit to how much anchor chain you can carry - but for most of us fair-weather seeking sailors, we have a reasonable chance of ensuring the chain never gets to that point, except in truly exceptional circumstances. For those who cruise high latitudes, and anchor in much deeper anchorages, then there probably is a practical limit in their ability to ensure this safety factor. For them, perhaps heavier chain, longer chain or kellets might need to be considered.

I was hoping that you would run the numbers and see for yourself. We discussed it in the old linked thread.


Playing with these numbers is very illustrative, and will show you that you objectively do not have a "reasonable chance of ensuring the chain never gets to that point". With 12mm chain, if you have 150 meters of it, and have all that chain out, then you can't pull the catenary out of it without breaking it. But let's take a more typical case -- a 36' boat using 60 meters of 1/4" chain, and displacing 8 tonnes.



This boat will be subjected to 762daN of force at 40 knots (per ABYC formula), and 429daN at 30 knots. But 60 meters of 1/4" chain in 6 meters of water needs only 218daN to lift the last link. That's only about 21 knots of wind.



Go up a size with your chain, to 5/16", helps this -- last link is lifted at 388daN instead of 218. But that's still not enough to keep the last link on the seabed even in 30 knots. Even 10mm chain will not keep the last link on the seabed in 40 knots, for a 36' boat using 60 meters of rode.


So you really can't count on just "putting out more chain" to solve all anchoring problems. This is an idea we've called "fetish for scope". You may not agree with Dashew and just give up and use lighter chain -- so, I don't -- but you still have to be ready, even if you're a "fair weather seeking" sailor, to anchor in a way that you are not risking your boat just because you can't keep the last link of chain on the bottom.


What that means is (a) a good enough and big enough anchor that a certain amount of angulation can be tolerated; and (b) a snubber, to absorb shock when catenary is no longer doing that.



You don't size an anchor to be barely adequate in perfect conditions -- prudent sailors, notwithstanding what some on here have said, oversize the anchor as much as is reasonably practical. It should not be 2x bigger than needed to give the exact holding power needed to prevent dragging, it should be 10x bigger if possible, even 20x. Every "x" is your margin of error for (a) non-ideal seabed, which can reduce holding force by 5x or more; (b) non-ideal angle of pull on the anchor, which WILL happen, when conditions get up to where the catenary gets pulled out of the chain; (c) non-ideal set, which can happen for a lot of different reasons. Murphy's Law states that these factor will tend to occur simultaneously.



Another reason for oversizing the anchor is a case which Dashew specifically writes about, and something I have, terrifyingly, experienced myself -- what if you are forced to anchor in short scope?


In Northeastern Greenland, the bottom slopes down incredibly, with depth of 1000 meters apparently sounded just a few cables from shore, in the back of Scorseby Sund. The bottom is rocky, the water is full of ice, and it is a hellish place to anchor. Your best bet is normally the DEEPEST place as far back in a cove as you can get, some place from which the bottom doesn't slope down anywhere, so that some silt accumulates to get your anchor into. This means anchoring in deep water, and close to the rocks. So the summer we were up there, we rarely anchored in less than 30 meters of water, and sometimes 40 or more, so we were often on as little as 2.5:1 of scope. We succeeded -- and weathered at anchor gales with gusts touching 50 -- only because the anchor was big enough to have enough excess holding power to still provide adequate holding power at that scope.



There is simply no such thing as too much holding power, and you never know when you'll need it, and that is why many of us use simply the largest anchor we can reasonably handle.

Originally Posted by Lodesman

I didn't take a fine-toothed comb to a 10-page thread looking for this conclusive proof, but I didn't see it in my cursory glance. Again I think this comes down to a way of thinking about it. The pull at the fairlead can be substantial, while there is still a significant amount of catenary. For example if the steady wind/sea is putting 3000 lbs of pull at the fairlead and the gusts spike that to 3500 or even 4000 lbs, then it's going to seem very abrupt and appear there's no spring left, but in fact it is still absorbing and releasing those loads.

If this were universally true, then we wouldn't need snubbers.



If you would look through the old thread -- someone somewhere (we have a lot of engineering talent on CF) reports on the force at which a given length of a given size of chain begins to behave like a bar -- so catenary giving less effect than elasticity of the steel. They are not such big numbers, in the case of moderate sized chains. And that's why we need snubbers. It is true that catenary ALWAYS works -- it would take an infinite force to make a piece of chain completely straight -- but the EFFECT of catenary vanishes.


This can be seen from this table from Peter Smith's site:






Even with heavy, 12mm chain in deep water, you can clearly see the bar-tight shape of the catenary.




Yet another reason to use a larger anchor, and a snubber, is soil mechanics. This post explains it: https://www.cruisersforum.com/forums...ml#post2853735


You want OVERKILL with your anchor for this reason, also -- repeated cyclical stress on soil causes it to lose its strength and fail. If your anchor is sized to give you barely enough holding power in that storm, it may pop out after a couple of hours of cyclical loads. The more oversized the anchor is, the less stress on the soil, and the less the risk of this kind of failure. This tracks real life experience.