Anchoring with 4:1 Scope?

[Dockhead]

Originally Posted by thinwater

Hmm....

In very soft mud, testing by several groups shows that the max reliable holding capacity in very soft mud is about 350 pounds for a 45# anchor. My 34' cat, that would ordinarily carry a 35# NG anchor (factory was 25# Delta) can have a storm load of 1100# in a squall with a 25' snubber (measured with load cell). That suggests an anchor of (1100/350)(45)2=283 pounds.

Good holding is about 10 times that, so a 35# anchor passes that test on sand. But poor holding bottoms don't yield to a simple plan.

So how do I avoid dragging? Shelter, to reduce the wind. There are some places you just can't anchor through a squall, and mostly, people don't anchor in the open around here. Use a Fortress sometimes, perhaps in a V, because it can hold 1100 pounds in that mud in a moderate size. But not at 3:1 either, and that is OK, because the water is probably shallow, so even 7:1 is not that much, like less than 70'.

Different ships, different long splices.

But that is exactly why I said "except in the poorest holding". There ARE times when you may need the max holding power of even an oversized anchor, and will need more than 3:1 scope, and there ARE some combinations of weather and bottom conditions where you can't even anchor at all -- as most of us know from experience.

But the point is that holding power varies so dramatically with bottom condition, that it would be a rare case that the 50% discount resulting from going from infinite scope to 3:1, will make the difference between holding or not holding. If you anchor is sized to just hold on long scope in soft mud in a given weather condition, for example, then you will have 700% of what yiou need in a good sandy bottom, so shortening up to 3:1 you still have 350% of what you need.


Anchor

I'm just trying to put all that in perspective. Some people, as I've written, fetishize scope, as if it were the only variable in anchoring. In fact scope, even within the range of 2.5:1 to infinite, is not at all the biggest variable. Much less within the range of say 4:1 to 10:1. If you are satisfied with a given anchor and 10:1, then all you have to do, for example, is oversize it by one size, and get equal performance at 3:1. Scope is not an absolute thing, contrary to what some people think.

[SaltyMetals]

This raises an interesting point as to why a vessel or superyacht which could be 100 x the weight & windage of my sailing boat does not have an anchor which is 100x larger. Is there a cube in the calc which explains this?
Andrew

[thinwater]

Originally Posted by SaltyMetals

This raises an interesting point as to why a vessel or superyacht which could be 100 x the weight & windage of my sailing boat does not have an anchor which is 100x larger. Is there a cube in the calc which explains this?
Andrew

a. A boat will not be 100x the windage and 100x the weight. It would only be 20x the windage if 100x the weight, and possibly much less. weight goes up slightly greater than cube of length, windage with the square, all things being equal, which they seldom are.

b. Waves are less of a contributor as size increases. They become smaller in proportion.


And the anchor likely will be >10x the weight of yours, if they plan on anchoring out.

[rslifkin]

Originally Posted by SaltyMetals

This raises an interesting point as to why a vessel or superyacht which could be 100 x the weight & windage of my sailing boat does not have an anchor which is 100x larger. Is there a cube in the calc which explains this?
Andrew

There's some indication that larger anchors work disproportionately better, at least in the 100+ lbs range.



In addition, larger boats are easier to anchor. They're heavier relative to their amount of windage, so they react less sharply to wind gusts, giving lower peak loads. They also gain less load from wave action as Thinwater mentioned.



Once you get to a certain size, the expectations of anchoring are also a bit different. The boat won't be left un-crewed at anchor, there will generally be someone on watch at all times. So the required safety factor is a little lower, as there's crew available to react if the anchor drags. That's not the case on many of our smaller boats.

[Dockhead]

Originally Posted by rslifkin

There's some indication that larger anchors work disproportionately better, at least in the 100+ lbs range.



In addition, larger boats are easier to anchor. They're heavier relative to their amount of windage, so they react less sharply to wind gusts, giving lower peak loads. They also gain less load from wave action as Thinwater mentioned.



Once you get to a certain size, the expectations of anchoring are also a bit different. The boat won't be left un-crewed at anchor, there will generally be someone on watch at all times. So the required safety factor is a little lower, as there's crew available to react if the anchor drags. That's not the case on many of our smaller boats.

It's popularly believed, including by me, that bigger anchors work disproportionately well, especially from 100 pounds, but I don't think it's been proven empirically.

But one thing for sure, the bigger the vessel, the lesser are the dynamic loads. Why you never saw a ship with a snubber.

[Alan Mighty]

Originally Posted by thinwater

a. A boat will not be 100x the windage and 100x the weight. It would only be 20x the windage if 100x the weight, and possibly much less. weight goes up slightly greater than cube of length, windage with the square, all things being equal, which they seldom are.

Given the so-called 'modern' or 'new generation' of drag-embedment anchors, what is the current expert thinking about the parameter(s) that most influence holding power.

By which I mean (if my understanding is correct):

* after the invention of stockless anchors, in 1907 (published in 1915) Parsons argued that the holding power of an anchor was proportional to the surface area of the flukes multiplied by the depth of embedment;

* after the invention of new anchors, in 1934 Land argued, based on work done by Leahy, Farrin, and others 1932-5, that the holding power was proportional to the shank length cubed

* after the CQR, Danforth, Stevin and other anchors, in 1975 Saurwalt argued that the holding power was proportional to the square root of the surface area of the fluke(s).

So now with Excel, Rocna, and a dozen others, does expert consensus exist as to what, in terms of dimension(s) or parameter(s), holding power relates?

[Panope]

Originally Posted by Alan Mighty

Given the so-called 'modern' or 'new generation' of drag-embedment anchors, what is the current expert thinking about the parameter(s) that most influence holding power.

By which I mean (if my understanding is correct):

* after the invention of stockless anchors, in 1907 (published in 1915) Parsons argued that the holding power of an anchor was proportional to the surface area of the flukes multiplied by the depth of embedment;

* after the invention of new anchors, in 1934 Land argued, based on work done by Leahy, Farrin, and others 1932-5, that the holding power was proportional to the shank length cubed

* after the CQR, Danforth, Stevin and other anchors, in 1975 Saurwalt argued that the holding power was proportional to the square root of the surface area of the fluke(s).

So now with Excel, Rocna, and a dozen others, does expert consensus exist as to what, in terms of dimension(s) or parameter(s), holding power relates?

No claim of expert status here, but I believe DEPTH OF BURY to be most important to holding power.

Steve

[Lodesman]

It seems obvious that fluke size is a factor, and intuitive that depth of bury is important insofar as it's a determinant of substrate density/resistance - which I think has an effect on ultimate holding strength. The only way I can see shank length having an effect is on the ease of levering the anchor out of the substrate when weighing it.

[s/v Jedi]

About big anchors: we have an original 176lb (80kg) Bruce which is very oversized for our boat. In hard sand, when I pull really hard with the engine in reverse, I can get one fluke buried and the anchor is sideways with the opposite fluke pointing straight up.

When we get a 30-35 knot squall, the anchor is still like that, even when the wind shifts 180 degrees.

After hurricane Isaias where we got periods of 70 kts wind and 48+ hours of 40+ kts winds, the center fluke had buried as well, and the anchor turned from sideways to hallway right side up.

After hurricane Ivan in Grenada (120 kts sustained wind), anchored in muddy clay, we labored for two days to get the anchor back up, which had buried about 10’ deep into the seabed.

The seabed is a huge variable. When we anchor in nice clear beach sand, our anchor buries textbook and the worst of squalls will only move it a couple inches while burying deeper. Here, it must hold you reliably at 3:1 scope. When seabed is less ideal, or weather conditions are worse, you increase scope to compensate.

[Don C L]

I'd put my money on Parsons. Steve, can you do the math on all the anchors for us?

[thinwater]

Originally Posted by Dockhead

It's popularly believed, including by me, that bigger anchors work disproportionately well, especially from 100 pounds, but I don't think it's been proven empirically...,

I think we all believe this, particularly in weed and cobbles. It feels right, to me too....



But I have tested anchors from 2-55 pounds (same model, several brands) in snad and mud, and the formula Hold = Mass^X always holds, with an exponent close to or slightly LESS than 1.



I've reviewed data from the oil platform and ship anchor makers; the exponent is generally between 0.8-0.92, depending on who you ask and what bottom.


I would love to see empirical data that showed otherwise. In fact, if you look at Panope's tests, the big anchors don't generally do that much better than much smaller anchors, certainly not better than Mass^0.9.


(BTW, all of the listed factors, certainly including depth of bury, are valid and included in this one relationship. It seem to just work out that way, when metal is made thicker to cope with stresses. It is NOT just mass, it is all of the things that go with it in an equivalent design.)



(The difference in performance between alloy and steel anchors is an interesting question. You would think, once the rode tension comes on, the mass of the anchor would make little difference. Yet it does matter, even when the difference is only a few percent of the total force picture. A small change in mass or just a few degrees in any angle can have a significant effect on bury depth.)


I have seen this testing (forget where) in cobbles. Some of it seems to be weight. However, much of it is that the fluke can get into the soil under the cobbles in larger sizes before the shank strike the surface of the cobbles.


Interesting topic.

[Bill_Giles]

More and heavier chain is useful.

[MathiasW]

Originally Posted by thinwater

I think we all believe this, particularly in weed and cobbles. It feels right, to me too....



But I have tested anchors from 2-55 pounds (same model, several brands) in snad and mud, and the formula Hold = Mass^X always holds, with an exponent close to or slightly LESS than 1.



I've reviewed data from the oil platform and ship anchor makers; the exponent is generally between 0.8-0.92, depending on who you ask and what bottom.


I would love to see empirical data that showed otherwise. In fact, if you look at Panope's tests, the big anchors don't generally do that much better than much smaller anchors, certainly not better than Mass^0.9.


(BTW, all of the listed factors, certainly including depth of bury, are valid and included in this one relationship. It seem to just work out that way, when metal is made thicker to cope with stresses. It is NOT just mass, it is all of the things that go with it in an equivalent design.)



(The difference in performance between alloy and steel anchors is an interesting question. You would think, once the rode tension comes on, the mass of the anchor would make little difference. Yet it does matter, even when the difference is only a few percent of the total force picture. A small change in mass or just a few degrees in any angle can have a significant effect on bury depth.)


I have seen this testing (forget where) in cobbles. Some of it seems to be weight. However, much of it is that the fluke can get into the soil under the cobbles in larger sizes before the shank strike the surface of the cobbles.


Interesting topic.

Thanks for sharing this! Experimental data always wins...

Naively, I would have assumed the exponent X to be even smaller, the argument being that it is the surface of the anchor that interacts with the sea bed, and so holding forces should be proportional to that surface. Now, if the anchor design is fixed and larger anchors are just scaled up versions of smaller anchors, then, when the anchor doubles in length, its weight would go up with a factor of 8, but its surface only up by a factor of 4. From this consideration one would thus expect X = (2/3).

Clearly, the experimental data point at other factors being very important. It seems reasonable to me that one of these factors is that the seabed gets more solid as one digs deeper into it. If that is the case, the scaling approach from above gets flawed and one would indeed expect an exponent larger than 2/3.

My 2/3 cent...

[Panope]

Originally Posted by thinwater

..........(The difference in performance between alloy and steel anchors is an interesting question. You would think, once the rode tension comes on, the mass of the anchor would make little difference. Yet it does matter, even when the difference is only a few percent of the total force picture. A small change in mass or just a few degrees in any angle can have a significant effect on bury depth.)...............

I have been wondering if the difference in FRICTION between aluminum and steel makes a difference. If, repeat, IF the seabed "sticks" to aluminum more readily than steel, then this will likely cause a reduction in holding power (in sea beds like the "sandy mud" that is common to my area).

Steve

[noelex 77]

Originally Posted by thinwater

I think we all believe this, particularly in weed and cobbles. It feels right, to me too....

But I have tested anchors from 2-55 pounds (same model, several brands) in snad and mud, and the formula Hold = Mass^X always holds, with an exponent close to or slightly LESS than 1.

I've reviewed data from the oil platform and ship anchor makers; the exponent is generally between 0.8-0.92, depending on who you ask and what bottom.

I would love to see empirical data that showed otherwise.

Here are a few of the articles written by Professor Knox. He had a refreshing scientific approach to anchor testing. His contributions will be missed. These are some of his published works.

His anchor testing produced a significantly greater than 1.0 ratio for modern anchors. For example, the ratio for the Spade was 1.4, the Manson Supreme was 1.5 and the Rocna was 1.9. The ratio for older anchors such as the Delta and Bruce was lower but still above 1.0. He suggested an overall ratio of 1.0, which of course is a simple ratio to remember and understand (doubling the anchor weight will double the holding capacity).


http://www.spade-anchor.com/IMG/pdf/...CAL_BOAT_OWNER.


https://static1.squarespace.com/stat...ticle+2012.pdf


http://www.stfeurope.com/pdf/Practic...Owner-2002.PDF

My own impression is that in most good substrates, 1.0 is close enough given the other many variables. However, in some difficult substrates, especially weed, larger anchors do significantly better. Here I think the ratio is well above 1.0 (therefore a 40kg anchor would have more than double the hold of a 20kg anchor).