Fortress scaling plot

[thinwater]

This is a good point. It is easy to underestimate the effect of the slope of the seabed on the holding ability of an anchor. One simple way is to calculate the "effective scope".

The maths is quite easy. Below is a typical example with an anchor dropped on a section of seabed sloping at 5.7 degrees (10m drop over 100m) with a nominal scope of 5:1.

The effective scope varies from 10 :1 to 3.4 :1 depending on the direction of pull.

View attachment 183283

This is an interesting topic, and I am going to start a new thread for it. When I started sailing, as a new engineer, I understood the above calculation, found it obvious and reassuring. Most sailors do. But after years of engineering and sailing, I think it sometimes simulates the situation, but I'm going to have to offer my apology for this. I think it misses the real point entirely and is most often not the important factor.

The real problem with slopes is not their effect on scope at all, but

• why they exist
• their effect on soil shear strength

I would really love for a real soil expert to weigh in. I have some experience related to engineering foundations from mountain (rock but also horrible moraine fields), to Peidmont (typically good strong clay), to the Mississippi delta (very soft--even the sand flows like water). Mud, clay, and sand can vary by multiples in strength, depending on micro structure, how they were laid down, and chemistry. The topic is way more complicated than anchors.

---

First, unless it is a severe slope (channel edge), the slope is typically quite small. Perhaps 3 feet in 200 feet, or 1.5%. At 5:1 scope this is less than 10% of the angle, and at 10: scope the chain is probably staying on the bottom (zero bottom angle) both ways. Not a zero effect, but minor.

Let's imagine an extreme case. First, a field of level fine gravel. Place an anchor at infinite scope and it digs. Then place the same anchor on the same gravel, but on a pile that is at its angle of repose (won't get steeper without sliding). Pull the anchor down slope with infinite scope, and it holds nothing at all. The gravel just slides. The rode angle did not change. The material did not change. But the down slope holding is nothing, no matter how much scope. For the same reason you cannot climb up the pile. If you then drag the anchor uphill (same rode angle) you get a very different result. It holds. But it was not the rode angle that mattered.

Why is there a steep slope?

• Dredging nearby. Probably a bad place to anchor because of traffic.
• Strong tide. Often this makes the bottom very hard because anything loose has been scoured away. I know some slopes in the Chesapeake like this; good fishing but terrible anchoring. They're not even steep, just mudstone and super hard clay.
• Outflow "pile" from a river. The sediment may be fresh and poorly consolidated. It may also be near its angle of repose.

Angle of repose. For angular (crushed stone) gravel this is pretty steep, maybe 35-45 degrees. Natural gravel, perhaps more like 30-35 degrees. Same with sand. Bank sand maybe 35 degrees, wet bank sand maybe 25 degrees. But what about river sand, such as found in the Mississippi basin and used for proppant in fracing? The grains are round and the angle of repose is less than 10 degrees wet. My first introduction was a construction project in New Orleans. They brought it what is called pump sand. One day it was in a pile, it rained, and it spread as thought it was cake batter, covering the lot. Lesson learned. sand is not all alike.

[Notice how smooth the proppant sand is at top. No sheer strength. Crappy for anchoring, but easy to pump down fraced wells.]


Coral sand. Not great because it is so light.

Angles make it strong. Good for foundations. Fine angular sand is the stuff anchors makers dream of.

So what if the slope has an angle of 1:30 and the angle of repose of such sand is 1:20? The slope is not geometrically important, but the slope is close to landslide geometry and can hold very little without sheering out.

I don't think it is the maths of a slope. I think it is

• why the slope exists. Tidal souring or outflow.
• The type of soil, its angle of repose, and other factors.

Discuss. Probably way too complex to define. But I think the effect on rode angle is not the main thing in most cases. The math is reassuring (I thought so for years), and it's not wrong, but it's not the main thing in many cases. It's more complicated.

 

[Tranona]

Thanks for that. It is indeed very different from the anchoring that I do. But look on the bright side, you do have certain advantages:

You know that people habitually anchor there, so you can assume that the seabed is fairly forgiving, or nobody would anchor there.
Your boat is not subject to yawing, and so the direction of pull on the anchor is pretty constant.
You have no tide to worry about, either in terms of a varying depth, or a change in the direction of current.
The fact that the seabed shelves up towards the shore is a benefit. Anchors are much less liable to drag uphill.
Presumably you choose to anchor so that you can "spend the night moored up in front of the taverna".

Each to their own. Our anchoring is more like wild camping, while yours is more like being in a camp site. "Vive la difference"

You asked about different techniques, not about whether one was more difficult than another. Your experience and technique is no different from the tens of thousands of people who anchor their boats in similar situations. However it is disingenuous to claim that it is in any way more difficult or more demanding and try and belittle the challenges other people face. Coming to the conclusion that maybe it is not only the anchor on your good ship that is ignorant.

 

[NormanS]

You asked about different techniques, not about whether one was more difficult than another. Your experience and technique is no different from the tens of thousands of people who anchor their boats in similar situations. However it is disingenuous to claim that it is in any way more difficult or more demanding and try and belittle the challenges other people face. Coming to the conclusion that maybe it is not only the anchor on your good ship that is ignorant.

I'm not at all sure why you're coming across so argumentatively. Nowhere have I even suggested that my anchoring or that of "tens of thousands of people who anchor their boats in similar situations" is more difficult or more demanding. Nowhere have I tried to belittle the challenges that other people face. Please do not put words in my mouth, and please do not misquote me.
I am merely trying to get more information. You suggested that "some people" need to get out more, implying a lack of experience, and that there were (unspecified) techniques, apparently known to you, which might help others when anchoring.
To say, "maybe it is not only the anchor on your good ship that is ignorant", is childish and unbecoming.

 

[boomerangben]

This is an interesting topic, and I am going to start a new thread for it. When I started sailing, as a new engineer, I understood the above calculation, found it obvious and reassuring. Most sailors do. But after years of engineering and sailing, I think it sometimes simulates the situation, but I'm going to have to offer my apology for this. I think it misses the real point entirely and is most often not the important factor.

The real problem with slopes is not their effect on scope at all, but

• why they exist
• their effect on soil shear strength

I would really love for a real soil expert to weigh in. I have some experience related to engineering foundations from mountain (rock but also horrible moraine fields), to Peidmont (typically good strong clay), to the Mississippi delta (very soft--even the sand flows like water). Mud, clay, and sand can vary by multiples in strength, depending on micro structure, how they were laid down, and chemistry. The topic is way more complicated than anchors.

---

First, unless it is a severe slope (channel edge), the slope is typically quite small. Perhaps 3 feet in 200 feet, or 1.5%. At 5:1 scope this is less than 10% of the angle, and at 10: scope the chain is probably staying on the bottom (zero bottom angle) both ways. Not a zero effect, but minor.

Let's imagine an extreme case. First, a field of level fine gravel. Place an anchor at infinite scope and it digs. Then place the same anchor on the same gravel, but on a pile that is at its angle of repose (won't get steeper without sliding). Pull the anchor down slope with infinite scope, and it holds nothing at all. The gravel just slides. The rode angle did not change. The material did not change. But the down slope holding is nothing, no matter how much scope. For the same reason you cannot climb up the pile. If you then drag the anchor uphill (same rode angle) you get a very different result. It holds. But it was not the rode angle that mattered.

Why is there a steep slope?

• Dredging nearby. Probably a bad place to anchor because of traffic.
• Strong tide. Often this makes the bottom very hard because anything loose has been scoured away. I know some slopes in the Chesapeake like this; good fishing but terrible anchoring. They're not even steep, just mudstone and super hard clay.
• Outflow "pile" from a river. The sediment may be fresh and poorly consolidated. It may also be near its angle of repose.

Angle of repose. For angular (crushed stone) gravel this is pretty steep, maybe 35-45 degrees. Natural gravel, perhaps more like 30-35 degrees. Same with sand. Bank sand maybe 35 degrees, wet bank sand maybe 25 degrees. But what about river sand, such as found in the Mississippi basin and used for proppant in fracing? The grains are round and the angle of repose is less than 10 degrees wet. My first introduction was a construction project in New Orleans. They brought it what is called pump sand. One day it was in a pile, it rained, and it spread as thought it was cake batter, covering the lot. Lesson learned. sand is not all alike.

[Notice how smooth the proppant sand is at top. No sheer strength. Crappy for anchoring, but easy to pump down fraced wells.]

View attachment 183294
Coral sand. Not great because it is so light.

Angles make it strong. Good for foundations. Fine angular sand is the stuff anchors makers dream of.

So what if the slope has an angle of 1:30 and the angle of repose of such sand is 1:20? The slope is not geometrically important, but the slope is close to landslide geometry and can hold very little without sheering out.

I don't think it is the maths of a slope. I think it is

• why the slope exists. Tidal souring or outflow.
• The type of soil, its angle of repose, and other factors.

Discuss. Probably way too complex to define. But I think the effect on rode angle is not the main thing in most cases. The math is reassuring (I thought so for years), and it's not wrong, but it's not the main thing in many cases. It's more complicated.

I agree. Anchoring is everything to do soil structure, which might explain the variance in anchor test results and forumite opinions!!. Anchor makers no doubt understand (either empirically or theoretically or a mixture of both) the geometry required of anchor and scope to harness the property of soils. Then manage the compromised anchor design to accommodate differing soil properties. A fascinating subject about which I know almost nothing!!