Possibly Original Thought About Chain Catenary - or - The Myth of the Bar Tight Chain

[Panope]

How much energy is absorbed by the change of the rotation of the earth?

Steve

[conachair]

Originally Posted by Dockhead

Exactly. That's exactly what I was confusing :headbang:

Worth bearing in mind the boat speed in the bigger picture as well imho. It's four times harder to stop a boat going twice as fast.

[conachair]

Originally Posted by Panope

How much energy is absorbed by the change of the rotation of the earth?

Steve

Doesn't the moon get some rotational energy from the earth as the friction of the tides slows the earth rotation down? Or something like that.

[evm1024]

If we assume that we all do our "work" in a 10 mile thick layer of air and water then if we look at a globe of the earth 12" (30 cm) in diameter....


.... Then that 10 mile thick zone would be 0.015" (0.38 mm) thick.

So we humans from the tops of the mountains to to depths of the oceans occupy the paint on the globe.

[Delfin]

Originally Posted by Dockhead

OK, thanks. I think I got that somewhere up the thread, in fact I knew it before, but this is a nice explanation. When catenary doesn't materially change the angle at the bow and anchor, and when it no longer absorbs energy materially, I think we can say that it's "bar tight".



The original idea of this thread was just that perhaps that happens later than we think, and that in some cases might happen later than the breaking strength of the chain. This idea of mine was BUSTED pretty quickly, for any normal size and quantity of chain we might carry on cruising boats.

Your puzzle was pretty interesting, so I dove in with a response before reading every post of the thread, so apologies for repeating what others already noted.

However, this got me thinking about a solution to the dreaded discussion of the importance of scope. The shape of an anchor chain, when viewed fractally, can be described as a Koch Snowflake or Curve, which describes something with finite boundaries, but infinite length. Therefore, when someone insists that more scope when anchoring means you are a savvy mariner, and less scope means you are a dangerous fool, you can simply tell them that you always anchor with infinite scope, which at the fractal level, you do.

https://en.wikipedia.org/wiki/Koch_snowflake

[Dockhead]

Originally Posted by Delfin

. . .However, this got me thinking about a solution to the dreaded discussion of the importance of scope. The shape of an anchor chain, when viewed fractally, can be described as a Koch Snowflake or Curve, which describes something with finite boundaries, but infinite length. Therefore, when someone insists that more scope when anchoring means you are a savvy mariner, and less scope means you are a dangerous fool, you can simply tell them that you always anchor with infinite scope, which at the fractal level, you do. . . .

. . . .

[jhulmer]

Originally Posted by Panope

How much energy is absorbed by the change of the rotation of the earth?

Steve

Originally Posted by conachair

Doesn't the moon get some rotational energy from the earth as the friction of the tides slows the earth rotation down? Or something like that.

Originally Posted by evm1024

If we assume that we all do our "work" in a 10 mile thick layer of air and water then if we look at a globe of the earth 12" (30 cm) in diameter....


.... Then that 10 mile thick zone would be 0.015" (0.38 mm) thick.

So we humans from the tops of the mountains to to depths of the oceans occupy the paint on the globe.

I have to chuckle. You guys were on the bloody Mary's early this morning!

[Pelagic]

Originally Posted by conachair

None goes to the anchor if it's a biggie and doesn't move. You need force and distance, just force doesn't transfer energy. :

But in reality, the anchor Does Move. It digs in deeper and deeper which we all feel when trying to break out the anchor after a really heavy blow.

Perhaps only a mm at a time when the boat comes hard up and bar tight, but in big ships, that controlled drag in a typhoon is all about where you are positioning you ship prior to storms arrival.

[conachair]

Originally Posted by Pelagic

But in reality, the anchor Does Move. It digs in deeper and deeper which we all feel when trying to break out the anchor after a really heavy blow.

Yes, of course, everything affects everything else even if just a little. But if you want to get a handle on how one part of a system behaves often it helps to just ignore the other bits and look at that part in isolation. I've no idea about soil mechanics, does seem that it can be quite complex.

[transmitterdan]

Soil mechanics are totally relevant too. If you keep compressing soil/sand over and over its resistance gradually decreases until suddenly it “fails”. Maybe the first time this was observed as a serious problem was during an earthquake when a building fell not because the structure was poor but because the soil under the foundation failed after the building swayed several times. The oscillation compressed and released the soil many times and eventually the soil turned to “jelly” and the building fell.

In a big blow I think something like this is possible as these new anchors compress the soil over and over.

As an undergrad I worked in a soil mechanics lab. We had an “earthquake” machine that compressed soil over and over. Each compression we measured the resistive force and plotted it vs. displacement. After several cycles the resistance dropped by about an order of magnitude. We controlled the surrounding “soil” with hydraulic pressure so we could isolate the test sample. We cataloged many soil types. Some lasted longer than others. But we could make any soil composition fail with enough force and cycles.

[Dockhead]

Originally Posted by transmitterdan

Soil mechanics are totally relevant too. If you keep compressing soil/sand over and over its resistance gradually decreases until suddenly it “fails”. Maybe the first time this was observed as a serious problem was during an earthquake when a building fell not because the structure was poor but because the soil under the foundation failed after the building swayed several times. The oscillation compressed and released the soil many times and eventually the soil turned to “jelly” and the building fell.

In a big blow I think something like this is possible as these new anchors compress the soil over and over.

As an undergrad I worked in a soil mechanics lab. We had an “earthquake” machine that compressed soil over and over. Each compression we measured the resistive force and plotted it vs. displacement. After several cycles the resistance dropped by about an order of magnitude. We controlled the surrounding “soil” with hydraulic pressure so we could isolate the test sample. We cataloged many soil types. Some lasted longer than others. But we could make any soil composition fail with enough force and cycles.

Yet another reason to oversize the anchor - less stress on the soil.

Incidentally, I've been puzzling over why holding seems so much better in deep water - I wonder if being compressed under deep water affects the soil mechanics? That's a whole lot of pressure at 30 meters. What do you think?

[transmitterdan]

The extra pressure at 30M (100 feet) is about 3 bars (~50psi). While not insignificant I guess it doesn’t make a huge difference just on that factor. But it is likely that the bottom is more firm due to having been less disturbed by waves and having that pressure for millennia.

[Kelkara]

Originally Posted by Dockhead

I wonder if being compressed under deep water affects the soil mechanics? That's a whole lot of pressure at 30 meters. What do you think?

The water pressure makes no difference ... sediments at the bottom of the deep ocean kms deep are just a soupy ooze, with little compaction.



To compact sediment you need a differential pressure between the pressure in the sediment grains and the pressure of the fluid in the pore spaces ... or to put it simply, you need something to squeeze the water out from between the grains ... the weight of the sediment column can do this (as can waves and current action) ... which is why the sediment becomes more compacted as you go deeper below the sediment surface, but not with water depth.

[Juho]

Originally Posted by Dockhead

Incidentally, I've been puzzling over why holding seems so much better in deep water - I wonder if being compressed under deep water affects the soil mechanics? That's a whole lot of pressure at 30 meters. What do you think?

The water pressure might not make much difference. The first thought in my mind, when wondering why the anchor might hold better in deep water, is that the long rode, with its catenary and long distance in water, is likely to make the jerking movements, caused by the boat, softer at the anchor end of the rode.

[Dockhead]

Originally Posted by Juho

The water pressure might not make much difference. The first thought in my mind, when wondering why the anchor might hold better in deep water, is that the long rode, with its catenary and long distance in water, is likely to make the jerking movements, caused by the boat, softer at the anchor end of the rode.

Maybe. I just don't understand it.


ALL of the experts comment on less scope being required in deep water, and I (who anchor a lot in deep water, last summer hardly ever in less than 30 meters) have noticed it concretely and practically.


As I've spent a lot of time over the last 6 years in rocky places with steep-to shores, I've ended up anchoring more and more in deep places -- as that is often where you find a bit of flat bottom with good sediment. Anchoring in the Swedish and Finnish archipelagos is very challenging; a challenge thank God moderated by the almost completely absence of really bad weather in the summer, and an abundance of great shelter because of all of the islands. Iceland and Greenland, on the other hand, were the hardest -- little shelter, and the shores are so steep in many places that you have to get within a few boatlengths of the actual shore, to find a depth where your anchor will even reach the bottom. :shudder: