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

[wsmurdoch]

Originally Posted by valhalla360

I did a quick and dirty using my traffic signal cantenary spreadsheet with bow roller to bottom of 33' and swing radius of 100' and and 1000lb horizontal force on the boat. The connection to the anchor is angling up at almost 45 degrees (visually assessed as my spreadsheet doesn't directly calculate it). Even down to 250lb of horizontal force, there was a substantial angle at the anchor shank.

45 degrees sounds high. arctan(33/100)=18 degrees.

You might want to try Alain's static spreadsheet http://alain.fraysse.free.fr/sail/ro...tic/static.htm from his site http://alain.fraysse.free.fr/sail/rode/rode.htm

Bill

[valhalla360]

Originally Posted by wsmurdoch

45 degrees sounds high. arctan(33/100)=18 degrees.

You are correct...I didn't adjust the graph to be to scale in both vertical and horizontal so the visual was way off.

Went back and pulled the underlying data for the last link and did the arctan...16.7 degrees...which is still pretty steep.

[Dockhead]

Originally Posted by valhalla360

I guess, the issue I see with this "thought experiment" is it can get a little misleading for typical cruisers...sure there are the rare cruisers who have 150m of chain and regularly anchor in 30-50m of water.

Most don't have nearly that much chain (I would bet 50-75m is more typical with rope used for rare extreme depths) and generally try to anchor in less than 10m of water. If these more typical cruisers don't grasp this key point, they may walk away thinking cantenary going "bar tight" and lifting the anchor shank vertically won't be an issue but in reality it very well may be in most common anchoring situations.

I did a quick and dirty using my traffic signal cantenary spreadsheet with bow roller to bottom of 33' and swing radius of 100' and and 1000lb horizontal force on the boat. The connection to the anchor is angling up at almost 45 degrees (visually assessed as my spreadsheet doesn't directly calculate it). Even down to 250lb of horizontal force, there was a substantial angle at the anchor shank.

Well, have a look at these tools: Tuning an Anchor Rode


You won't need to use a traffic signal catenary calculator.



Don't forget that once catenary is gone, the angle on the anchor shaft (called "angulation") will follow normal geometry rules, so:


3:1 19.47 degrees
4:1 14.48 degrees
5:1 11.54 degrees
6:1 9.59 degrees
7:1 8.21 degrees
8:1 7.18 degrees
10:1 5.74 degrees




So you won't get "lifting up the anchor shaft vertically" until you get to much shorter scopes than 3:1. A 45 degree angle on the anchor shaft appears at 1.41:1 scope without catenary.

[Dockhead]

Originally Posted by valhalla360

You are correct...I didn't adjust the graph to be to scale in both vertical and horizontal so the visual was way off.

Went back and pulled the underlying data for the last link and did the arctan...16.7 degrees...which is still pretty steep.

See post 89 in this thread, for some data about the relationship between angulation and holding power:


http://www.cruisersforum.com/forums/...-215250-6.html




16.7 degrees (3.5:1 scope without catenary) is only 7 degrees or so more angulation than you get at 6:1 without catenary. Not a huge difference in holding power.

[Juho]

I made a small experiment to study the whipping behaviour. I tied a loose bowline on the table leg. Then I tugged the line (that was partially catenary, partially lying on the floor) lightly, horizontally. It was quite easy to make the knot jump upwards. I think this is an example of a whiplash effect in an anchor chain. Well, except that when we move to boat scale, and real life wind and wave action, where the chain is in (viscous) water, maybe one can not reproduce any such whiplash effects. My guess is anyway, that unlike in my miniature model, in real life anchoring this probably is not a problem that one should prepare for.

[groundtackle]

Originally Posted by thinwater

He brings up a point that you seldom hear. As you approach the WLL of the chain, the chain stretch become an important portion of the equation. I've run the numbers and measured it. Obviously, this is more true of high grade chain than BBB, since strain is proportional to load, not % WLL. This is just one of the reasons that transport chain must be grade 70 or higher.

Interesting. See my posts #39 & #47. When measuring what did you observe?
Chris

[transmitterdan]

Chain on the bottom is producing resistance. If you don’t believe it try dragging 25M of 5/16 HT chain along the beach.

[Dockhead]

Originally Posted by transmitterdan

Chain on the bottom is producing resistance. If you don’t believe it try dragging 25M of 5/16 HT chain along the beach.

Naturally it does. But it is a very small force compared to an anchor. Try dragging a 25kg Rocna set in the beach, or even a 10kg one.

[evm1024]

I did some quick math for the elongation of G43 (astm1022) anchor chain.

With 165 feet (50m) of chain out loaded to 2000 # I get 1.13" of elastic stretch.

(2 * 165) / (0.17 * 23000) = 0.0844' = 1.128"

If I did the math correctly (and the constants were accurate) then if we are getting more stretch than that by measurment then it must be structural stretch. The links fitting together more tightly.

[nmccubbin]

Our worst experience was on our old 40 foot boat in the Alligator River.
Blew about 50 knots, and the chain was definitely bar-tight, making each small wave hit the anchor hard. We had 180 feet of 7/16" chain in under 12 feet water. That is 15 to 1 scope.
Our 2 foot long nylon "snubber" (really an attempt to avoid load on the windlass) snapped at a point with no chafe. 4500 lb breaking strain rope. We dragged a bit but had plenty room

A few years later, same boat, we anchored in St Martin's Simpson's lagoon through two hurricanes, one a cat 4 We had 90 ft 7/16 chain with a 90 ft 7/8" nylon snubber. We could see it stretch, but the boat did not drag while many around us did. Of course the rope felt bar-tight.

[transmitterdan]

Originally Posted by Dockhead

Naturally it does. But it is a very small force compared to an anchor. Try dragging a 25kg Rocna set in the beach, or even a 10kg one.

Small compared to a good anchor. Try dragging a 45 lb CQR on hard sand. It can be done by a couple strong guys. When we chartered in BVI the nice guy doing our checkout sail gave strict warning. “At anchor put out all the chain you can. The chain has more holding power than the 45 lb CQR at the end.” He was right.

[Dockhead]

Originally Posted by transmitterdan

Small compared to a good anchor. Try dragging a 45 lb CQR on hard sand. It can be done by a couple strong guys. When we chartered in BVI the nice guy doing our checkout sail gave strict warning. “At anchor put out all the chain you can. The chain has more holding power than the 45 lb CQR at the end.” He was right.

Ha, ha. OK, fair enough.


I'm an official member of the CQR Survivors Club, so can't argue with that

[valhalla360]

Originally Posted by Dockhead

16.7 degrees (3.5:1 scope without catenary) is only 7 degrees or so more angulation than you get at 6:1 without catenary. Not a huge difference in holding power.

7 degrees is a big increase if you are starting at 9.8 degrees.

If you look at the upward force vector on the anchor shank, for the same tension, it will be almost double.

[groundtackle]

Originally Posted by evm1024

I did some quick math for the elongation of G43 (astm1022) anchor chain.

With 165 feet (50m) of chain out loaded to 2000 # I get 1.13" of elastic stretch.

(2 * 165) / (0.17 * 23000) = 0.0844' = 1.128"

If I did the math correctly (and the constants were accurate) then if we are getting more stretch than that by measurment then it must be structural stretch. The links fitting together more tightly.

I'm struggling with this math. I see 15% elongation vs UBL, its regulated. Not inches but feet per hundred. 1 1/8" per 100' ?
Please correct me, I don't understand.
Chris

[Dockhead]

Originally Posted by valhalla360

7 degrees is a big increase if you are starting at 9.8 degrees.

If you look at the upward force vector on the anchor shank, for the same tension, it will be almost double.

Sure, but "double", "half", or whatever, is not very meaningful without knowing how much you are starting with. Double of diddly squat is still diddly squat, for example Double of "some" is a bit more



You don't get double the holding power at 6:1, compared to 3.5:1. It's very roughly a 35% increase, depending, of course, on the anchor.