SailNet Community

SailNet Community (http://www.sailnet.com/forums/)
-   General Discussion (sailing related) (http://www.sailnet.com/forums/general-discussion-sailing-related/)
-   -   Caternary & Chain... (http://www.sailnet.com/forums/general-discussion-sailing-related/105387-caternary-chain.html)

Maine Sail 11-07-2013 09:16 AM

Caternary & Chain...
 
Just read an interesting piece on anchor snubbers in the latest Practical Sailor. What really jumped out at me was not about snubbers but rather just how little load was required to lift the last link of chain off the bottom of the ocean and affect the anchor shank angle.

If the chain is laying on the bottom, as people often assume it is, the anchor shank is parallel to the bottom or in optimum holding orientation. When the last link of chain lifts it can change shank angle to match that of the scope being used. If on short scope.......:confused:

PS used a 100' section of 5/16" chain set to a 5:1 scope to test how much load was needed to lift the last link off the ground..

To apply the load they used a chain-hoist / come-along and measured it with a calibrated load cell. They physically pulled the chain to see how much load was required to lift the last link of chain eg: the anchor shank, off the bottom.

The shocker for me was that it took just 190 pounds to lift 100' of 5/16" chain at a 5:1 scope off the ground out of the water. The in water load calculation to do the same, when taking the density of the chain into consideration, would be just 158 pounds to lift the last link off the bottom...

Seeing as I own a digital load cell and have physically measured the loads of our 36' sloop at 140 - 218 (218 was peak loads) pounds of load in 17-19 knots the idea of chain holding your anchor on the bottom is really considerably less than where I and many books and experts suggest it would be....? Just 17-19 knots on our boat is enough to lift our chain at 5:1 !!!!!:eek: 17-19 knots is not even a stiff breeze.......

I actually just ran the numbers through an anchor load calculator and it does not show the chain lifting until a load applied of 242 pounds yet based on the PS actual test data it takes just 158 pounds of load to do this... Does this mean that all the theoretical data we've been using for years has been skewed???? It would be one thing if it was skewed in the safer direction but it has been skewed to the unsafe direction....

This means the wind conditions to affect anchor shank angle, with all chain, seem to be considerably lower than originally thought...

So if I am anchoring at 5:1 with 100' of 5/16" chain in approx 17' of water it will take just 17-20 knots, on our boat, based on actual measurements, to begin to lift the chain so the anchor shank angle is affected.

Interesting stuff to say the least and certainly some interesting data to ponder........

Quote: Practical Sailor

"We fixed one end and then tensioned the chain with a come-along until the last links at the lower end had lifted free of the ground. Lifting this required a load of 190 pounds, which translates to 158 pounds in the water. Based on data from last year’s test (PS, May 2012), this would be the equivalent of about 15 knots of wind on a 40-foot boat anchored in about 15 feet of water with 100 feet of 5/16-inch chain."


We've all heard the old axiom that 5:1 on chain is okay but 7:1 on rope/chain rode is needed. With this physically tested & measured data it appears that this is simply not entirely true..

It seems prudent that scope should be set irrespective of all chain or rope/chain rode because chain alone really will not help shank angle when you really need it most, in high winds...??

manatee 11-07-2013 09:37 AM

Re: Caternary & Chain...
 
Now we know what doesn't work.

What *does*?

Kellet?
Heavier chain?
More scope?
Better anchor?
Longer snubber?
All the above?

Maine Sail 11-07-2013 10:07 AM

Re: Caternary & Chain...
 
Quote:

Originally Posted by manatee (Post 1116052)
Now we know what doesn't work.

What *does*?

Kellet?
Heavier chain?
More scope?
Better anchor?
Longer snubber?

All the above?

I think the bolded items will be the most beneficial. A kellet can help prevent sailing at anchor etc. but in a storm the only things that will reasonably work IMHO are:

*Buy the best performing anchor your money can buy (there are lots of great anchors out there today compared to just 10 years ago)

*Use proper setting technique

*Know your bottom and choose the correct ground tackle

*Be prepared to quickly deploy back up tackle

*Use an anchor alarm

* Use GPS cookie trails as PROOF you stayed put when that other clown hits you and then tries to claim it was you who dragged!! (never know when you might need them for insurance purposes) Our GPS is ALWAYS on & laying trails when at anchor!!!

*Use proper calculations for scope and don't forget tides and bow height

*Check your set/setting technique with a strong power set by backing down at full throttle. If your sailboats engine can drag or un-set your anchor, by backing down hard, YOU ARE NOT SET....

*Recheck/power-set again before you go to bed

*If the wind shifts power-set in the new direction to re-test your new set direction. Some anchors are very poor re-setters..

*Use as much scope as you can for the swing room you have

*If you don't feel comfortable with how much scope you can lay, a kellet can certainly help, in moderate winds.

*Get where you are going earlier in the day not later.....

*If you don't feel comfortable with the scope you can lay & swing to, move on......

chucklesR 11-07-2013 10:18 AM

Re: Caternary & Chain...
 
Funny, I just read an interesting letter in Practical Sailor about batteries.

Written by some guy who owns Compass marine :)

Nice article MS.

capecodda 11-07-2013 10:21 AM

Re: Caternary & Chain...
 
This data is interesting.

Even if the last link is off the bottom, I would think that the angle at which the chain meets the anchor shank would be more horizontal with a weighty rode like chain than rope. So I'm postulating that even though the chain is "off the bottom" it is just barely off the bottom at the shank and therefore pulling mostly horizontally.

What would be of interest is what that angle is. You could then calculate the horizontal (good) and vertical (bad) components of force on the anchor.

All that said Maine, I'm surprised at this data. When I look down over the bow in a blow and the chain is bar tight, I'm getting even less sleep than before. And to argue against my hypothesis, I guess if it's bar tight, it's a straight line down to the anchor, so the angle is the angle no matter what the rode.

Maybe the real benefit is that in real life, it's not a straight even pull like a come-a-long. And as you swing back and forth, yanking on everything, the chain absorbs the shock before jerking the anchor out?

denverd0n 11-07-2013 11:02 AM

Re: Caternary & Chain...
 
Didn't read the article. Were they lifting the chain straight up? If so, I would think that the dynamics of a boat pulling on the chain would be different. I would expect it to take more effort to lift the last link when pulling at a 5:1 angle (11.5 degrees).

denverd0n 11-07-2013 11:18 AM

Re: Caternary & Chain...
 
Quote:

Originally Posted by capecodda (Post 1116065)
What would be of interest is what that angle is.

Easy enough to figure out...

http://i1017.photobucket.com/albums/...ps2f0363df.png

When a=1 and c=5 (a 5:1 scope), the angle at A is 11.5 degrees. Given that a link of 3/8" chain is about 1.75" long, that means that the last link would be lifted less than 3/8" off the bottom. At least, until the shank of the anchor also starts lifting.

Maine Sail 11-07-2013 11:43 AM

Re: Caternary & Chain...
 
Quote:

Originally Posted by denverd0n (Post 1116091)
Easy enough to figure out...

http://i1017.photobucket.com/albums/...ps2f0363df.png

When a=1 and c=5 (a 5:1 scope), the angle at A is 11.5 degrees. Given that a link of 3/8" chain is about 1.75" long, that means that the last link would be lifted less than 3/8" off the bottom. At least, until the shank of the anchor also starts lifting.


The important part is that this "lifting" starts at just 158 pounds at 5:1 with 5/16" chain, a very common size on 30-40 boats. On our boat, a 36 footer, we see peak loads of 218 pounds in 17-19 knots and the loads vary up and down between 140-218...... Get above 25 knots and these loads escalate, rather dramatically....

I have never believed, based on my own all-chain experience, that chain gave the caternary one desired when the winds get blowing or when you really need it...

I did not however believe the loads were so small as to when you began to affect shank angle. I find it rather shocking, and reassuring, that I have always believed in scope first before caternary, when you really need it most......

I think this, and the previous data conducted by PS, really points to longer scope and longer and more elastic snubbers...

We know from the previous data that the shock loads increase dramatically as scope is shortened.....

pdqaltair 11-07-2013 01:41 PM

Re: Caternary & Chain...
 
Note that the article was writen by a multihull sailor. There are several reasons why that happend:
* The lighter the boat, the more you feel the chain snap tight. Not that force is greater, but you are more aware of it.
* Multihulls commonly anchor in shallower water, because they can. Perhaps the only free spot in a crowded anchorage is thin. Perhaps there is cove no one else can enter. Either way, multihulls often have less chain out.
* Shallow water gets rough first; some cats have been lost because the owners forgot this. It doesn't matter if you have 2' draft, if the water is <10' it will start breaking fast.
* Multihulls use a bridle/snubber EVERY TIME, even lunch breaks. Thus we tend to be rather obsesed about the best answer. It is also very reasonable for multihulls to use long bridles.
* We tend to like G43 chain to save weight. We have good anchors but would rather not get rediculous on weight as a solution.

The math is the same, I'm just saying the subject is more interesting to cat sailors. A 2' chop in 5 feet of water with only 50' of chain out gets pretty jerky. NO catenary. A 25-30 snubber really takes the sting out.

I too have started running bridle lines down the side decks when in an exposed anchorage; it allows for a long bridle in shallow water. Not always needed, but I often anchor in dubious holding ground and the less the ahchor is horsed around, the better.

Sail Delmarva: Long Bridles

GeorgeB 11-07-2013 02:29 PM

Re: Caternary & Chain...
 
Now I’m really confused. If it takes so little of a force to lift chain off the bottom, what is the advantage of chain over rope rode? I have a 34’ boat weighing 15,000# and was considering ¼ BBB coil all chain rode for the west coast of Mexico. Can I get by with a chain/rope combo and save the weight on my bow?


All times are GMT -4. The time now is 04:53 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012