Caternary & Chain

[Maine Sail]

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..

PS used a 100' section of 5/16" chain set to a 5:1 scope. To apply the load they used a chain hoist and measured it with a 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.

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 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....?

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

So if I am anchoring at 5:1 with 100' of 5/16" chain in approx 20' 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....

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 rode is needed.With this data it appears that this is simply untrue and that scope should be set irrespective of chain or rope/chain rode because chain alone really will not help shank angle when you really need it most........

 

[Tim R.]

Does this make a case for kellets?

 

[Stu Jackson]

Tim, basically no. My experience is that kellets or sentinels are good for keeping the rode (chain or line) DOWN when current changes, but LOTS of research has shown that they simply don't work when it blows. Rocna has done a lot of posts on their website.

 

[Maine Sail]

Does this make a case for kellets?

To me it spells the need for adequate scope. If your shank angle can change in winds that light, due to the chain lifting and providing little caternary effect to the shank, then the only real fix for storm or heavy winds would be longer scope....

Kellets can certainly work to minimize sailing at anchor in lighter winds but I suspect they will add only minimal value in holding the shank down in higher winds....

 

[Stu Jackson]

Kellets can certainly work to minimize sailing at anchor in lighter winds but I suspect they will add only minimal value in holding the shank down in higher winds....

'Zactly. :dance: Summarizes about 87 years of discussions.

 

[Ray Bowles]

Maine Sail, Did they, or do you know the figures for a 7:1 scope? Does the use of a kellet or weight hanging on the chain add enough of an improvement to make it worth while to do this?
We had experences while in the Keys with boats next to us. They had a 4:1 scope and dragged anchor in winds of 35 to 45 mph. We did not with a 7:1 scope. We all had boats around 38 feet, but all had 3/8ths chain and 45# anchors. I had 125 ft of chain out and they had approximately 75 ft. We got together after this happened and talked about it. We then dove on my chain to see how it handled the load. At about 100 ft the chain had a 30 degree change of direction. That meant that less that 100 ft was not enough scope. I've always wondered about the use of a kellet as sometimes there is not enough room to lay down a lot of scope.
Ray

 

[Ray Bowles]

MS, I waited too long to post that last post and only then did I see that you had answered. Sorry.
Ray

 

[Stu Jackson]

Ray, 4:1 is nonsense and not prudent. What else is there, based on your story? Sentinels wouldn't have helped unless the wind was lighter. Simple. Am I missing something?

"Oh, no" as they say on www.crusiersforum.com - Not another anchor/kellet thread...

Perhaps some newbies can learn.

Ray, WADR, if it's "too crowded" you might be safer finding another anchorage.

 

[Ray Bowles]

Stu, As I said, I had a 7:1 scope and the others had 4:1 and in no way would I use less that 7:1. I can say that there are a lot of folks dragging in the Gulf using 4:1.
Next time I'll try to be a little less "simple"
Am I missing something?
Ray

 

[thinwater]

Clearly ultimate holding must be rated at known scope. A practical anchor needs to be strong at 5-7 scope. Repeatable answers generally require initial setting at longer scope, but power setting at the listed scope.

The corollary is that chain does very little absorb shock once the wind reaches important (over 30 knots) levels. The chain is so straight that the difference between straight and the small amount of curve is only 1-2% of the chain out. This is particularly true when anchoring in modest water depths (6-10 feet).

A few years ago I switched from 25' chain + rope to all chain in the process of getting a new anchor. The ride at anchor was MUCH rougher with all chain, until I switched to a longer, softer snubber. The chain slows reaction to wind shifts and thus probably aids resetting, but sharp gusts and waves are a different matter. Any rumor that chain absorbs as well as line in moderately shallow water in strong conditions will be hard to support with numbers.

 

[Maine Sail]

The problem as always is more complex than just 4:1 vs 7:1 etc.. While performance wise 7:1 will outperform 4:1 all else being equal there is always more to it...

What anchor type dragged at 4:1? Were these apples to apples comparisons other than scope or were there other factors that changed?

There are plenty of good anchor designs that won't drag at 4:1 and some designs that will still drag at 10:1 so what is on the end of the chain matters as much as setting technique, rode calculations, caternary, power setting and testing the set, snubber etc. etc...

I can paint a scenario of two identical boats both with different anchoring gear, techniques and scopes and if the 4:1 boat had better technique, a high performance anchor I would prefer to be on that boat vs. the 7:1 and poor performing anchor...

While more scope does always reduce anchor shank angle if you have a poor performing anchor no amount of scope can fix that, only place a Band-Aid on its performance. If you did not properly set the anchor and properly back down and power set it then more scope can't always fix that either.

One thing we know is that with less scope the chain straightens even earlier. With shorter scopes the snatch loads on deck gear and snubbers increases tremendously. However we also know that in many anchorages your only option is 4:1 or move on.... As I said complex and no easy answer other than....

*Buy the best performing anchor your money can buy
*Use proper setting technique
*Use proper calculations for scope and don't forget tide and bow height
*Check your setting technique with a STRONG power set by backing down at full throttle
*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.
*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.
*If you don't feel comfortable with the scope you can lay, move on......

 

[txtowman]

Maine-sail.... Well said as usual. I had fits for several years because it is not easy to back down with a catalina 22 and an outboard. got myself a manson supreme and can't believe the difference. And no I don't endorse them it's just what they had in my budget.

 

[Chris Patterson]

Tim is right on with that one. The one time I tried all chain, when a larger wave made the chain tight, I thought it would snatch the bits off of the boat.

With MYSELF.....I from now on like a piece of three strand in there somewhere for the shock loading.

(All things considered, anchoring tends to be the biggest PIA for me, and about the most "nerve racking" as well)..

 

[Captn TJ]

Shouldn't bottom factor in as well? We have all chain on a lewmar delta and use 7 and sometimes 8 - 1 depending on wind conditions. A year ago we were in a squall 60+ winds in the north channel (mud/clay bottom). Didn't drag - however we had a tough time getting the chain and anchor up in our 42 because it was buried in the bottom.

 

[Maine Sail]

Shouldn't bottom factor in as well? We have all chain on a lewmar delta and use 7 and sometimes 8 - 1 depending on wind conditions. A year ago we were in a squall 60+ winds in the north channel (mud/clay bottom). Didn't drag - however we had a tough time getting the chain and anchor up in our 42 because it was buried in the bottom.

Bottom should ALWAYS factor in, as well as which anchor you choose for that bottom. In really soft bottoms plow types generally "plow" when loaded hard but other types such as Danforth/Fortress, Spade, or others perform considerably better. Certain anchors have a hard time setting in hard substrates so these anchors should not be chosen for known hard bottom conditions. Of course the best bet is to get an anchor that is known to perform well in the widest varying conditions.

Scope much beyond 7:1 or 8:1 changes very little in shank angle in relation to the bottom. Beyond 8:1 the diminishing returns on shank angle are rapidly apparent but if you have the swing room it can't hurt... It is when we begin to shorten scope that the effects change in shank angle more rapidly..

2:1

4:1 scope translates to about a 13 degree shank angle:

This is 5:1 and it results in roughly a 10 degree shank angle. Keep in mind that with 5:1 / 110' of 5/16" chain, on our 36' boat, we are lifting the chain like this at UNDER 20 KNOTS!

At 7:1 you drop to a 7 degree angle a full 3 degrees less than 5:1. There really is no substitute for a shallower shank angle especially when your chain is lifting the last link at WELL BELOW strong wind conditions.

At 8:1 you only lose another degree and are at about 6 degrees....

 

[Ross]

When I anchor I set the CQR down when the boat is stopped and we back down paying out rode. I snub at 5:1 and sometimes it grabs then I continue to pay out to 7:1 and it always grabs then I payout a couple more marked lengths to give us 8:1 or even 9:1. There is always room for such a long scope.

 

[Bob 04 H260]

I think the Rocna site suggested using lightest chain possible and putting the weight into a bigger anchor for better holding.

I believe it was Dodds book on Seamanship where he does a calculation showing the limited effect on catenary of an all chain rode. I believe Dodds suggests a boat length of chain with a small float to keep the nylon rode off the bottom. He feels the nylon rode will act like a bungee and prevent shock loading the anchor and will result in better holding than all chain. Having been on a boat that was set up that way the nylon rode would wrap around the keel with each tide change :cry:, it can disadvantages as well, but may work well in the Great Lakes.

Bob

 

[Stu Jackson]

Yes, you need to match the chain and the anchor based on your boat.

Anchor System Sizing Tables (Reply #6) http://c34.org/bbs/index.php/topic,4990.0.htmlhttp://c34.org/bbs/index.php/topic,4990.msg30400.html#msg30400

A friend who anchored a lot (took his C34 from Vancouver, BC to Mexico, wrote this:

The Rocna. All 20kg of it with 100ft of chain. The rest of the world can debate all they like. When I pull into a place like Bodega Bay at midnight and the fog is so thick I can't see the jetty 50 feet away to make an entrance, I drop my hook in the rolling ocean swells with the surf crashing (Foster says it's like staying in a cheap Best Western beside the highway), and I sleep. And in the morning I have a windlass to pull the beast up and I wouldn't trade it for anything. (I also wouldn't add more chain - this works perfectly in 25 to 30 feet of water - you let all the chain out and you tie off nylon at the preferred scope and don't bother with snubbers and chain hooks and all that stuff...)

This was our best upgrade.

 

[Gunni]

The chain may lift off of the bottom, but that doesn't mean that the anchor shank will lift...unless you have a poor set, or a poor anchor . Most anchors are too small. I'm looking at a 50 foot Passport across the way - two 35# CQR's on the bow. Ridiculous.

 

[Ross]

And I carry a 35 # CQR on my Islander 30.