Chain Versus Rope Scope ?

[cwyckham]

Originally Posted by wdkester

1. Tiea string between your hands
2. Hang a 5# weight from the string
3. You will feel a vertical pull on each of your hands of 2.5 # (and a horizontal pull of something lests say .1 #)
4. If you were to step on a scale you would way 5 # more
5. Pull, your hands apart using as much force as possible lets say a horizontal pull of 100#, the string would be "bar" tight and you would still have 2.5# of vertical force on each hand
6. If you were to step on a scale you would way 5 # more no matter how hard you pulled your hands apart

Pulling an anchor chain bar tight does not change the vertical component of the forces due to the weight of the chain; it only increases the horizontal forces. The chain still has weight that acts on the bow of the boat and anchor attachment.

I'll take my reply offline.

I haven't quite wrapped my head around this one. I agree that the best thought experiment is probably to concentrate the weight of the rode at the center and consider it to be otherwise weightless. Your string and 5# weight are perfect for that.

Your experiment is different from the anchoring set-up in one important way, though. One of your hands should be below the 5# weight. Now, you can't push a rope, so I would submit that the rope in this case will have an upwards and a sideways component of force on the anchor, and zero downwards force. The complete downwards force would be taken by the boat.

If we were anchoring with an actual rigid bar of steel, then the down force would be half on the anchor and half on the boat, but that's not what we're doing at all.

Another way to look at it: Instead of an anchor, you have slid the last link of chain over a long rod fixed vertically on the sea bottom. You have 500# of chain out, and the wind is blowing just enough to lift the last link of chain off the sea bottom. You dive down and lift up on the last link, sliding it up the bar. Assuming no friction, do you really think that you will need to lift up with 250# of force?

Let's say that the boat doesn't drift backwards as we lift up. If we lift the chain up a couple of feet, then there will be a small "sag" of chain at the bottom and we'll have to use a couple pounds of force to hold the chain off the sea bottom. It's not until we lift the bottom end of the chain up to the height of the bow roller that we will be holding up 250# on our end (the other 250# taken up by the boat), and we now match the thought experiment described above.

Conclusion: The chain does not "hold down" the anchor at all, ever. The only positive effect on holding once the chain is off the sea bottom is in the catenary effect. This effect becomes less and less usefull the higher the wind speed gets.

Having said that, I like all chain for the other good reasons already described in this thread.

[s/v Jedi]

Originally Posted by cwyckham

Conclusion: The chain does not "hold down" the anchor at all, ever. The only positive effect on holding once the chain is off the sea bottom is in the catenary effect. This effect becomes less and less usefull the higher the wind speed gets.

Having said that, I like all chain for the other good reasons already described in this thread.

Correct. The chain is hinged at every link. With just the last link at the anchor touching the seabed, all the weight of the chain is taken by the bow and a horizontal pull on the anchor. This actually means that a heavy chain puts more pull on the anchor....

The mistake in the example ith the hands is that the seabed is missing.

ciao!
Nick.

[Seahunter]

Chain acts like a damper. The more chain the more dampening effect. It all depends on the weight of your boat. We're 47000lbs +- a thousand or 2, so I like to use all 350' of chain if anchored for any length of time. We also have 300' nylon anchor rode with 100' of chain, I call the picnic hook.

[Lagoon4us]

Originally Posted by s/v Jedi

Correct. The chain is hinged at every link. With just the last link at the anchor touching the seabed, all the weight of the chain is taken by the bow and a horizontal pull on the anchor. This actually means that a heavy chain puts more pull on the anchor....

The mistake in the example ith the hands is that the seabed is missing.

ciao!
Nick.

Agree there totally
My thoughts FWIW IS also the anchor design has an angle of attack, the scope will come close to matching that designed angle. If scope is correct and the rode is taut the anchor will merrily plow its way with a horizontal or downward attack.
When the boat drifts into shallower water the scope improves and the attack allows the anchor to dive deeper.
However should the drift go into deeper water the scope decreases with a similar result to weighing anchor,,, it will pull out.

Ideal Scope must match design brief for the anchor for all to be in harmony, 7:1 is for want of a better phrase 'The industry standard'.

Lunch hook + staying on board = minimum you can get away with. 2:1 etc
Big blow = we let it all hang out. 7:1 minimum

Only my thoughts no arguments with most theorys and practices expoused in this long taut thread! Cheers

[s/v Jedi]

Originally Posted by Seahunter

Chain acts like a damper. The more chain the more dampening effect. It all depends on the weight of your boat. We're 47000lbs +- a thousand or 2, so I like to use all 350' of chain if anchored for any length of time. We also have 300' nylon anchor rode with 100' of chain, I call the picnic hook.

Yes chain acts like a damper.... for relatively benign conditions. As soon as the wind goes up enough to pull the chain tight, the damper has become overpowered and it's effect is completely lost... Just when we need it most!

Also, I would argue that if you put 350' of chain out in shallow water of 10-20' or so, you are actually at more risk of incidents than when you put out just 60-120'. Possible incidents are collission with other boats, snagging something with the chain etc. I have seen too many incidents that were caused by too much scope.

ciao!
Nick.

[CAELESTIS]

So, in theory . . a 450lb anchor attached to 250' of 3/8 Amsteel (not accounting for chafe) would be superior to a 60lb anchor attached to 250' of 3/8 HT chain (again not considering chafe)? Both would be similar in weight.

[jewel114]

Isn't this fun? Nobody agrees with anybody! Sounds like Religon and Politics.

The fact is that even tho we're all doing it wrong, we all seem to have few problems with what we're doing.

As for obscure words--catenery--I don't think a chain rode hangs in one. The Golden Gate Bridge is a catenery, load equal at both ends. Hmm??

Is a catenery really just a upside down trajectory?

Harley

[s/v Jedi]

Originally Posted by CAELESTIS

So, in theory . . a 450lb anchor attached to 250' of 3/8 Amsteel (not accounting for chafe) would be superior to a 60lb anchor attached to 250' of 3/8 HT chain (again not considering chafe)? Both would be similar in weight.

Yes of course the 450lb anchor wins. Even on CF, I don't think anyone would dare to argue otherwise oh wait, they already did...



ciao!
Nick.

[SailFastTri]

Originally Posted by CAELESTIS

So, in theory . . a 450lb anchor attached to 250' of 3/8 Amsteel (not accounting for chafe) would be superior to a 60lb anchor attached to 250' of 3/8 HT chain (again not considering chafe)? Both would be similar in weight.

Yes, it would hold better... but the Amsteel should have a nylon snubber to absorb shock loads -- it's very low-stretch. Also Amsteel floats, so if it went slack you'd have to worry more about passing boats but less about bottom abrasion.

[sailorboy1]

Originally Posted by CAELESTIS

So, in theory . . a 450lb anchor attached to 250' of 3/8 Amsteel (not accounting for chafe) would be superior to a 60lb anchor attached to 250' of 3/8 HT chain (again not considering chafe)? Both would be similar in weight.

I vote for the 450# anchor combo

[Jim Cate]

Originally Posted by SailFastTri

Just thinking... I looked it up and 3/8" chain weighs 1.67 pounds per foot, so in the bow of your fine entry 36-foot boat you had 511 pounds of ground tackle up forward. That would have a significant impact on performance and trim in that size boat. 5/16" G4 is 1.1 pounds per foot and there's always the option for some degree of mixed rode, even if mostly chain.

I respectfully acknowledge that there is no such thing as "too much" anchor when a squall comes over the anchorage at 0-dark-hundred, but every decision involves trade-offs.

Given the limits of whatever weight you're willing to add I would rather concentrate weight in the anchor where it increases holding than in the rode.

G'Day again SFT,

I think that your numbers are about correct. So, by going to the 5/16" or 8 mm chain we would have saved on the order of (1.68-1.1)*280, or 162 lbs. Not inconsiderable for sure, but probably not going to make a huge difference in performance. The experiment with incremental weight forward I suggested earlier would have been interesting to do on our boat, but I never worried enough about it to do it!

At any rate, I agree that our anchoring power would have been better with the smaller chain and a bigger anchor. As it happened, once we got rid of the 45 lb CQR and replaced it with a 20 kg Bruce we had no anchoring difficulties. On the other hand, we were able to use the original 3/8 " chain for the full 17 years that we cruised in that boat. By the third regalvanizing, there was visible wear in the chain. If I had had the smaller diameter wire (ie 5/16 ") it would have been harder to convince myself that the chain was still usable with that much material worn off. I'm sure, BTW, that many experts would have condemned the chain by then, but on our budget replacement was not attractive... and in fact the current owner is still using the chain AFAIK. Maybe we were lucky, or maybe the extra margin of oversized chain helped... who can say for sure?

With the superior anchors available now our security has surely been improved, and if I was to start over from scratch on this boat I might well consider lighter chain and a bigger anchor (than our 60 lb Supreme).

Cheers,

Jim

[wdkester]

Originally Posted by cwyckham

I haven't quite wrapped my head around this one. I agree that the best thought experiment is probably to concentrate the weight of the rode at the center and consider it to be otherwise weightless. Your string and 5# weight are perfect for that.

Your experiment is different from the anchoring set-up in one important way, though. One of your hands should be below the 5# weight. Now, you can't push a rope, so I would submit that the rope in this case will have an upwards and a sideways component of force on the anchor, and zero downwards force. The complete downwards force would be taken by the boat.

If we were anchoring with an actual rigid bar of steel, then the down force would be half on the anchor and half on the boat, but that's not what we're doing at all.

Another way to look at it: Instead of an anchor, you have slid the last link of chain over a long rod fixed vertically on the sea bottom. You have 500# of chain out, and the wind is blowing just enough to lift the last link of chain off the sea bottom. You dive down and lift up on the last link, sliding it up the bar. Assuming no friction, do you really think that you will need to lift up with 250# of force?

Let's say that the boat doesn't drift backwards as we lift up. If we lift the chain up a couple of feet, then there will be a small "sag" of chain at the bottom and we'll have to use a couple pounds of force to hold the chain off the sea bottom. It's not until we lift the bottom end of the chain up to the height of the bow roller that we will be holding up 250# on our end (the other 250# taken up by the boat), and we now match the thought experiment described above.

Conclusion: The chain does not "hold down" the anchor at all, ever. The only positive effect on holding once the chain is off the sea bottom is in the catenary effect. This effect becomes less and less usefull the higher the wind speed gets.

Having said that, I like all chain for the other good reasons already described in this thread.

You are correct. It's a good thing that I was a EE not a ME.

[wsmurdoch]

Let me ask two questions whose answers (I think) rest more on experience rather than science.

1. Any all chain rode will weigh more than a (full or partial) line rode. When the the wind blows and the rode goes tight all of the weigh of the deployed rode plus any undeployed rode sill stored in the bow rests on the bow pulling it down. Does the weight keep the bow from rising over the waves that come with with the wind and contribute to the jerking that could dislodge the anchor? Is this a significant problem with all chain rodes?

2. Nylon is recommended for anchor rode or snubbers because it is stretchy. When the wind gusts or waves strike the boat, the nylon stretches and the boat goes backwards. On my boat when that happens the bow blows off to one side or the other presenting the side of the boat with its greater area to the wind. How much stretch is optimum? Can you have too much? Can you have too little? Should you have 100 lb per foot, 500 lb per foot, 50 lb per foot...?

Bill Murdoch

[s/v Jedi]

Bill,

As to the weight holding the bow down... No that does not really happen because the force upwards from the buoyancy is higher. But something must give and the boat will move forward, closer to the anchor. This is why a nylon snubber should be used.

Stretch: the biggest problem is that after stretching, the nylon will pull the boat forward so much that the rode goes slack. This amplifies the problem with the next gust.

I experimented a lot with snubbers. My boat weighs 55,000lb or 25 metric tons. I found that the best snubber is a 20' long 5/8" 3-strand nylon line. When I go down to 1/2" nylon, I break the snubber during 30-40 knot squalls. At 5/8" I never broke the snubber and that includes squalls up to 60 knot winds.

I advice everyone to find that right diameter and length for the snubber. Let out some more or shorten up and see what changes in the boat's behaviour behind he anchor. Tie the snubber to the chain or these tests so that you don't loose a chain hook when it breaks (I had to dive for mine...)

When the boat sails back and forth, try out a riding sail using a cheap piece of tarp or sheet or something. Experiment

ciao!
Nick.

[dandrews]

Originally Posted by Don Lucas

I vote for the 450# anchor combo

Hmmm. Not sure I would be able to get a 450 lb anchor back on deck. ;-)

Don