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Old 01-07-2016, 12:16 PM   #1
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Anchor Rode Weight Bias

This is a discussion on the merits of adjusting the weight of the rode's chain so as to ballast the rode to best advantage re the catenary and more importantly the angle of the rode on the bottom to horizontal. I'm going to copy and paste several posts that I made in an attempt to communicate this function.

Post #62
"Food for thought about the catenary because catenary is claimed to be the reason to use all chain.

Imagine a line attached to a tree about 10' up the tree. The other end is attached to a well set anchor about 50' away from the 1st tree. Lets assume about 3' of line droop (catenary) at about the middle of the rode. Place a heavy weight on the line about 3/4 of the way up the line from where it's attached to the anchor. What happens to the angle of pull at the attachment point on the ground at the anchor? Does the angle of pull on the anchor increase, decrease or remain the same?

Post #66
"Reality is that if you add weight to the upper end of the line (about 3/4 of the way up the rode) in my tree example and on a rode of a boat at anchor the rode near the anchor will rise providing it was not on the bottom or strung out straight. What does that tell you about the catenary effect of weight on the upper end of a rode? Simple logic will indicate that the added weight above will raise the rode a bit near the anchor having the opposite effect that chain huggers think happends. Or you can go fine two trees and get out some line and a weight ... and try it.

Post #78
"What I'm trying to communicate is that the bias of weight along a rode has a large effect on the capabilities of the rode re minimizing the angle of pull at the anchor.

I'll try it one more time. Go back to the tree. Tie a rope to two trees so there is considerable droop in the rope. Then put a heavy weight fairly close to one tree and mentally observe that now the rope will form a "V" shape if the weight is fairly heavy. The angle of the rope at each of the two trees will be different. Depending on the location of the weight the angle will vary. If the rope is horizontal the difference of the angle on each end will be maximum.

Here's another illustrative example. Lets say you're anchored and can attach a 200lb weight to the anchor 3' from the anchor. One hell of a Kellet one could say. A dramatic increase in holding power will result. Now take that 200lb weight and attach it to the rode 3' down from the bow roller. It will have practically no effect at all.

So now that one has a choice of where to put chain and line in the rode you can have a great effect on the rode's performance w no increase in cost. Of course there actually be a decrease in cost both in money and weight. This was not very practical before splicing to run through a gypsy was introduced. But it is here and seems dependable. So weight savings, cost savings and performance increases all at once are available w anchor chain weight bias along the rode. And if you are happy w the weight of an all chain rode just use 1/2 to 1/4 as long a chain of the same weight as the all chain rode for considerably more performance.

Lastly I should add a paragraph that Makobuilders wrote after my post #78.
"I believe that all your statements are accurate. That is why historically the boating references recommend at most one boat-length of chain or half the weight of the anchor. I also recall statements told from many commercial fishermen in Seattle about adding a fathom (or less) of extremely heavy chain as a leader, which seemed to solve alot of problems with dragging."
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Old 01-07-2016, 12:21 PM   #2
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Are you trying to say that partial chain on a combo rode is more effective than an all chain rode because there is more weight concentrated near the anchor?
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Old 01-07-2016, 12:30 PM   #3
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psneeld,
Yes if the rode weight remains the same .. or somewhat so. But it could be lighter depending on the "stack up" of chain. The reduced angle is a direct result of more weight concentrated near the anchor.
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Old 01-07-2016, 12:39 PM   #4
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A chain catenary from a tree is a static system. An anchored boat is a dynamic system.
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Old 01-07-2016, 12:45 PM   #5
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Spy,
OK use a boat. Not going to change.
However if the rode is pulled straight it makes no difference.
And if the rode is almost straight down still makes no difference.
But if in-between .. much advantage.

Right now I have 3' of chain in my rode. 3/8ths chain that is very heavy for my sized boat. The chain as you may guess is at the anchor. Imagine if I attached the 3' of chain to the bow roller? A waste to be sure. What if I attached it to the center of the rode? Some advantage to be sure. But most significant advantage placement is near the anchor. Dynamic on a boat or sitt'in in a back yard attached to trees. The weight bias advantage slowly diminishes as the rode shortens until when the rode is vertical it's gone.
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Old 01-07-2016, 03:57 PM   #6
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If you think that 20 feet of 1/2 inch chain plus 100 feet of nylon is a better setup than 120 feet of half inch chain....have at it....

Not saying one is bad but I think most would vote 120 feet of chain if they don't mind the other attributes of chain.

Many of us don't seem to...it is usually boat size and windlass dependent.


I agree that a 200 pound weight 5 feet from the anchor is an awesome setup...just not practical.

It is all the other attributes of all chain that make people choose it or not....

Only the rare cruiser goes with the science that for day to day anchoring a combo rode is incrementally better and usually only in a huge blow.
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Old 01-07-2016, 04:21 PM   #7
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A chain catenary from a tree is a static system. An anchored boat is a dynamic system.

Exactly. The advantage of catenary is two-fold. It provides a degree of shock absorbing as long as it's there, and it reduces the angle of pull on the anchor.

The important question is where along the rode will an extra weight help the most in reducing the angle of pull on the end of the anchor shank? In a static example--- Eric's tree thing---- the geometry and physics involved do make the angle of the rode where it meets the anchor shank less than if the weight is hung somewhere further toward the boat.

But in the dynamic real world---- Spy's world---- the boat is surging up and down and pulling back on the rode. This greatly alters the situation. Now what we need to know is in what position will the weight do the most in spite of the boat's pulling back hard and the bow surging up and down to reduce the average angle of the rode where it meets the end of the shank?

I'm not sure being down next to the shank will do it unless one has a hell of a big weight. One so large as to be impractical from a user standpoint as psneeld points out.

But if one has a twenty or thirty pound weight, where will it do the most good? I agree with Eric that a few feet down from the bow roller won't accomplish much or anything. But maybe halfway down the rode will do more to reduce the angle to the shank from the surging, pulling boat? Or a third of the way? Because unless the whole rig is pulled bar tight, the chain will sag from it's own weight as long as it is hanging free.

So as the weight is pulled up and down by the boat's motion and tugging, the chain will curve down to the weight and continue to curve down to the anchor shank. If the weight is right next to the anchor and the boat pulls it up off the bottom, there will be very little chain between the weight and the anchor shank to curve. It will be more of a straight line.

Will the angle of the short amount of chain between the lifted-off-the-bottom weight and the anchor shank just a few feet away be less, more or the same as the angle of the sagging chain between the same weight and the anchor shank 50 or a 100 or more feet away under the same amount of surge and pull from the boat?

That's the question that actually matters.
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Old 01-07-2016, 04:28 PM   #8
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I'm not trying to win a popularity contest. We already know what is most popular .... the bandwagon os loaded.

I'm saying a combination rode w about 1/2 to 1/4 in chain weighing .. say 200lbs ..is better than a rode all chain weighing 200lbs. It puts the weight where it will do the most good .... minimizing rode angle at the anchor.
That's it .... and w that in mind it makes all chain a poor choice. But I'll admit it's popular.

OK I'll drop the tree example scenairo and say the same thing in "real world" terms. If you had a 300lb chain rode and made a combination rode of the same weight w about 275lbs of chain (probably about 1/4 the length of the original rode) and the rest nylon you'd have a much better performing rode.

And you're right Marin the best place for the weight is not at the anchor but about 10 or 15% of the way up the rode. But if you had a combination rode w 1/4 of it in chain you'd be very close to perfect. I really don't know if that's fact but close enough IMO.

Very yes Marin.
Surely you can recall I've always said given a specific weight rode the weight is best "spent" in the anchor.
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Old 01-07-2016, 04:54 PM   #9
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I'm saying a combination rode w about 1/2 to 1/4 in chain weighing .. say 200lbs ..is better than a rode all chain weighing 200lbs. It puts the weight where it will do the most good .... minimizing rode angle at the anchor.
I can't think of any reason to disagree with that. You don't need the catenary of all-chain to provide shock absorbing since a good portion of the rode is nylon. It can introduce some operational challenges depending on the size of the boat, the chain, the anchor, and the method used to deploy and retrieve it.

And in the long run a better solution than the nylon with heavy length chain at the end might be to have a larger and/or heavier anchor that will get a better grip on the bottom regardless of the angle the rode meets its shank, with a relatively short length of lighter chain between the rode and the anchor to minimize chafing or cutting issues.

After all, the objective of the exercise is to keep the anchor stuck in the bottom and the boat connected to it. Whatever the best way to do that is given the three variables of one's boat, ground tackle handling system, and bottom characteristics is the right solution, right?
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Old 01-07-2016, 05:17 PM   #10
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Eric,

Not sure of the point you are making. Here in SE we have been slipping 40 or 50 lb cannon balls down the rode in order to increase holding and reduce surge. It is important to keep the cannon ball off the bottom and assure it does not foul.

Many of the commercial fishing boats here do this routinely, even in good weather in order to get a more comfortable ride at anchor along with dropping their fish over the side.

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Old 01-07-2016, 05:18 PM   #11
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A comment. The tree analogy doesn't apply because the boat is not a fixed point. Consider that the pull on the anchor and rode will not lift rode completely off the bottom until the pull exceeds the weight of the rode.

Now consider that 1/2" 3 strand nylon rope weight about 0.065 lbs/ft and 5/16" HT chain weighs about about 1.09 lbs/foot. The density of nylon is about 1.15 g/cc and steel is about 8 g/cc. The result is that the effective weight in water of nylon rode in only about 0.0085 lbs/ft while chain is about 0.954 lbs/ft. So if you have 100' of 5/16" chain on the bottom in calm conditions its effective weight is 95.4 lbs. In other words wind or current action on the boat will have to put a pull of 94.5 lbs on the rode to lift the rode off the bottom. So the angle of the rode to the bottom at the anchor is 0 degrees until the load on the rode exceeds 94.5 lbs. Now consider a mixed rode with 30' of 3/16" chain and 70' of 1/2" nylon. The load required to lift the 100' rode off the bottom is about 30 pounds, so the rode angle at the anchor goes above zero at only 30 lbs load on the rode. To achieve the same performance as the all chain rode, the mixed rode needs a 75-80 lb kellet located far enough down the rode to sit on the bottom.

Basically the weight of the nylon is inconsequential. Unless the short chain leader at the anchor is equal in weight to the all chain rode the mixed rode will be worse than the all chain rode. To achieve the same weight of rode as all 5/16" chain my 30' chain - 70' nylon combo rode needs the 30' of chain to be at least 5/8".
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Old 01-07-2016, 05:48 PM   #12
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So you are saying that regardless of the type of rode (all rope, all chain, or combo), the anchor will hold better when more weight is near the anchor. In your tree example, a weight is added near a tree. In your combo setup, the short section of chain attached to the anchor is the extra weight forward. Those examples seem reasonable to me. I have to disagree when you say a combo rode is the best. In my case, I have 300 feet of chain. The first 50' weighs 80 pounds and the last 250' weighs 400 pounds. I doubt that substituting 250' of nylon rope weighing 10 pounds would do a better job anchoring my boat than the all chain rode.
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Old 01-07-2016, 07:56 PM   #13
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1025,
I made a mistake putting out the tree scenario.
You've got 480lbs of chain? If you went to an all line rode you could have an anchor over 350lbs. Think that would hold your boat? Here's a pic of a boat in Alaska w a 500lb anchor. Don't know what his rode is. Simple fact is that if you substitute the chain for nylon and got an anchor that would bring the weight back up to what it was w all chain your boat would be very much more secure than it was before. Or if you took the upper half of the chain rode off and substituted it w nylon, weighed the chain you took off and got an anchor as heavy as the removed chain and your old anchor you'd still have the mother of all anchors. You may then sit tight in 100mph winds w no increase in rode weight.
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Old 01-07-2016, 08:07 PM   #14
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Go to marinas all the time and the anchor is relegated to only emergency use.

While holding is important... multi-purpose ground tackle is my goal and seems to be others as well.

Yes gigantic or super holding anchors with 200 pound kellets next to them are one mean ground tackle setup.... not fo me and most reasonable cruisers I know or have read about.

While the theory holds bottom...the practicality of the suggestion is for those that live in that concept.
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Old 01-07-2016, 09:11 PM   #15
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My 300' of galvanized BBB weighs 495 pounds, but I rounded for simplicity.
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Old 01-07-2016, 09:55 PM   #16
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Up to a point the weight of the chain is significant in affecting the rode seabed angle. Unfortunately when the wind picks up and the chain becomes taught the concept is not so relevant as would be greater scope all chain-rode- or combo. If you want to stay put provided a set anchor the first rule is greater scope thus affecting the critical angle of pull. This also makes a very good argument for a combo rode because greater length and less weight up forward can be had that way. There are not many 30-45 foot boats that can handle 400-600 foot of all chain while many can do so with a combo.
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Old 01-07-2016, 10:07 PM   #17
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Quote:
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1025,
I made a mistake putting out the tree scenario.
You've got 480lbs of chain? If you went to an all line rode you could have an anchor over 350lbs. Think that would hold your boat? Here's a pic of a boat in Alaska w a 500lb anchor. Don't know what his rode is. Simple fact is that if you substitute the chain for nylon and got an anchor that would bring the weight back up to what it was w all chain your boat would be very much more secure than it was before. Or if you took the upper half of the chain rode off and substituted it w nylon, weighed the chain you took off and got an anchor as heavy as the removed chain and your old anchor you'd still have the mother of all anchors. You may then sit tight in 100mph winds w no increase in rode weight.
Interesting take on it. Weight is fine, but heavy chain is good too. Take my mooring for example. It is three tons of granite. By your logic I could just run a suitable line up to the boat. However, local convention requires a two stage all chain rode up to the mooring ball. My mooring has 35' of 1-1/4 inch chain then 45 feet of 5/8" chain. I have a 25' x 1" double braid pennant shackled to the chain just below the mooring ball. The mooring has held my 20,000 lb boat through several 60+ knot storms with 6 foot seas at the mooring. The weight of the chain (about 700 lbs) helps prevent shock loading that might break the granite block out of the bottom or rip the cleats off my foredeck. Shock loading would occur with an all line rode since the line would be bar tight even in moderate winds (30-35 knots). Locally there are a couple of places that have all line rodes on moorings, but those are only found in hurricane hole type harbors where waves don't develop.
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Old 01-07-2016, 11:05 PM   #18
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Lets not forget the testing that has proved anchor weight is not as important as setting- digging ability and area and shape of buried anchor. A small aluminum or SS anchor with an all rope rode can and often does outperform heavy with or without all chain. While centenary angle is important using heavy chain to get the angle down is probably only justified in tight anchorages in low wind(very common East and West coast). In conditions where a boat is at risk greater scope, chafe protection and shock control are the best solutions. The best argument against a long all chain set up is the effect of the weight in the bow and strain on the windless. I think the best compromise is enough chain for the tight anchorages backed up by a nylon section allowing for 10/1 scope in your anchoring area. In PNW on way to Alaska 10/1 not realistic but long line definitely needed.
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Old 01-07-2016, 11:26 PM   #19
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I'm just say'in those w all chain would be better off w a combination rode of the same weight.

This thread has turned into a forest of details 80% of which are off topic. This is not an accusational statement as I know how easy it is to do. The above sentence in this post is the subject. To bias the weight of an all chain rode well fwd toward the anchor to get a better rode angle. The operative word is BIAS.
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Old 01-08-2016, 03:25 AM   #20
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Well, I'm not biased either way, and would readily agree that for smaller craft, like the 26 ft yacht we once had, combination rode is fine, we had a 26 ft length of quite heavy chain, then all nylon the rest. I also agree that if you have to anchor often in very deep water, then a combination with 50 odd metres of chain, then brait nylon is reasonable, but for a boat 30ft up, anchoring in shallow to moderate depths, the all chain rode with appropriate windlass is probably the best all round set-up, as it is all so straightforward to use, and with no bottom chafe or spliced join concerns.
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