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Old 08-03-2013, 12:12 AM   #81
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Quote:
Originally Posted by manyboats View Post
............"I think you're barking up the right tree Craig".

Yes, yes...finally..by George I think he's got it..!

"The attached point on one end is obviously the anchor and the attach point on the other end is the anchor cleat on the bow deck. The force against the boat MUST match the force (pull) on the anchor."

No, that bit's not right - that is only the case when almost all the rode is off the seabed, as Rick has pointed out previously, also.

"That's a given I'd say." That so-called 'given' has been your blind spot Eric.
"It seems to me now that the force would be the same nylon or chain rode. But they will differ re what AusCan says. But the load on the boat doesn't end at the bow cleat. It's passed on to the hull. I'm thinking now that the forces on the chain from the weight of the chain are felt only 'along the active length of the chain'." YES!

"Unless someone else comes up w something better I'm going to accept that. So when at anchor in a wind the forces on the bow and anchor w a chain rode and a line rode are equal. But the forces of tension are much more along the chain itself just due to the weight of the chain. Yes, but well within the chain's strength, and mostly transmitted back to the unfixed end, ie, the boat...drawing it always back towards the fixed point, the anchor. This is why your chain tensioned between cars or posts scenario is not comparable, Eric.

"So if two boats were anchored w one chain and the other line and both line and chain were equal in strength the chain rode would break first. Just because of the added weight of the chain."
Not necessarily at all, as strength per length of the correct sized chain would probably more than account for it's own weight plus equal the strength of the nylon, I would think. However, I would defer to others more expert on that.
The main point being the forces on the boat have to be much higher with the heavier chain rode than a lighter synthetic rode, before they lift enough rode to to be able to exert any real tension directly on the anchor.

"So my theory was flawed or w/o merit.
As I said before I knew it must be but just couldn't figure it out. The extra forces are contained within the active part of the chain."
Exactly... yes. I think you do see it now Eric.
However, knowing how you love a real world example, and to maybe set your mind at rest...just think how much less as a fraction of boat size/length/displacement, (let alone windage), an ocean-going ship's anchor is compared to our boats, yet they seldom drag anchor, as they have such humungous chain links in the rode that the anchor itself usually bears no weight at all, it's all taken up in the chain. and they don't usually put that much out either.

It's been fun, but are we there yet..?
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Old 08-03-2013, 04:00 AM   #82
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Eric,

While I certainly respect your opinion and appreciate your posts, we are going to have to respectfully disagree on this issue. I have listened to your points, but disagree with your assumptions.

I approach this matter not as a marine engineer, but as an aeronautical engineer with a flyboy mentality with training in mathematical analysis including vector analysis and physics. No doubt there are flaws in my analysis or description, but I feel the basic foundation of this argument is solid.

1. I feel you are confusing the issue in thinking of 2 forces being applied to the rode. Think of it as 1 force being applied to the anchor. This single force applied from the boat end acts through the rode to the anchor. The manner in which the anchor reacts is dependent on the weight and mechanical properties of the rode and the anchor. Weight, elasticity, drag resistance and strength all come into play as to how the force applied at the boat end will be transmitted to the anchor shank. The reaction of the anchor is a result of the DIRECTION of the forces being applied to the anchor shank.

2. When the tension force is applied on the rode, the anchor presents a restriction to that force in the way of anchor 'friction' or drag which we refer to as 'anchor hold'. Anchors present the greatest hold when the tensile force from the boat pull is applied horizontally, or parallel to the bottom (assuming a level, flat bottom).

3. The tensile force on the line and anchor are presented from the boat in 3 dimensions; up/down, right/left and forward. Since the rode is not a rigid connection like a rod, there is no aft push in the anchor or line.

A theoretical non-elastic rode of infinite strength and zero diameter/water drag like a piano wire will apply the load immediately, directly and without resistance from the water or medium (i.e. mud) in which the rode lies. This rode would tighten like a…well…piano string and apply all the force to the anchor exactly as a sum of the directional forces being applied to the boat end. In other words, if I stood on the bow and applied 20 Lbs of force at a 45 degree vertical angle, 0 left/right force, then 14.1 lbs would be applied horizontally and 14.1 lbs vertically at the anchor shaft. The rode would be straight as an arrow at a perfect 45 degree angle. (Pythagorean’s theorem: A2+B2=C2) If we can keep the transfer of force in the horizontal plane for as long as possible, we can maximize the effectiveness of the anchor.

In the real world, we all get the benefit of two principles that help us keep this force horizontal:
a. Catenary which, thanks to gravity, forces the rode to the bottom and applies much of that force in the horizontal plane. (more from chain, less from thin, floating poly line)
b. Rode drag though the bottom mud which provides additional resistance, much like a well set anchor, to the horizontal (and to a lesser extent the vertical) forces being applied to the boat end of the anchor line. (more from chain, less from piano wire)

For the purposes of this discussion, let’s leave out the following:
1. the anchor design and its resistance to the boat’s pull since we’re really talking about the transmitter of the boat’s forces…the anchor rode,
2. abrasion issues…chain resists abrasion better than nylon rode, and
3. the bottom is level and perfectly consistent medium of mud. No rocks, no rebar and no lobster traps to foul the discussion.

Let’s focus our brains on the DIRECTION of pull and the resulting components of that force…the lateral (left/right) and vertical (high/low angles) components.

With a light, yet somewhat elastic rode like a 3-ply nylon braid, the rode will apply the load in the vertical plane sooner than a heavy chain rode would since it lacks the weight and mechanical properties to resist the vertical component of the force applied. But its elastic properties will provide shock absorption to the vertical and horizontal components of the load as it is applied.

A chain, by comparison, will lie along the bottom longer due to its greater weight and bottom “stickiness” and apply that tension in a more horizontal plane for a longer period of time than the nylon rode. But, without gravity, its lack of elasticity would apply that load more immediately and directly, without the same shock absorption. The saving graces of chain are its catenary caused by gravity which serves as a lateral and vertical shock absorber until it becomes (theoretically) piano wire tight and its “stickiness” which resists primarily the horizontal tensile force component. It’s mechanical property of acting as independent segments is also a benefit to be discussed.

An anchor provides its best resistance when the forces applied are in the horizontal plane. If all forces were horizontal, we’d all be carrying much smaller anchors. The problem is that they are not just horizontal forces. Start pulling vertically and it releases easily, like when we retrieve our anchors. Keep the forces horizontal as long as possible and we retain the greatest resistance to the tension on the rode.

If we were to create a backyard comparison to test the principles on a smaller yet more observable scale, I’d set up two wagons as the boats. Behind the wagons I’d set up two rodes: one of ¼ nylon cord and another of swingset chain. Each rode would be set 3 inches deep in wet sand and the anchor on each would be represented by a single red housing brick buried perpendicular to the rode in that sand. Each wagon handle would be attached to a scale to measure the pull (tensile) force being applied. In the first experiment, we will apply the force of the wagon smoothly, slowly and consistently to observe the differences.

Wagon A with the poly cord rode would be pulled and the first thing we’d observe is the poly cord being pulled tight out of the sand in one fluent motion as one piece since it is a single-element component and its mechanical properties limit its hold on the sand. Once the cord was tight, it would stretch ever so slightly, providing a moment of shock absorption. Additional forces applied would be transmitted directly to the brick in the sand. The vertical component would lift the brick out of its shallow grave and drag it along the surface of the wet sand at a force measured on the wagon handle scale.

Wagon B with its swingset chain rode would be pulled and the first thing we’d observe is the chain being pulled in segments out of the sand. The lifting would occur slower than the poly cord since each chain segment weighs more than its corresponding poly cord length and each segment acts independently of the other. As segments closer to the brick encounter the pull from preceding segments still buried in the sand, they apply that force in the horizontal plane until that segment begins to lift from the vertical force component. Only then does THAT segment of chain begin to lift, hinging freely on its anchor end which delays the following chain segment from lifting. This additional weight, resistance to horizontal pull and segmented lifting action postpones the transmission of the vertical component of this force, allowing the brick to remain embedded in the sand for a longer period of time. The force measured on the scale at the moment of brick release would be greater than that of Wagon A.

Now let’s look at it as a force applied not smoothly over time, but as a shock in a very short burst of force:

Wagon A rode releases from the sand and stretches, delaying the full transfer of force to the brick. When the stretch reaches its maximum, the rode then applies the force in a linear manner to the brick, pulling it out of the sand.

Wagon B rode resists the pull ever so slightly, but the rapid application of wagon pull minimizes its segmented transfer of forces and causes the chain to rip violently out of the sand in a very short period of time. There’s still a bit of catenary in the chain providing minimal shork absorption, but the effects are not pronounced. The brick pulls out of the sand in a shorter period of time with a similar force on the wagon handle. While the catenary helped in this example, its effect was reduced with the rapid application of tensile force – a shock load.

How do we regain the advantage of the chain weight and catenary when the force is applied rapidly? Add a bungee (elastic) cord to Wagon B to spread the transfer of rapidly applied forces over time. From a force analysis standpoint, it doesn’t matter if the bungee is inserted at the anchor end or the boat end. But from a rode abrasion standpoint, it makes sense to insert the stretchable bungee at the boat end.

Now Wagon B with its unstretchable, but heavy chain has a mechanical component to deliver the force over a period time which is precisely what gave the poly cord the advantage when the force was applied as an immediate, shocking tension. The advantages held by the chain rode during the normal, gradually-applied, give-and-take forces experienced in most anchoring situations is fully retained with the addition of the bungee cord. The bungee itself becomes a wear item with a limited finite lifespan, but its minimal cost and ease of replacement make it an easy component to add and replace as needed.

Wagon B’s performance can be improved with the addition of a kellet, but the added complexity to deployment and retrieval prevent its frequent use. It also is not something that can be added easily during rapidly changing conditions without hauling the anchor and starting the anchor set procedure all over again.

Another option is to utilize components of each system, rope and chain, to achieve a balance in your anchoring system to meet the needs of your waters, and style of boating. For my purposes usually anchoring in less than 20 ft of water, I find 120 ft of 5/16 chain and 240 ft of 8-ply Brait to meet my needs quite nicely. Of course, it helps to have a windlass that can handle the combo rode. YMMV.

I’m sure there are holes in my simplistic analysis and look forward to hearing about them all. But in the meantime, I’ll continue to seek solace in my (primarily) chain rode and kellet-free Claw anchor system.
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Old 08-03-2013, 09:59 AM   #83
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Chain catenary (dare I say a nylon line would have less?):


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Old 08-03-2013, 10:51 AM   #84
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Al and Peter,
It seems I've got you guys going now and I'm all worn out. It takes a lot of mental effort to make posts like this. Lots of editing and thinking. My head's so full of this stuff now I'm having a hard time getting into it enough to add things up.

Other than my recent load theory (that I now think is flawed) I've never strayed from thinking chain is better for anchoring boats. But I also think it's extremely heavy and very inappropriate on the bow of a boat. I think nylon line does almost as good and so why not use it? And if one must have better anchoring performance simply get a bigger anchor. I've said this many times ... that a pound spent on one's anchor results in far a greater performance increase than the same increase in weight of the chain rode.

Mark that just shows how little it takes to pull that barge.

So I'm really on the same page as everybody else but more willing to think of things not the norm ... and basically more analytical as well.

And yes Peter I can't believe what ships get away with. Perhaps the Reynolds number gets into the act. It looks like the anchors in size are small compared to the ships. It looks like if we had the same size anchor relative to the size of our boats we'd be anchoring w 8lb anchors.

Al you place heavy importance on the angle of pull on the anchor as a result of chain catenary. But we don't even know under what conditions the chain comes up off the bottom. We know now that probably the catenary is much more operative or "in the picture" than we thought before. I say this re what Rick B posted. But having a lot of catenary is far from necessary as I have anchored extensively up and down the coast w only 10 or 15' of chain. But I only use anchors that do well at short scope and therefore aren't as dependent on catenary.

I think if we were to uncover many more truths regarding scope and catenary we'd need Practical Sailor to do a rode test (line v/s chain) and a comprehensive test on scope. psneeld talked about something that addressed the issue of scope as I recall.

There is much to be learned. There is always surprises to be found and often not where we expect them. The world was once flat ... right?
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Old 08-03-2013, 12:39 PM   #85
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I’m sure there are holes in my simplistic analysis and look forward to hearing about them all. But in the meantime, I’ll continue to seek solace in my (primarily) chain rode and kellet-free Claw anchor system.
WOW! Well done! I look forward to the next edition of Chapman's where this treatise is sure to appear.
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Old 08-03-2013, 12:42 PM   #86
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"So I'm really on the same page as everybody else....
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Old 08-03-2013, 12:53 PM   #87
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Oh please stop... You can't help it if you are not more analytical....
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Old 08-03-2013, 02:21 PM   #88
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Walt OK maybe the same chapter.

psneeld I'm going to try out the ignore list.

HaHa it worked. You're gone.
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Old 08-03-2013, 04:15 PM   #89
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Perfect...I love covert operations...
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Old 08-03-2013, 09:40 PM   #90
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Thanks everyone, Flywright and Eric especially, for the well developed thought provoking contributions and intelligent constructive criticism. You helped everyone with an open mind, to understand and learn, as well as each other. It is a standout TF thread.
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Old 08-04-2013, 10:39 AM   #91
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Craig wrote;

"what I'm mainly getting at is if a half a boat length of chain induces droop or centenary like a kellet, would it not be reasonable to assume an all chain rode would do the same. Right or wrong I look at the chain in my rode as a shock absorber, if all chain was practical on my boat I'd do it. If for nothing else the shock absorbing qualities alone."

Indeed but I think it's more useful for the catenary. And w very light loads on the rode (as when setting the anchor) the cat gain w a little chain is probably very significant. So I think chain is important for setting performance.
As a shock absorber the advantage of chain is obvious. But the advantage of nylon line as a shock absorber is super obvious if you take a 30 or 40' line, tie it off somewhere and pull on it's full length. It's like a big bungi. Hard to believe unless you've done it. The only rode w/o good shock absorbing ability is cable.

Craig wrote;

"I wonder if much of this debate boils down to a big boat/small boat sorta thing? All chain is not very practical on boats 30' and under, it starts coming into its own after 32 feet in my mind. Below that the boats can really feel the weight difference on the bow. Above say 40' the line diameters increase to the point that I really wouldn't wish to mess with rope of the diameter required for the job personally."

Very true Craig but to what degree. It is dependent on numerous things. I agree objectivity is one to be sure as I certainly do focus on my own anchoring problems and situation. I try to stay out of the box though.
But I suspect that the percentage of the weight of rode and anchor compared to the disp of a boat would be a constant. An all chain rode should be the same percentage of the total weight of a boat irregardless of the boat's total weight for equal anchoring performance. This is new ground. We've never discussed this before. Perhaps this relates to what Peter and I have said about the ground tackle of large ships.
But getting back to your point Craig that all chain on 30-32' boats or smaller is impractical. Not "impractical" I think just not necessary. You can put a windlass on any small boat w all chain and it should work just as well if it's the same percentage of vessel weight as larger boats.
There are lots of advantages to being on the bow w the anchor line in your hand. You can see where the line goes in the water, if there's weed attached to it or if your'e bringing up a big branch from the river bottom. Your'e much more aware of the whole anchoring event. But it's much too inconvenient w 3/8ths chain and a 40lb anchor. So I think it's the ability to anchor by hand that separates the two worlds of anchoring by winch and by hand.

Bruce K wrote;

" You helped everyone with an open mind, to understand and learn, as well as each other."

Yes and unfortunately no. I've played down the importance of chain and it's clear benefits to the anchor rode. Someone could buy into this too much, use an undersized line w no chain and an anchor w poor short scope performance and come to grief on the beach or worse. Standard lengths of chain recommended in Chapman's and many other places are useful, important or very important depending on many variables. Chain is not something to dispense with but to use to our advantage. Now that we have windlass's that handle chain AND line on the same rode over 50% chain on a rode is just a waste. The only reason for it is if you don't trust your line spliced into the last link of your chain.

Thank you very much for the compliment. Very appreciated.
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