How Much Chain Rode

[waddenkruiser]

Wadden,
My experience reading about wind forces produces results all over the map.

I've thought of tethering a boat in a steady wind w an accurate tension meter or scale.

Measuring frontal area is useless because there are so many little things that increase drag. How much drag is that cleat or windshield wiper. Sounds silly but there are hundreds or possibly thousands of little things that cause drag. Some things just can't be made into numbers. And approximations could be 75% (or worse) off of reality. And then there's the bottom. The sea floor. The biggest variable. By far.

Eric,
agree, all text books will give us only rough estimates on the loads, the interaction of anchor and ground etc.. We shall not take those theoretical and empirical figures as absolute numbers. These are more or less "house numbers", giving us only an order of magnitude of that what we have to expect out there.
And I also agree with the posts above claiming that all those nice theories are not valid to replace masters experience. If it comes to decisions out there I also trust on my gut and act according to experiences made in the past. My own experiences or those heard from other reliable experienced boaters.
But these theories might give us guidance and indication if we are designing a new anchor equipment for our baby. Or if we are in the planning phase for a trip to new to us destinations asking ourselves "are we well equipped?" Then theory might give us a second opinion to base our decision not only on recommendations.

And last not least: I'm tired to see anchor neighbors dragging their ground tackle in a windy night throughout the bay and telling me "don't know, I applied exactly 5:1 as I was instructed during captains course, so what went wrong?" ...


best regards / med venlig hilsen
wadden

 

[caltexflanc]

Sorry if anyone takes it personal, that's the interweb for you. I get concerned that impressionable newbies (and I certainly was one once) might actually buy into some of this stuff. If someone's posts make it seem apparent to me that they have little if any practical experience (or they've just been lucky doing the wrong thing), and that my own extensive experience couple with that of experienced cruisers that I know personally counters that, I feel obliged to counter. Much of this "experience" is based on making a lot of mistakes, be they through ignorance, misinformation, inattention, impatience or just basic ineptitude.

I don't consider myself any sort of master mariner, but just a regular guy and his regular wife who ultimately learned to be pretty OK at boating, though far from perfect.

There are a lot of topics here I don't have much if any experience with, and thus never weigh in on. But I do feel obliged to " pay it forward" for all the help I got along the way myself. If you want to throw some theoretical trial balloons out there, don't get insulted if some people endeavor to pop them.

 

[Nomad Willy]

waddenkruiser,
That's a good point. Many people aren't familiar enough w boats to notice while looking at a boat "that's about right for a boat of that size" and many want specifics dripping w specififications ("X" disp, "X" frontal area one needs an "X" weight anchor for such and such wind). Many will walk the dock and observe most 30' boats have anchors that are about 20lbs or whatever. Few will have the experience to say the've anchored 100 times in this or that wind on various bottoms and not dragged. Many on this forum have that experience but almost all head for the harbor and a stout dock when the wind is expected to blow. And w modern weather forecasts we almost always know.

Many will consider one that eyeballs the situation reckless or at best uninformed and the eyeballer will consider the by the book guys to be insecure. Some think for themselves w little knowledge or experience to guide their thinking. And others are not comfortable unless widely accepted standards are adhered to. These folks would be likely to say "according to hoyle" ...

Ideally though IMO all these sources of guidance will be used to help one make decisions that are important. It's hard to go far wrong using a lot of all the information availible, observations and personal experience plus recomendations by people and anything else we think is credible or some may even throw in "that anchor looks cool".
But re the chain question even widely accepted expert sources of information are at opposite ends of the range of possibilities. And then some say (like me) that all chain is best for anchoring performance but use and recomend short chain lengths. Chapman says "a few feet of chain" and other credible sources say all chain is best. What to do? Hence this thread.
The obvious response is to notice even the experts disagree so one is inclined to error on the safe side ... bigger anchor and more chain. And that (I think) guides most of the thinking here on TF. Also very few want to be viewed as reckless, uninformed or not very bright so the bigger (or more) is better philosophy rules the most common and accepted numbers and practices of the trawler community and TF.

 

[psneeld]

I think the main issue is some think that one anchor or another, or anchoring one way or another, or one type rode or another is better.... even in the smallest way....yet there's no proof in most cases.

Some thing that generically adding bits or pieces or subtracting them from ground tackle are weak points or hindrences.....well....no one can actually prove it and others with experience dispute it.

It's just a discussion with many possible facts or errors....not science only, not art only....

But those that use the collective wisdom of many and practice within their equipment's and experience capabilities have the best handle on how they should probably operate.

For every anchoring success story, someone can probably dispute it.

 

[cafesport]

I'm from the school of use enough chain to anchor the boat,and then a good all around anchor to anchor the chain. While danforths/ fortresses have great setting and holding power when things like little rocks, bags, reversing tides or currents don't happen, these things do happen often enough where we go, to relegate them to number two status. Im not sure how this looped video is going to work but it shows a 60' boat anchored in 50' of water in 30 knots of wind and building seas. Catenary force is probably around 1800 pounds while the wind force is considerably less leaving much of the chain laying flat on the seabed. Having dove lots of anchors throughout my life and having seen in real time how anchor rode's behave in extreme conditions, as well as having done all the math with regards to catenary forces, wind forces, rigging angles and leverage calculations, we seem to be able to get it right about 99 percent of the time...as long as you hold your mouth right.


Spell check via iPhone.

 

[Nomad Willy]

cafesport wrote;
"Danforths as #2 status"
Number two is rather high but there's no doubt about their holding power and their faults. Since most bottoms are mud or mud/sand a strong case could be made for a Dan (or Fortress style Danforth) as primary and a Claw as secondary. I've never had a Danforth pick up anything but seaweed but the fluke gap is an obvious problem.

What on earth is "catenary force"?
I keep hearing about chain anchoring boats and pass on to isle #2.

psneeld,
How does one "generically add pieces"?

 

[psneeld]

Post 53....

Sorry...I don't get it.....

 

[waddenkruiser]

Tried to post a small spreadsheet calculating the catenary (all chain and chain + rope) but failed unfortunately. XLS files are not supported as attachment.
I used it together with some text book data as a sizing tool for our equipment, refer to #91. It is definitely not intended to be used out there to decide whether it is safe or unsafe to anchor or to calculate rode length to be applied under actual conditions or whatever ...
Please PM if you are interested, would send it via e mail.



best regards / med venlig hilsen
wadden

 

[Nomad Willy]

wadden,
I don't need to calculate anything.
Just wondering what catenary force is.

 

[Capt.Bill11]

Catenary Engineering Solutions

http://home2.fvcc.edu/~dhicketh/Dif...ard_Kirk_Gordon/1Project/Kirk&ToreysDifEQ.pdf

 

[waddenkruiser]

Yep, that's it.


best regards / med venlig hilsen
wadden

 

[waddenkruiser]

wadden,
I don't need to calculate anything.
Just wondering what catenary force is.

Eric,
Don't know either. A catenary is given by
- the horizontal force in the rode which is constant throughout its length and
- the vertical force in the rode which is at any point equal to the weight of rode that point is carrying.
These are the two components of the only force a flexible rode can transfer, the tensile force. So which one should be the "catenary force" ?


best regards / med venlig hilsen
wadden

 

[Nomad Willy]

wadden,
Disagree ...
The tension on a rode would be the same along it's length if the rode weighed nothing. But the weight of the chain should vary because w catenary the angle of pull or tension will be different all along the rode and the water density deeper would make the chain lighter ect.

Also I would think the rode would need to be straight for the tension to be equal along it's length.

The link says;
"The weight of the mooring lines activly contributes to the ultimate holding capacity"
It contributes to the tension but not the capacity IMO.
Don't know if I can buy that. The weight of a chain rode pulls on each end. If the rode were level (both ends being at the same height) the tension would be the same at both ends. But on an anchor rode w catenary more pull will be on the boat. But the rode applies pull to both ends because of the rodes weight and that pull would IMO decrease the holding power of the system. The anchor is preloaded by the pull from the rodes weight decreasing the amount of windage that would be necessary to break the anchor out.

Just say'in
Not being an engineer the link is probably true but it seems to me it would'nt be.

So the catenary force is that tension in the rode that is a result of the rodes weight?

 

[Nomad Willy]

Bill,
Now I see the link most meaningful .. I think.
The math is beyond me
On page 13 the author shows a catenary curve and I question it's shape. Each half should'nt be reversable. That is if you fliped the catenary line end to end it would be the same ... in the cat curve shown. I don't think so. The catenary should'nt be a liniar line. It should be more curved near the anchor and straighter near the boat. Like most of a fish hook or one quarter of an elipse. The catenary in the drawing lackes this feature. Perhaps it's excluded only to simplify the drawing?

 

[markpierce]

My experience: lower an anchor with a short chain and rope rode or with an all-chain rode, and the anchor holds. Have good holding ground here.

 

[Peter B]

wadden,
I don't need to calculate anything.
Just wondering what catenary force is.

Ok, I think this is another one of those, 'knots per hour' type of semantic arguments. A knot is a combination term that means, nautical mile per hour. So, if you say knots per hour, you are in effect saying knots per hour per hour...

In the case of catenary, it is a function of weight in the rode and its downwards force, counteracting the horizontal pulling force, or words to that effect, I think. So catenary, is by definition a 'force', so if one says 'catenary force' you are in effect saying catenary force force...
How you calculate it?..well, it's in those formulae, which I will leave to better mathematicians than me...I just know it's there, and I love it.

 

[cafesport]

Well it would be less on the moon and less on Mars, due to their smaller masses thus having less gravitational force. But the cool thing about catenary is it works the same as hydraulics, it's all about leverage. It's how my 90 pound wife can apply 4000 pounds of force to a spring line just by standing on it, in order to reposition our boat in a slip on a windy day. It's why suspension bridges can support more weight than the cables suspending them can actually lift without breaking.

I wish you could see the animated gif I posted previously in action as the boat is literally leaping 8 footers while securely anchored in place. If it was a nylon rode the boat would be slingshotting fore and aft until the rode fatigued internally and possibly failed without warning.

I admire Steve Dashew and have read his thoughts on anchoring as well as the stuff Peter Smith posts on the rocna site but I disagree with much of it. Steve Dashew designs and constructs boats that are extremely light and perform very well as long as they stay light, much like Colin Chapman did with lotus years ago. Dashew needs to save weight and likes using ultra light, ultra strong, 1 size down chain and 6:1 scope . There is no doubt that 50 knots of wind would certainly cause that catenary to disappear. But if he had just let out at least 86 meters of chain in the in the first place you wouldn't see the famous almost bar taught chain in the photo he uses to justify his methods. If your willing to put on a mask and do some experimenting or perhaps crunch a few numbers you might be surprised at what you find out about anchors rodes and anchoring.


Spell check via iPhone.

 

[psneeld]

Well it would be less on the moon and less on Mars, due to their smaller masses thus having less gravitational force. But the cool thing about catenary is it works the same as hydraulics, it's all about leverage. It's how my 90 pound wife can apply 4000 pounds of force to a spring line just by standing on it, in order to reposition our boat in a slip on a windy day. It's why suspension bridges can support more weight than the cables suspending them can actually lift without breaking. ......

cut for space......

catenary to disappear. But if he had just let out at least 86 meters of chain in the in the first place you wouldn't see the famous almost bar taught chain in the photo he uses to justify his methods. If your willing to put on a mask and do some experimenting or perhaps crunch a few numbers you might be surprised at what you find out about anchors rodes and anchoring.


Spell check via iPhone.

I beach anchored the other day and the pros of ever inch of chain and catenary were evident....

http://www.trawlerforum.com/forums/s3/interesting-work-26604.html


Sure there are limitations on ground tackle and preferences for all sorts of reasons...but usually not the backyard guessing that abounds here and even some pros seem to do based on their point of view.

 

[MYTraveler]

Ok, I think this is another one of those, 'knots per hour' type of semantic arguments. A knot is a combination term that means, nautical mile per hour. So, if you say knots per hour, you are in effect saying knots per hour per hour...

In the case of catenary, it is a function of weight in the rode and its downwards force, counteracting the horizontal pulling force, or words to that effect, I think. So catenary, is by definition a 'force', so if one says 'catenary force' you are in effect saying catenary force force...
How you calculate it?..well, it's in those formulae, which I will leave to better mathematicians than me...I just know it's there, and I love it.

It seems to me (not an engineer) that the total force on the chain (ie, its tension) can be completely described by two components -- horizontal and vertical (net downward only), and that (at least if friction is ignored) the tension on the chain is constant along its length and that it is equal to the vector sum of those two forces. I suspect (at the risk of stating the obvious) that the ratio of those two forces is equal to the tangent at any particular point along the chain.

So, if tangent is important, being able to precisely describe the shape of the rode (which is a parabola in any event) is important, since that shape defines the tangent at every point. Knowing length and weight of the chain rode and its tension, should be the only variables necessary to mathematically describe the shape of the rode.

With that information, and if the strength of the chain is also known, it becomes trivial to calculate, the force necessary to lift the anchor end of the chain off the sea floor, and similarly, the scope beyond which the force necessary to lift the anchor end of the chain off the sea floor exceeds the chain's breaking strength.

It is also then possible to demonstrate that the chain can never be pulled upon hard enough (even disregarding its breaking strength) become "straight". As arbitrarily close to straight as one might want but not perfectly straight. Moreover, when the chain's breaking strength is regarded, not even close to straight, at least not for any typical anchor chain for a decent sized boat.

 

[waddenkruiser]

Please let me try to convince you of the laws of mechanics.
A flexible rode can transmit only a tensile force which is at any point tangential to its curve shape. No forces perpendicular to its shape and no moments can be transferred via the flexible rode (as long as there is no friction between the chain links).
This tensile force T can be divided in one vertical component V and one horizontal component H. Thus
T^2 = H^2 + V^2 (law of Pythagoras)
while obviously
tan(alpha) = V / H
with alpha as the local slope angle measured against the horizontal plane.

For the time being we don't need more formulas and we will make instead two experiments. Let's imagine we are sitting vis-a-vis at a table. Between us lies a chain on the table, 6 ft long with a specific weight of 1 lb/ft (weight obviously 6 lbf). On both ends we have mounted identical spiral springs which will later indicate the force in the chain.

First experiment:
I'm holding my end tight on the table and ask you to pull at your end of the chain without lifting it. Since the chain lies on the table which is taking the weight of the chain the only force in the chain is your pulling force, let's say 4.5 lbf. Obviously on both ends we have only a horizontal component of the tensile force. The spiral springs on both ends will be elongated by the same amount according the spring constant, let's assume your pull has caused 10 inch on both springs indicating that the force at both ends has the same magnitude.
I will mark now the length of the spring on my end on the table in order to prepare our set up for the next experiment.

Second experiment:
I will ask you now to lift your end 3 ft above the table and to pull so at the chain that only one last chain link on my side touches the table while the rest is completely lifted. The chain will establish its catenary curve and the horizontal pull on my end will be again exactly 10 inch, thus the tensile force on my end which is a truly horizontal force is again 4.5 lbf (I picked the numbers accordingly). But the spring on your end will now be elongated by 16.7 inch indicating that the tensile force on your end has increased to 7.5 lbf.
Why? Because you hold in addition to the horizontal component of 4.5 lbf the complete weight of 6.0 lbf in your hand:
4.5^2 + 6.0^2 = 7.5^2
Second observation: the slope angle alpha of the chain is 0 degrees on my end and increases continuously to 53 degrees on your end. This corresponds to
tan(0) = V / H = 0 / 4.5 on my end and tan(53) = 6.0 / 4.5 on your end.
The increasing slope angle from my end to your end indicates that the vertical component V of the tensile force and therefore the tensile force itself increases along the chain because every chain link has to carry the weight of all chain links below.

To summarize: H constant along the chain length while V and consequently the resulting tensile force T increases from chain link to chain link with the weight each link has to carry.



best regards / med venlig hilsen
wadden

 

[waddenkruiser]

The anchor is preloaded by the pull from the rodes weight decreasing the amount of windage that would be necessary to break the anchor out.
?

Eric,
Definitely not. The weight of the lifted chain gives the vertical component V and contributes via
H = V / tan(alpha)
to the horizontal holding force H at the bow. No preload on the anchor, it rather pulls our bow back towards the anchor. And H is constant throughout the applied chain ...


best regards / med venlig hilsen
wadden

 

[Nomad Willy]

Wadden,
Remember cafesports wife on the spring line? Every bit of weight on a rode above the seafloor is pushing down on the rode like cafesports wife on the spring line and pull is exerted on the line. Either the boat moves or the anchor .. or both. But when the boat is held in a specific place as by the wind the pull on the line will be felt down the rode to the anchor. And the weight of the rode develops pull on both ends of the rode. The boat is usually pulled closer to the anchor but if you could instantly change the rode from all chain to all nylon the pull from the weight of the rode would be far less and the rode would rise up and the pull that was exerted by the chain weight would all but disapear.

All that would mean is that the boat would drift upwind and the tension on the rode would be the same .. so I was wrong. The thing that would be different is that the rode would assume more catenary .. more droop in the rode. So w an anchor rode the boat would just be pulled closser.

Only if the rode was attached to a dock (fixed point) and a set anchor would the weight of the chain increase the tension on the rode. Yes I was wrong.

 

[MYTraveler]

wadden,
Disagree ...
The tension on a rode would be the same along it's length if the rode weighed nothing. But the weight of the chain should vary because w catenary the angle of pull or tension will be different all along the rode and the water density deeper would make the chain lighter ect.

I believe you are correct. i must rethink.

 

[Nomad Willy]

MYTraveler,
I agree the chain can never be pulled straight.
But can it be for all practical purposes? More or less straight at the anchor end?

I think so. But w the rode at low pulling forces like when you're setting your anchor there is usually lots of catenary and the anchor sets more readily and probably deeper. But in a gale and w a reasonable amount of chain weight the chain rode would become more or less straight and therefore have no advantage over nylon line. I think that this is true to the degree that just as Chapman states "a few feet of chain" is all that can reasonably be used to any significant advantage.

So if it's going to blow one should depoly "a few feet of chain" to insure good setting and a big anchor to hold the boat.

 

[waddenkruiser]

If the horizontally pulling force H exceeds significantly the weight of the chain the ratio
V(x) / H
with V(x) the vertical component of the tensile force T in the chain at the horizontal distance from the anchor x will become more or less independent from the position x. Thus with
tan(alpha(x)) = V(x) / H
the slope angle alpha of the chain will become independent from the local distance x to the anchor. Meaning the chain has become stiff!
We will not be able to recognize this excessive load situation by a significantly increased distance to the anchor e.g. by monitoring our anchor alarm since the difference between (ultimate) catenary and stiff chain will be in the range of 2-3 feet only under typical anchor situations.
Indication could be the significant change of the slope angle at the bow. But is there anybody monitoring this? Me not.


best regards / med venlig hilsen
wadden