Anchor Chum

[hylas]

Re:I apologize..

Sorry the calculation was wrong..

The pulling angle related to scope is as follow..
1/1 = 90°
2/1 = 30°
3/1 = 19° 30’
4/1 = 14° 30’
5/1 = 11° 30’
6/1 = 9° 30’
7/1 = 8° 10’
8/1 = 7° 10’
9/1 = 6° 30’
10/1 = 6°

Or sin  (pulling Angle) = rode lenght / Water height

<hr width=100% size=1>

 

[hylas]

ReMathematics..

I just discovered a VERY interesting web page talking about "tuning an Anchor Rode" wiich doesn't talk about behaviour but about MATHEMATICAL FACTS..

Please have a look at:

http://alain.fraysse.free.fr/sail/rode/forces/forces.htm

<hr width=100% size=1>

 

[MainlySteam]

Re: ReMathematics..

Thanks Hylas, that looks a very interesting site.

As I said I was going to try and have a closer look at the situation some day when I get time. Towards that I wonder if you have looked at the accelerations in the anchor warp involved when the boat surges , yaws, etc. I was just interested in looking at the force required to accelerate the mass of the rode (F = M x A) so accelerations in 3D are required (for example in a straight pull the chain is accelerated in the direction of the pull plus also in a direction opposed to the catenary as the catenary is pulled straighter). I suspect the force is low as the mass is comparatively small, and I assume that the accelerations are also, but was just going to look at them in case.

John

<hr width=100% size=1>

 

[hylas]

Re: Will others please add their experiences

What about thin sand on rock, the ultimate test of an anchoring system?

Sorry, but again I have to disagree.. thin sand on rock, is absolutely not “the ultimate test of an anchoring system?” but the type of anchoring ground to be avoided..

No anchor, including the “best” or the Fisherman anchor.. will work on such a ground..

On the August issue of “CRUISING WORLD” on the article “Staying put in Tricky Anchorages” John Harries, well known for his travels in high latitudes (www.morganscloud.com) stated:
“… bottoms what I call the Three Impossibles:
- small round rocks that act just like ball bearings
- smooth rock
- light, soupy mud

When faced with one of these bottom types, we don’t waste our time trying to anchor – We go someplace else…. “

And I fully agree. This is the “Ultimate” solution..

To ponapay

Oh Yes, I will accept the “practical approach” if it is well documented.. not the kind of sentence I will mostly read “Once I did anchor… “

But more than the “Practical approach”, I fully agree with the Maths.. but since the beginning of this thread I NEVER did see any mathematic formula.. only “general” beliefs..

In this case “maths” means “Catenary formula..” Not only I use it, but I have a small Excel program wich can compute all infos such as length of chain, weight of the chain scope.. ETC and I can tell you EXACTLY when the last link of chain will not longer exert an horizontal pull..

If you are using another mathematical theory, please tell me which??

“Most practical sailors know that a weight lowered down the cable improves the
holding of most anchors”..

OK.. can you make the mathematical explanation of this “BELIEF”?? and first, “most practical sailor” will disagree with the location of this weight..
To increase holding should it be placed near the boat’s bow, near the middle of the rode or near the anchor’s shank?? Do you have any idea of the difference of holding increase related to the position of that “weight”?? (should it be at ¼ or at ½?? On my side.. using mathematical formula, I will answer “as close as possible to the anchor’s shank”!..)

Considering the best solution for the location of this weight?? Do you have any idea of the percentage of increase of holding..

If you will be using “Mathematics” you will answer these questions.. Will you??


To MedMan

Yes, again I agree with you..

with no wind, or light wind “extra weight
will keep the pull on the anchor horizontal” but who cares.. The problem with anchoring is not light winds. It’s strong winds.. and then “THE” solution is not weight but SCOPE.. (and elasticity) and veering more rode decreases the pulling angle, and therefor increases the holding.. a “chum” is then absolutely useless..

My own experience?? As a liveaboard sailor for nearly 12 years now. I have also a “chum” onboard.. well blocked into the bilge and I NEVER had to use it..

And then I FULLY agree with you.. to veer more chain is so much quicker and easier.. and that was exactly my statement at the beginning of this thread..

Once more I also agree with you “a chum on a rope rode will be very useful in order to keep it well under the water and out of harms way.”


To MainlySteam, John

“Chain has a significant effect on anchoring performance” again this is a old WRONG belief..
You FEEL safer with an all chain rope as for years you have been teatched that holding was related to weight of both the anchor and the chain.. and YES it was true.. with, for example, the “Biblos” anchor.. but not longer with the modern anchoring tackle..

I will say “SCOPE” has a significant effect on anchoring performance, and I can EXACTLY tell you how much is this “significant effect” related to the angle of pull..

In his book “the complete book of anchoring and mooring” Earl HINZ says:

As the wind load increases, the CATENARY tends to disappear, virtually eliminating the shock-absorbing feature and also increasing the lead angle of the anchor shank.. at this point, a sudden gust of wind or a large wave would cause the boat to jerk against the taut anchor chain resulting either in upsetting the anchor or breaking part of the ground tackle..”

On a former answer, I gave the force of wind required to remove the catenary effect.. between 30 and 40 knots..

Earl Hinz also said:
“the catenary is seen to flatten very quickly and by the time 20 knots of wind are reached, there is virtually NO EXTENSION left in the chain and at 30 knots, the sag for all practical purpose has also disappeared. At 40 knots of wind, the chain is ramrod straight…

Hank Halstead had this to say about the role of chain in the November 1984 hurricane Klauss Disaster:

“Interesting enough, chain proved to be far less efective as a mean of securing anchorages than did the Nylon anchor rode”

Now, as I said before, if you use an all chain rode, a chum or a Nylon rope.. this will not change anything on my level of earning and living.. But I hope that my contribution will convince some participants of this forum and if I can only contribute to the safety of one single boat, that will be enough to make me happy..

By experience, I know how difficult it is to change old beliefs, but I also know, that time after time, mentality will change..

You also said “I would expect that adding significant weight, appropriately placed along the warp for most advantage would be worthwhile.”
That’s exactly the king of belief I’m fighting against.
- Can you precise what means “significant weight”?? What is exactly the “ appropriated position” and how much it will be “worthwhile”??

“I can only assume that those claiming that the weight of the anchor warp is not relevant and so, have missed something important out of their analysis”
Yes.. it is only an ASSUMPTION.. I remember ten years aggo, when I started explaining that weight had no relation with the holding and that holding was related to the surface area and the shape of this surface.. I had very often the same comments.. but now, nearly everybody agree with this theory.. I have to be patient and to wait another ten years.. :0)


<hr width=100% size=1>

 

[Guest]

Re: Anchor ideal and chain ....

My quote was ORIGINAL theory ........

Trouble is the rest si lost in the verse ...... yachting has altered the anchoring to that of easily handled anchor, lighter chain and limited scope. And yee gads - rope rodes !

These have brought about a need for a 'holding anchor' and defeated the original concept.

I don't like comparisons using horizontal pulls on beach's and such as they are not really epresentative of snatch and pull. lift and drag etc. of real boat behaviour.

I like chain rode on a reasonable weight anchor - in my case a CQR ......... the amount paid out being to some excessive - to others prudent !!! I would use a 'chum' if I thought necessary and be very happy to do so.
I have spent many happy hours swinging to the hook and will continue to do so..

My statement comes from my shipping days and that is a fair statement when taken in the context of which it was given.



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

Re: Anchor ideal and chain ....

Sorry - Chain not rope for me always.

Only time I will EVER use rope is on a stern kedge for temp use.

I also like anothers reply to your 'chain pulling stright in a blow' ........ try this : take a piece of string and a small fishing weight ........ now take each end in your hands and straighten out the string - not so easy is it ....... think about chain and a chum somewhere down that chain and now you understand why I say ----- sorry mate ------- that chain will not straighten out ......... if it does - you are definitely out in waether that is past the limits of yachts !!!!! whatever the anchor sytem you use !!!!!

Finally I think that the subject is so open for personal preference and own experience that it could be debated drunk, sober, night, day and forever !!!! Even the so-called experts fail to agree IN PRINT let alone forum DJ's !!!!!!

<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

Pulling straight ...

Now if I remember rightly from my Maths and physics days ....... as the line or item you are pulling to straighten with a weight supended at its modpoint approaches the straight - the force required to straighten that item further is not linear increase but multiplies considerably as you approach the straight - to the point that it becomes infinite at the straight .........

Meaning that you would have to have incredible force applied to straighten the chain / item....... given its weight and if with a chum - boy its blow me down - impossible.

Sorry but the old school trick of the weighted line is as true today as it ever was - and it still wins bets from unsuspecting smart-asses who think they can straighten it !!!!!



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[MainlySteam]

Re: Will others please add their experiences

>>>Earl Hinz also said:
“the catenary is seen to flatten very quickly and by the time 20 knots of wind are reached, there is virtually NO EXTENSION left in the chain<<<<

Well I can tell you that if Earl Hinz said that applies generally (it obviously will apply with short warp lengths) then he is just plain wrong. We commonly anchor in 20 m water (thats just what a lot of the achorages are here) and with 3:1 scope. To full extend the catenary means that the angle of the chain is approximately 20 degrees below the horizontal. I doubt, from my observation in around 20 knots wind, that it is even pulled up to 45 degrees below the horizontal in our situation (10 mm chain, 1.4 m forward freeboard wide beam yacht with solid dodger).

Not being a frequent occurance, I cannot recall what angle the chain is pulled out to in around Force 9 winds, but as I said before, the criticsms of shock loadings with chain in heavy wind conditions are not ones we have ever experienced. Because of depth, we are rarely able to use much more than 3:1 scope (we carry 75 m chain on the main anchor) and while we use good holding ground in heavy weather we have never dragged to the extent that it has been measurable.

The maths may be the maths, but there would appear to be strong evidence that there is something else going on that is not being accounted for. Instead of discarding the observations of what experienced people have observed, maybe we should be looking at why the maths (as it has been presented) may not represent experience.

John

<hr width=100% size=1>

 

[MainlySteam]

Re: Pulling straight ...

I agree, to pull a catenary out straight requires an infinite force. However, in the anchoring case we do not have a whole catenary and I have not seen that case analysed (a case of my not looking, rather than such not existing), as the depth of the catenary is fixed by the water depth and and we are attempting to pull what was half of a catenary straight from the bottom at the anchor to the boat once we are past the point where the last of the chain has been lifted from the bottom. Sense would indicate this force would also be infinite to for a non weightless warp.

We can look easily at a few things which put to one side the claims for nylon such as that it has an advantage of elasticity.

If we take a boat in 20 m water with 3:1 scope (60 m) of 10mm chain (2.25kg/m, but perhaps a little less in the water) then the horizontal force required to be exerted by the boat to pull all the chain into a catenary from bow to the anchor itself, that is the state where the last of the chain is lifted from the bottom, is around 180 kg. This is probably that from about a 30 knot wind for a 12 m yacht facing into the wind. Up until that point there is more "elasticity" in a chain warp than a nylon one, assuming that the nylon rode is essentially weightless and pulled out to a straight bar (ie non lying on the bottom) with next to no wind force. Beyond this point the chain maintains "elasticity" from the sag in the catenary until it is close to bar tight. Like nylon then the greater the tension the less give there will be for an increase in force, but there will be give.

It may be that when some people calculate the forces in the anchor rode catenary they use the length of chain from the bow to the anchor, whereas in fact they should, at the point where the last of the chain at the anchor end is just lifted from the bottom, use that which would apply for the full catenary ie 2x the scope. At that point the anchor is providing the opposite force that would exist at the bottom of a full catenary. Good sense would tell us that until the last of the chain is lifted from the bottom the catenary forces are less, and after the last of the chain is lifted (leaving a new more complex catenary from the anchor to the bow) the forces would be greater.

With chain, up until the catenary from the anchor is formed (ie no chain lies on the bottom), a boat on a chain rode can be expected to range around much less either by circling the anchor (because the chain on the bottom has to be dragged sideways around) and fore and aft (again because of the need to drag chain, but mostly because of the catenary that exists in that part of the chain from the boat to where the chain is lying on the bottom) provides a restoring force.

So without even exploring the effects of the modified catenary where the bottom fixed point (the anchor) is below the other end point (the bow) we can see that it takes a considerable force to even lift the last of the chain off the bottom. Up until the last chain is lifted a lesser catenary exists but as more force is applied to it at the bow end all that happens is more chain is lifted from the bottom and the boat moves astern.

Other things which come to mind and which I have not explored (and may prove to be insignificant) are the forces required to accelerate the mass of the chain in 2 dimensions at the accelerations found in the catenary when it is extended (the chain is accelerated both tangentially to the catenary and at right angles to it) and the energy required for the vertical motions in the rapid pulling out of the catenary against water resistance when the boat surges - neither of these are significant with a rope warp as it is not in a catenary.

I would contend then that all vessels, and especially light ones, will range around their anchor much less with a chain warp than one on a nylon rode and that it will take a considerable wind force (perhaps around 30 knots for a 12 m yacht) to even get to the stage where the last of the chain is lifted from the bottom. These fit in with my own observations. In the case the water is shallow, say 5 m the catenary advantages of chain are much diminished because of the reduced weight of chain laid if the same scope (say 3:1) is maintained. However, I would have to say that I would be most unhappy anchoring in 5 m of water in a 2m draft yacht in heavy wind conditions due to the likely proximity of the shore. If the shallow depth was because of a wide shallow bay, so one could anchor clear but still be in shallow water, the obvious solution would be to lay more chain.

About time I dived for cover!!

John

<hr width=100% size=1>

 

[qsiv]

Re:I apologize..

This can never ever be correct for anchoring systems on a planet with gravity, as it assumes zero catenary curve. A trivial application of calculus will prove that this state (of zero catenary curve) can never be achieved, even with infinite pull on the chain.

A simple look over the bow will also suffice - if you check the angle at which the rode enters the water (with 3:1 rode), you just wont see it at 21 degrees. If your stem fitting is 5 feet above water, then the chain would enter the water more than 13 feet ahead. I seldom see mine more than 5 feet forward.

<hr width=100% size=1>

 

[MainlySteam]

Re:I apologize..

What an excellent demonstration of what is found in practice. Worked it out for us and the chain would enter the water 12' 6" ahead of the bow, equivalent to more than a quarter of our overall length - like you, never seen anything like that at all, ever.

John

<hr width=100% size=1>

 

[Guest]

Dragging chain across seabed .....

Please put up your hands all those that have dragged mooring chain acroos the seabed to fix to sinkers etc. ?? I have and its not funny. I have done it on relatively hard ground with limited silt to quite deep silt / mud that needed extreme care and anothers watchful eyes in case of problems.

I can honestly say that dragging that chain was extremely difficult and convinced me years ago of its worth in anchoring.

Second - I have seen many boats snatching and ranging on rope rodes to convince me of its lack of stabilising effect as in chain. Why for ages have combination chain / rope rodes been advocated if a need for light - easily managed anchor system ??? Are ALL mariners wrong as HYLAS suggests ???? Why for ages have experienced blue-water and professionals stuck to chain and more chain supposedly misguidedly as HYLAS suggests ??????

Sorry but I am definitely not convinced, as maths is great on a blackboard etc. - but riding an anchor is a serious matter and I would like to feel safe and secure.

Catenary is a matter that is fact, weight is fact ....... nylon rope is nylon rope and should stay where its intended - mooring and other rope duties - not risking owners boat and others. Yes I am serious and I am very serious about others mistakenly learning false advice.

Stow the rope and listen to experience - not some farm tractor and beach experiment that is flawed before it even gets started.

For me its QED - chain and a chum if needs even more curve / catenary to dampen ......

Das ve danya


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Jacket]

Re: Mooring rode

<with my car,(when I had one!) when the parking place I have found is smaller than the car.. I always go somewhere else.. So do I also with my boat..>

You obviously weren't trying to anchor over the bank holiday weekend. I had to look in three anchorages just to find one with a space that was too small! Crowded anchorges like this seem to be a fact of life in much of northern Europe, at least at weekends.

<they are much more boats using Nylon anchoring rode than all chain rode.>

I wish someone would tell the boats I meet. I always seem to be the only boat using a nylon anchor warp, and it causes chaos in a crowded anchorage when the tide changes, or if there's a gusty wind.

I agree with you that Nylon has many advantages, but for crowded anchorages, untill everyone strats moving to nylon, using chain is the way o go, just because you'll swing the same as everyone else.

<hr width=100% size=1>

 

[vyv_cox]

Re: Mooring rode

The argument is now entering its third circle. I posted much the same as you and Nigel a couple of weeks ago.

I still maintain that there is no single answer. Very few of us anchor in the situation that we need elasticity in the rode to take the shocks that might be caused in onshore winds of 50, 60 knots. If I was in this situation I would include a length of nylon in my chain, but not instead of it, just hooked to form a bight in the chain.

As you say, in crowded anchorages, boats on rope warp are a pain in the bum. Even if all are on rope they move very considerably and there will inevitably be clashes. In the vast majority of anchoring cases the chain lies on the bottom and restricts boat movement to a little yawing. The swinging that results from rope warp is a very different thing. But the weight of a big anchor and chain is considerable and it takes some recovering. So I carry both arrangements. If the situation requires it, and for me this is always for overnight stops, I use my Delta and chain. If not then I use the Fortress, 7 metres of chain and the rest Anchorplait.

Maybe it's time for a new angle - what about a chum on rope warp for crowded anchorages?

<hr width=100% size=1>

 

[AlanPound]

Re: Will others please add their experiences

The maths may be the maths, but there.... <snip>

... I think that is the issue ... the maths is provably correct, but is it the right maths?

The forces and conditions of a boat of such a weight and with such windage, in a blow and a chop, with such a length of rode of such a weight, with an anchor of such a holding, would be so, so difficult to model convincingly. The scientific method would be to eliminate as many variables as possible in order to inspect each one in turn. But basically that is never, ever a realistic situation...

In theory, there is no difference between theory and practice - but in practice, there usually is....

... in the mean time, most people will prefer to use an all chain rode; with a length of nylon as a snubber attached using a chain hook, and properly chafe-protected oved the bows, and maybe an anchor chum as well - if they can be bothered, or are sufficiently concerned (ie shit scared)....

Alan

<hr width=100% size=1>

 

[Jacket]

Re: Will others please add their experiences

Not only is it a question of is the maths right, but every "Scientific test" to do with anchoring that i've seen so far has been seriously flawed in one way or another, so like you I'll go with practice.

Just a thought. For many centuries, all ships used rope rather than chain anchor rodes, because making chain by hand was ecpensive. When the industrial revolution made chain affordable, all these sailing ships went over to chain. Now given that this was in the days (just) before ships had engines, they spent much of their time at anchor, rather than tied up alongside, and so must have known a bit about anchoring.

So I wonder why they changed from rope to chain. Was it because they found from practical experience that chain was better? Or did they just think that it was, because (flawed?0 theory suggested it was? Or was it purely because the natural ropes of the day were prone to rot, and so chain was more reliable? It would be interesting to know the answer, but I haven't a clue where to start looking.

<hr width=100% size=1>

 

[hylas]

Re: Mathematics..

To Nigel Luther

I don’t believe we could find any agreement on this subject as we are using two completely different theories :

Yours, would be called “conservatisme” as I’m trying to demonstrate mine using “mathematics”..

I have to admit that I’m quite surprised by the reactions to this thread, I was expecting much “open minded” contributions.. but from my experience, I perfectly know how it should be difficult to introduce new “disturbing” theories… mainly when they are quite different from old beliefs that have been in use for centuries.. People have believed that the earth was a flat disc for centuries… and it has not be easy to convince them that it was round…

“ I like chain rode on a reasonable weight anchor, the amount paid out being excessive, I would use a 'chum'. I have spent many happy hours swinging to the hook and will continue to do so”

Yes, I understand well.. and I believe I will loose my time trying to convince you that it could be better alternatives..

“Think about chain and a chum somewhere down that chain and now you understand why I say that chain will not straighten out .........”

You are prefectly right.. to have the chain perfectly straight, needs an infinite pulling force. But this is pure theory.. Like the effect of the scope: to have a perfectly horizontal rode, you need an infinite length of rode.. but as I have demonstrated before, with more than 6/1 scope, the pulling angle change very little, and with a scope greather than 10/1, any additional large length of rode will have nearly no effect on this angle..

It is amazing to see that you don’t have a precise idea of where you will use your “chum” “somewhere down that chain..”

Do you want “mathematics”??

You can consider several alternatives:
If you vant to know the length of rode (Lup) to pay out to keep the anchor shank horizontal, in relation to the pulling force you may use the following formula:

With the height H, the active rode length L, the rode weight (in the water) per unit of length w, and the pulling force F.
Any additional length will not have anny effect in keeping the pulling angle horizontal.. then, take your computer with you and you can keep your swinging circle as small as possible without loosing efficiency of your anchor..
Another way, is to compute until which pulling force you will keep your anchor’s shank horizontal, in relation with the length of rode you paid out?? The formula is:

Or if you prefer, use the following formula to compute the minimum scope N (ratio L/H) to keep the shank horizontal for a pulling force F:

This equation shows that, contrary to a popular belief, the minimum scope depends on both parameters F and H. An example will be more convincing:
Let's take a 8 mm (5/16") chain. What is the minimum scope to withstand F = 89 daN (200 lb)?
Depth H Minimum scope N Minimum chain length Lc
4.6 m (15 ft) 5.6:1 26 m (84 ft)
9 m (30 ft) 4.0:1 37 m (120 ft)
18 m (60 ft) 2.9:1 54 m (176 ft)
Thus, choosing N = 4 is adequate for a 10 m (30 ft) depth, but it is noticeably insufficient for 5 m (15 ft) depth, and overabundant for 20 m (60 ft)!
More generally, with an all-chain rode, shallow anchorages need relatively large scopes, while deep anchorages make the best of moderate scopes.

About “Chum”???
The formula give the critical value Fc that lifts an homogeneous rode completely:

If we add a weight K at distance Lk from the bow, what improvement can we expect beyond Fc?
There is no simple expression of the result, but if we assume the scope is greater than 3:1, an acceptable approximation (with less than 6% error) is given by:

These expressions show the improvement equals the chum weight K multiplied by the ratio Lk/H. Consequently, the best improvement is obtained by putting the chum NEAR THE ANCHOR, where it equals the chum weight K multiplied by the scope N.
With the same chum in the middle of the rode, the improvement would be halved.
An all-textile rode with a chum close to the anchor has the same performance as an all-chain rode of same length, with only half the total rode weight. This confirms that concentrating the weight down the rode, if possible, would be much more effective than spreading it along the rode.
Unfortunately, handling capabilities limit the weight of actual chums around 22 kg (50 lb), which is insufficient in severe wind conditions unless the scope is very large.
In addition, using a chum does not significantly improve the swinging radius R.

MainlySteam

Let’s take your example: “a 12 m boat in 20 m water with 3:1 scope (60 m) of 10mm chain (2.25kg/m) (the weight of the chain in the water will be 135 kg)

The force required to lift the last chain’s link from the horizontal is 190 daN or equivalent to a static force exerted by a 32 knots of wind.. (your calculation of 180 kg is nearly right..)

With this strenght of wind, the maximum dynamic overtension (wind gusts) will be around 1330 daN and the angle of the last chain’s link with an horizontal sea bottom will be around 17°.. as the maximum angle achieved with a 3/1 scope and a straight rode will be 19° 30’ we can say that with this relatively moderate wind, (32 knots) the rode is nearly bar tight..

With a wind force of 50 knots, the chain will form one angle of about 12 ° with static pull and 18° during the maximum dynamic overtension..

Then, Yes it will take a considerable force to reach the theoretical 19°30’ angle, but with a relatively light 32 knots of wind, and during gusts, the efficiency of the anchor is largely diminished.. (pulling angle of 17°)

To nigel_luther (again)

I gave the figure we have measured when pulling a chain on hard sand.. very low by comparison with the holding of an anchor…

??? Are ALL mariners wrong as HYLAS suggests ???? Why for ages have experienced blue-water and professionals stuck to chain and more chain supposedly misguidedly as HYLAS suggests ??????

Sorry but again you are confusing things.. The invention of synthetic fibers is relatively recent.. and their use in the yachting world even more recent.. Before, and when only “natural” fibers have been in use for ropes, the resistance of ropes was very low.. and that’s why chain has been so largely used.. Also with old “fisherman” anchor designs, the weight of the chain was very important as a part of the anchoring tackle..

Today, if Bristish and even more French sailors are quite conservatist.. and mostly use an all chain rode, Americans are more using a combinaison or some chain against chafing and then Nylon rope.. considering the relative size of these markets, I will say that they are more sailors anchoring on rope than with all chain rode…
Are American sailors less wise than European ones??

Maths is a precise science.. Feeling secure and safe is “psychology” and has noting to see with precision..
The fact that you don’t agree about my theories doesn’t necessary means that I’m mistakenly giving false advice. And please consider that I’m not only talking about theories, but I’ve been sailing for about 45 years and I’ve been a liveabord for about 12 years now.. My anchoring knowledge doesn’t come only from pulling anchors horizontaly on a beach.. but I have some “blue water” practical experience too…

And my advice is some chain (twice the length of the boat) and then NYLON rope and NO chum..

(ALL MATHEMATICAL FORMULAS ARE EXTRACTED FROM THE WEB SITE: http://alain.fraysse.free.fr)
(Sorry the mathematic formulas didn´t show up.. I may find another way to publish them.. more tomorrow..)


<hr width=100% size=1>

 

[AlanPound]

Re: Mathematics..

I have to say that Hylas may well have a point or two here, although he does himself no favours by the way he presents it. But there does seem to be some contradictions too...

The point I accept as probably correct (to a degree) is from:

"with more than 6/1 scope, the pulling angle change very little, and with a scope greather than 10/1, any additional large length of rode will have nearly no effect on this angle.. "

... Ok, sin-1 of 6:1 scope is 9.6 degrees (assuming a bar-taut rode, which although impossible, represents the worst case), of 10:1 is 5.7 degrees. If a particular anchor shows no tendency to break out with a shank angle below, say, 10 degrees (and I cannot imagine that break-out would be a sudden catastrophic event at a precise angle), then veering more rode to decrease the angle of pull from 9.6 to 5.7 degrees will provide no benefit (although this does disregard the fact that there *will* be a catenary, and that *will* buffer the anchor from the extremes of the impulsive loads caused by the action of wave and wind). But moving on....

He doesn't show the formula, but gives the examples of depth, scope and lengths (of *chain*, not *rope*):
"4.6 m (15 ft) 5.6:1 26 m (84 ft)
9 m (30 ft) 4.0:1 37 m (120 ft)
18 m (60 ft) 2.9:1 54 m (176 ft)"

... so what is the deal here? In deeper water (assuming the formulae, and the figures to be correct) we need less *scope* than in shallower water. Why? As this is not simple straight-line geometry, the only other variable beyond simple triangles and the ratios of the sides, is the *weight* of chain deployed is greater in deeper water than is shallow water. Without considering whether it is an effect of the catenary or not, I think Hylas has provided counter-evidence to one of the points he was actually trying to establish (that weight of chain has no benefit...) And I think this also implies that a "chum" has a benefit as well, as it adds to the weight.

... however, in respect of "chum", he says

"Unfortunately, handling capabilities limit the weight of actual chums around 22 kg (50 lb), which is insufficient in severe wind conditions unless the scope is very large."

... now I thought your view, Hylas, was i) a chum did nothing of value, and ii) scope over a certaim value did nothing of value.... Hmmmm

And then you say:

"In addition, using a chum does not significantly improve the swinging radius R."

... well, if as I somewhat suspect myself, and I am prepared to argue the point, (without the benefit of an Excel spreadsheet), a chum has the effect of breaking the catenary up into *two separate* catenaries, and the catenary closest to the anchor having the appearance of a catenary produced by a rather longer rode than is actually present, and with the corresponding improvement in angle of pull on the anchor. If this is so, then we can say that adding a chum is rather equivalent to veering extra scope. Accordingly, adding a chum would allow you to shorten scope whilst retaining the same angle of pull on the anchor. ie. You would lessen your swinging circle....

In fact, right now, I am not quite sure what points you are actually trying to make, apart from the one that commonly held views that have been established by very many people, over centuries (probably), and widely believed today, are in fact wrong.

I would say that, although such a situation is obviously possible, I don't feel you have made the case with any clarity here...

Alan



<hr width=100% size=1>

 

[AlanPound]

Re: Mathematics..

... all of this above does make a rather unhealthy set of assumptions that:

- it is only angle of pull that makes an anchor break out
- the nature of the bottom is such that the anchor will not drag
- the impulsive forces from wind and wave are not considered
- the boat sheering around, or tide, won't cause the anchor to roll out
- the anchor won't break (period)
- given the conditions, I wouldn't rather be in a marina than anchored...
- or better still (personally) in the pub....

Alan

<hr width=100% size=1>

 

[MainlySteam]

Re: Mathematics..

>>>Yes it will take a considerable force to reach the theoretical 19°30’ angle, but with a relatively light 32 knots of wind, and during gusts, the efficiency of the anchor is largely diminished.. (pulling angle of 17°)<<<<

No. In the example, at 32 knots of wind the pulling angle is parallel to the bottom. It is at that point that the last of the chain is picked up off the bottom. Beyond that point the pulling angle is no longer parallel to the bottom and a complex catenary is formed where the extension of the catenary, for analysis purposes, extends down through the sea bed ie the depth of the catenary to be calculated increases, whereas up until that point the depth of the catenary, for calculation purposes, is the water depth.

I do not know how you are calculating the catenary, but just in case, I will mention that until the point all the chain is lifted off the sea bed the catenary width is 2 x the horizontal distance from the bow to where the chain first touches the bottom and the depth is the water depth. At the point when the last of the chain is just lifted, then the catenary width is 2x the distance from the bow to the anchor and the depth of the catenary is the water depth. In both those cases the chain weight to be used in calculationg the force is twice the length of the chain from the bow to where the chain first touches the bottom.

In the case after all the chain is lifted then the catenary, for analysis purposes, is deeper than the water depth and is wider than than 2x the distance from the bow to the anchor. The length of chain to be used for calculating the force in the catenary is the full length of the chain from the bow down through the catenary (whose depth is deeper than the water depth) up to the point at the same level of the bow - this horizontal distance is obviously a greater distance than that from the bow to the anchor. I have not sat down to try and work out a case, but I would suspect that the chain in a catenary would fail quite some time before it approached a straight bar between the bow and the anchor.

In all cases it is important to note that the chain weight to be used in calculating the catenary is not the weight of the chain in use, but rather the weight of chain in the fully extended catenary.

As you say, when the angle at which the catenary intersects the sea bed increases then at some time the holding of the anchor will fail as it will not be able to withstand the vertical component of the catenary force at it. However, that is not a disadvantage of chain over nylon as a nylon rode, if considered essentially weightless, as the angle of intersection of a nylon rode will always be greater than that of a chain warp (the difference between them decreasing as the wind strength increases).

When the angle of the catenary from the bow is large (ie at low wind loads) then because a nylon rode extends a smaller angle from the horizontal, the force in the chain will be larger than that in a nylon rode to give the same horizontal opposing force, however, as the horizontal wind load increases the angle in the chain approaches that of nylon, but in the end a chain road must always bee in greater tension that a nylon rode. Significantly, this effect is maost when it matters least ie at low loads. It does, however, mean that over and above the weight of the chain itself a chain warp is "heavier" to retrieve (it is noticable on our boat that in 20m of water, if one tries to manhandle the chain up it is virtually impossible alone if the boat is set back at all against the chain, even though the chain weight alone is only around 40kg).

In the end, the only advantage nylon has over chain is increased elasticity for very small scopes. However, that is overplayed because there is actually not that much extension possible from such short lengths, and as I have said before, I, for one, would not be wanting to anchor in such shallows in extreme conditions because of the likely proximity of the shore if one did drag. If that was not avoidable, because of a wide shallow anchorage, one would (whether using chain or nylon, prudently move into water further from the shore danger and lay out a longer warp length anyway.

As qsiv brought up, and I go along with completely, the acid test is that none of us have seen the chain extended out in the manner it would be if it was ever pulled straight, nor indeed anything approaching that. In the example of our own case used, for a straight chain the chain would enter the water a distance nearly equivalant to 1/4 LOA ahead of the bow. It would be the case that that is probable with a weightless rode and if one thing has come out of this is that I must remember to keep much further clear of the bows of vessels on nylon rodes in anchorages.

John


<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 28/08/2003 22:55 (server time).</FONT></P>