Anchor Rode Poll

[psneeld]

Scope angle is about anchor digging ability while catenary and snubbing are a different issue. While both are inportant, I don't see that they are interchangeable and yet both important to holding power.

 

[Nomad Willy]

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?

 

[jleonard]

Food for thought about the catenary because catenary is claimed to be the reason to use all chain.
?

Not for me. My main reason is so my line cannot be cut if a boat motors by too close or drifts.

 

[psneeld]

For the almost uncountable time...catenary is NOT the only reason why cruisers use all chain.

Plus I am not imagining my boat is a tree...I have plenty of real and theoretical experience in anchoring to know what is happening. Usually not what some try to imagine here.

My experience tends to match mainstream anchoring thoughts.

The best statement about anchoring so far was about variables that only lots of experience can evaluate...or extensive research and some common sense.

 

[Marin]

Plus I am not imagining my boat is a tree...I have plenty of real and theoretical experience in anchoring to know what is happening. Usually not what some try to imagine here.

But psneeld, if we didn't have all the armchair theorists we wouldn't have a forum.

You need to get with the times and imagine your boat is a tree and your anchor is an airplane. Only then will you truly understand how to properly operate your boat(s). Don't start sticking reality into these discussions because it will only confuse everyone.

 

[Nomad Willy]

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.

 

[Marin]

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..

This is true but that's not the important point in this particular armchair theory discussion. What's important is lowering the angle of pull on the anchor by the boat. What the weight does to the angle of the chain between the weight and the anchor is pretty much irrelevant. The weight is not the thing being blown around by the wind and whacked by the waves.

So, sure, hanging some weight along the rode will slightly raise the angle of the rode going to the anchor if the weight is sufficient to put some pressure on the chain between it and the anchor.

But that's not what matters. What matters is that you're lowering the angle of pull by the boat. This is why a kellet can be very effective in helping an anchor stay set if the winds and waves kick up.

 

[Old deckhand]

The best use of our very heavy spare anchore was to slip it down the tow line when towing another trawler in heavy seas. It smoothed out most of the heavy jerking created by the seas. Now if the othe boat was a tree........

 

[waddenkruiser]

Not sure whether I've got all your arguments due to my limited vocabulary. If not I have to apologize ... but I have to admit that I'm an engineer having a weakness for mechanics ...

Yes, the angle between the rode and the ground should be small, let's say below 10 degrees. Why? The more the rode force acts horizontally at the anchor the better the anchor holds in the ground. (That is behind textbook rules like "1:5 slope needed" ...)

But this is only relevant for the safety of our boar if the rode has already become "stiff" i.e. it has become a straight line from the anchor to the boat. As long as the rode force (the force resulting from the boat pulling backwards) is not high enough to lift the last meters of the rode from the ground there isn't any force acting at the anchor! And therefore no need to stay below a specific angle for the rode slope ...
... and that is one advantage of a chain compared to a rope: there is a higher force needed before the rode becomes "stiff" due to the higher weight per length.
... and here is the disadvantage of an all chain rode: if the chain has become stiff, there isn't any elastic effect as it would be with a nylon rope, the chain acts immediately hard and directly at the anchor without any "last margin" left.

But as long as the chain didn't become stiff the chain has a tremendous advantage: the chain compensates the kinetic energy of the moving boat with the potential energy needed to lift the chain from the ground - without any force acting at the anchor itself!
So, if we have enough chain out (enough rode weight to be lifted by the moving boat before the rode becomes stiff) it is all about catenary ("Kettenlinie" in German) and nothing about an angle.


best regards / med venlig hilsen
wadden

 

[Nomad Willy]

Deckhand,
You're not really old are you?
Because my ground tackle was light I made a Kellet form the ballast weights on fishing floats w a little mast/stick on top. The weight was a bit below the float. It was only 12lbs and I've never used it. But 12lbs is almost as heavy as many anchors I've used. Having almost all nylon rodes (big rubber band) I've never been short on shock absorbtion.

 

[markpierce]

... and here is the disadvantage of an all chain rode: if the chain has become stiff, there isn't any elastic effect as it would be with a nylon rope, the chain acts immediately hard and directly at the anchor without any "last margin" left. ...

To avoid that, use a snubber.

 

[Nomad Willy]

waddenkruiser,
Very good post.
Nylon line re catenary and the resultant lower rode angle of pull on the anchor is vastly inferior to chain. The catenary becoming almost gone with chain only in extreme conditions that would require an excellent bottom and a very high holding power anchor. But the advantage of chain only applies to the lower half of the rode connected to the anchor. Removing the upper half of the chain rode and instead using nylon line would not cause the holding power performance of the rode do suffer.

In posts years ago somebody brought up information (source unknown) that indicated the best place for weight on a rode per the desirable effect on anchor holding was about 15 or 20% up the rode from the anchor. At that point I assumed the optimum place for weight was right at the shank end.

During my 8 years in SE Alaska I found that the fishermen mostly had about 2/3rds of the rode in nylon line. The chain at the anchor shank was very heavy (sometimes studded chain) and one or two descending chain sizes followed that. Some had only two sizes but few had only one like pleasure boats. The reason for this arrangement should be obvious. Concentrate the weight of the rode near the anchor. Here are some pics that illustrate the practice. Many have so little chain you can see the line underneath.

I'm supportive of chain on anchor rodes as is Chapman. But only in limited amounts. Twenty to 30% chain should make an optimum rode. And if more catenary and a more optimum angle of pull is desired just use heavier chain.

waddenkruiser wrote;
"As long as the rode force (the force resulting from the boat pulling backwards) is not high enough to lift the last meters of the rode from the ground there isn't any force acting at the anchor"
I read this as saying that as long as there is chain on the bottom there's no pull on the anchor. I can't come close to agreeing with that but otherwise a good analytical post. Head and shoulders above the typical post on this form ... and others too probably. I look fwd to more analytical contributions.

 

[Old deckhand]

Deckhand,
You're not really old are you?
Because my ground tackle was light I made a Kellet form the ballast weights on fishing floats w a little mast/stick on top. The weight was a bit below the float. It was only 12lbs and I've never used it. But 12lbs is almost as heavy as many anchors I've used. Having almost all nylon rodes (big rubber band) I've never been short on shock absorbtion.

You are correct, 71 years young. We carry 30 fathoms of gold line for towing. That day the seas were about 15 feet and curling on top. When the two vessels were out of sync the line would come very tight and the anchor would shoot straight up about 30 feet in the air. Several times I thought we would lose the anchor and then the heavy jerking would pull a cleat out of our boat.

 

[Mako]

But the advantage of chain only applies to the lower half of the rode connected to the anchor. Removing the upper half of the chain rode and instead using nylon line would not cause the holding power performance of the rode do suffer.

The chain at the anchor shank was very heavy (sometimes studded chain) and one or two descending chain sizes followed that... Concentrate the weight of the rode near the anchor.

I'm supportive of chain on anchor rodes as is Chapman. But only in limited amounts.

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.

Even Steve Dashew and Peter Smith have commented about the realities of catenary, in heavy winds of course.

This discussion is about catenary and certainly does not apply to other advantage of chain like chafing.

 

[FF]

"This discussion is about catenary and certainly does not apply to other advantage of chain like chafing."

Once the breeze picks up the catenary on chain is long gone.

The elasticity of nylon remains to soften the anchor load in gusts.

Yes even 3/4 nylon and larger gets like a snubber in good breezes.

For a named storm , where you KNOW its gona blow , if there is a eye splice at the bitter end of the line a few ft of heavy (we use 5/8 chain!) can be shackeled on to secure to the boat with no chafe thru..

Better to have the bow damaged by the Heavy chain leading overboard than loose the boat.

 

[waddenkruiser]

I read this as saying that as long as there is chain on the bottom there's no pull on the anchor. I can't come close to agreeing with that ...

Eric,
you are right if you don’t agree, it was simplified too much and it is indeed not correct in every case. It is correct in case 1 (please refer the enclosed figure), it can be correct or not in case 2 and it is not correct in case 3. In a nutshell, valid for an all chain rode while anchoring on sand / mud ground and again very simplified: there is no force acting at our anchor as long as the weight of the chain laying directly on the ground is 1/3 higher than the force which is pulling our boat backward.



Let’s assume our boat at anchor (see figure), anchor at x = y = 0, boat at x = x0 and the bow at y = f above the ground. The rode has the length L and the weight per length q.
Wind or current or whatever is pulling at our boat with the horizontal force Hw. The result is that the rode becomes a catenary y(x) following the hyperbolic cosine between the boat position x0 and the position x1 given by the point where the rode loses contact to the ground. The length Lx of the rode which is not laying on the ground (formula for Lx given in the figure) may be equal or less the rode length L (case 1 in the figure: Lx = f < L, case 2: Lx < L, case 3: Lx = L).

The rode can only transmit a tensile force i.e. a force tangential to the catenary y(x). This tangential force is given by T = sqrt (H² + V²) where V is the vertical force component resulting from the rode weight and H is the horizontal force component. While H is constant along Lx the vertical component V is a function of the coordinate s(x) along the lifted rode, V(x) = q * s(x). Obviously V becomes a maximum at our bow with V = q * Lx i.e. the highest tensile load of the rode is located at our bow.

Back to the horizontal force Hw pulling our boat backwards and which has to be compensated by our ground tackle (anchor + rode) to keep our boat in place. To make it more handy I will give some figures which fit to our own boat as an example.
Let’s assume we are anchoring at a water depth of 4 m, our bow (1.7 m above water line) is then f = 5.7 m above the ground. Our 8 mm chain has a mass of 1.4 kg/m giving a weight per length of q = 13.7 N/m. We have put out all our chain, i.e. L = 55 m (180’), and since there is no wind and no current there is no horizontal force Hw acting at the boat, i.e. Hw = 0. The chain is hanging straight vertical, Lx = f = 5.7 m, and the position x0 of the bow is 49.3 m (distance from the anchor). This situation is case 1 in the figure.

Now a moderate breeze establishes. The breeze acts as a horizontal force Hw > 0 pulling on our boat. Hw may be estimated from the wind velocity, a drag coefficient and our boats area against the wind, e.g. I’m assuming Hw = 38 daN (85 lbf) for our AMS 40’ at a constant wind velocity of 15 knot. Hw will push our boat backwards. More and more chain will be lifted from the ground and the chain becomes a catenary with a certain slope angle alfa at our bow. Our boat will stop to move backwards when the horizontal component H = T * cos(alfa) of the rode force T(x0) pulling at our bow equals Hw. The corresponding data Lx and alfa of the catenary have to be derived by an iterative calculation from the formulas. For our example Hw = 38 daN it is Lx = 18.7 m (alfa = 33.9°). The wind has pushed us back 4.5 m (x0 = 53.8 m). This is situation 2 in the figure and there are still L – Lx = 36.3 m (or 50.8 kg!) of our chain laying directly on the ground. Since there is a certain friction between the chain and the ground the chain on the ground can develop a horizontal force Hc = (L-Lx) * q * fm, in our case Hc = 40 daN with a friction coefficient fm = 0.8 (text book data for a track vehicle on farmland).
In this example the chain laying on the ground alone is able to pull horizontally at our rode with the force needed to keep our boat in place, even without anchor!
The weakness of this is of course the friction coefficient. It will vary according to the ground material, vegetation, the chain geometry etc. It is uncertain to be 0.8, it might be less and I wouldn’t rely on it, but with fm = 0.75 it gives us at least a rule of thumb: there is no force acting at our anchor as long as the weight of the chain laying directly on the ground is 1/3 higher than the force which is pulling our boat backward.

However, we have an anchor at the end of our chain and we will need it definitely if the breeze develops to a 30 knots steady wind: the force is now Hw = 153 daN (335 lbf) and the corresponding catenary would be Lx = 36.1 m (x0 =54.4 m), alfa = 17.9°. There are only 18.9 m of chain left directly on the ground, even with our optimistic fm = 0.8 those 26.5 kg steel would only be able to pull with Hc = 20.7 daN, we need the anchor to hold the chain at the ground and us in place.
What is now the benefit of those 18.9 m chain laying on the ground? They have to be lifted if a gust or a high swell pushes our boat back. The kinetic energy of our boat is transformed into potential energy of the lifted chain and this is slowing down / damping the movement of our boat!

Wind develops further to a gale with 46 knots so that the complete chain has been lifted (case 3 in the figure). Corresponding data are Hw = 360 daN (810 lbf), Lx = L = 55 m, x0 = 54.6 m and alfa = 11.8°. Fine, but if we are facing a gale it will not be a steady load on the boat and our ground tackle due to swell and gusts. A small movement of our boat of just 0.1 m further backwards and our chain becomes complete stiff and all of a sudden there is a horizontal force of H = 725 daN (1’630 lbf) tearing brutally with a slope angle of 6° at our anchor. Now we have to rely severely on the holding power of our anchor, if might start to drag now.
And we wish to have an elastic element between the chain and our bow to reduce the impact loads on boat and ground tackle by damping effects, this elastic element can be the nylon rope of a combined chain / nylon rode or a snubber for an all chain rode.

Conclusions from our nice theoretic excursus?
If you want to anchor, use an anchor (! ) and a chain at the anchor.
The weight of the rode chain is useful since its catenary will slow down movements of our boat + it helps to ensure a small slope angle at the anchor shaft.
And at ultimate load conditions we want to have an elastic element between chain and bow.


best regards / med venlig hilsen
wadden

 

[READY2GO]

I really don't think what fishing boats do or don't do is relevant. As time is money I imagine they are either fishing or underway to unload their catch. In contrast we as pleasure boater/cruisers may anchor for days on end. Their is a recent thread about taking a beating on a mooring from unexpected conditions. This is more "real world" to us. When I anchor I anchor for a blow every time. We may intend to be at a place one night and it turns out to be five. We may expect to get 15 knot winds and get 30 knots with higher gusts.

To me setting the anchor properly with more than adequate ground tackle is worth the extra effort and or expense. It lets me sleep at night not having to worry about what may come at o'dark thirty.

I see no big disadvantages to using all chain. As far as weight goes, I don't care. An extra 200 lbs. of chain an a 24,000 lb. boat is nothing. As far as expense it is worth it to me.

But again, we all anchor in different areas under different conditions. The important thing is to do what you feel comfortable with in the conditions you find yourself in. If you are anchoring in the Bahamas I don't think you would want 10' of chain and the rest nylon. If you were anchoring in Alaska in 150' of water you may not want all chain. Lots of variables. There is no ONE right answer for every boat.

 

[Nomad Willy]

I'm making a new thread as this thread has been thoroughly hijacked.

And the above posts are addressing simply the value of chain .. not the bias of weight along the rode. Makobuilders is an exception.

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.

New thread is "Anchor Rode Weight Bias".

 

[psneeld]

Considering the title of the thread...not sure who the worst hijacker really is.

Physics problems using wrong or incomplete assumptions don't lead to facts... 2 trees remain stationary, wher my boat does not.....

Even with total agreement that 200 pounds, 5 feet from the anchor end might be best...good luck convincing real cruisers to use that setup over conventional wisdom...and I do use the word wisdom with purpose.

 

[waddenkruiser]

What is now the benefit of those 18.9 m chain laying on the ground? They have to be lifted if a gust or a high swell pushes our boat back. The kinetic energy of our boat is transformed into potential energy of the lifted chain and this is slowing down / damping the movement of our boat!

Additional remark regarding dynamic loads on our ground tackle.
The kinetic energy of our boat which is “stored” in the catenary can be calculated as the difference of potential energy between case 1 and case 3 in the figure above. Assuming the data from the example above one can derive the following table.
f = 2.7 m => H = 768 daN (1’727 lbf) for Lx = 55 m (180’) => Energy = 630 J (465 ft lbf)
f = 5.7 m => H = 361 daN (811 lbf) for Lx = 55 m (180’) => Energy = 1’218 J (898 ft lbf)
f = 10.7 m => H = 187 daN (420 lbf) for Lx = 55 m (180’) => Energy = 1’949 J (1’438 ft lbf)
While the specific number are not really of interest the trend behind might be:
The deeper the water (the lower the slope ratio at same rode length) the lower the maximum horizontal (holding) force H but the higher the kinetic energy we can tolerate!
(There is the more potential energy “stored” in a catenary the higher we have to lift the chain links.)

Conclusion from that: It might be beneficial to anchor in deeper (not in shallower !) water if we have to consider dynamic load situations (gusts, shifting wind directions, swell …). Precondition is that the remaining holding force H stays above the required static force.


best regards / med venlig hilsen
wadden

 

[READY2GO]

The intent of the thread was to see what kind of rode people use. It has fulfilled that and a discussion of why they use what they do has ensued. As the op of this thread I see no drift.

 

[BandB]

Wifey B: Can I say "damn" on this forum? I mean I thought anchors were a horrific topic of wild inflammatory self-endorsed expertise and opinion but anchor rode. Omg. You all think you have the one and only answer. I'm glad I'm not a scientist and just know I like what we've got and it works fine for us. And, yes, I've read tons on anchoring and such. Enough to know reasonable people can disagree and to know that in doing so they often become unreasonable. You guys have me laughing hysterically.

And, please, take this in the good humor it's meant. Proceed with your debate. It's fine. I just find it humorous to see the intensity with which it's conducted, but still think you're all great.

 

[BandB]

Wifey B: Oh and I did vote. All Chain. I'm in the majority. We win. Doesn't mean we're right...just we have the rest of you outnumbered.

 

[ranger58sb]

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.

Maybe; maybe not. Our anchor maker recommends X scope with Y rode because that usually -- apparently -- renders the best angle of attack (flukes to bottom).

In our case, adding that much weight that close too the anchor would probably change that angle of attack dramatically enough so results might not be predictable. Might not set best. Might not continue digging in further. And so forth.

I think that leads to the conclusion that "optimum" angle of attack is the goal... and that would vary greatly given specific anchor (design, weight, construction, etc.), rode composition, holding ground, maybe 5-6 other variables...

-Chris

 

[Pgitug]

I switch from 3/8" to 5/16". I think the 5/16" is strong enough. The 3/8" was hurting her back every time she pulled the anchor up. Hopefully this will be better for her.

 

[Nomad Willy]

Ranger,
Just an extreme example to show rode weight minimizes rode angle at the anchor if the weight is near the anchor. And is next to worthless near the boat.
See the thread "Anchor Rode Weight Bias".

Wifey B wrote;
"Wifey B: Oh and I did vote. All Chain. I'm in the majority. We win."
What have you won?

 

[Marin]

Ranger,
Just an extreme example to show rode weight minimizes rode angle at the anchor if the weight is near the anchor. And is next to worthless near the boat.

I don't think the extra weight is worthless when near the boat. It still contributes to lowering the angle of pull on the anchor. But I do agree that more weight down near the anchor can be more effective. Which raises the question, rather than wrestle with an additional weight and the hassle of installing and removing it, if one feels more weight is needed at that end of the rode why not simply opt for a heavier anchor in the first place?

 

[Britannia]

I don't think the extra weight is worthless when near the boat. It still contributes to lowering the angle of pull on the anchor. But I do agree that more weight down near the anchor can be more effective. Which raises the question, rather than wrestle with an additional weight and the hassle of installing and removing it, if one feels more weight is needed at that end of the rode why not simply opt for a heavier anchor in the first place?

The purpose of the weight is to reduce the angle of pull (i.e. increase the horizontal component) on the anchor. This is not the same as increasing the weight of the anchor. It's not obvious which would be more effective use of the extra weight.

Richard

 

[Marin]

The purpose of the weight is to reduce the angle of pull (i.e. increase the horizontal component) on the anchor.

We know that. The question Eric has raised is where on the rode would the extra weight be the most beneficial.

Assuming the same amount of weight and the same pull from the boat, where should the weight be to keep the angle of pull on the anchor the lowest?

Eric claims it should be down near the anchor itself. I don't have any reason based on physics and/or geometry to dispute that but I'm not sure it would be any more effective in reducing the angle of pull on the anchor in that position than it would if it was halfway or even a third of the way down the rode from the boat, and it may even be less effective down next to the anchor.

I think the only real way to know is to perform a test with an anchor, a rode, a weight, a boat, some wind and a diver or a camera. Otherwise it seems like it's mostly just speculation.

 

[psneeld]

While weight near the anchor is good, it doesn't mean that the remaining rode MUST be lighter.

I believe he is starting with a bad premise and coming to an incorrect solution.

Seem now and in the past others agree, especially trying to use the tree example as Spy pointed out...boats are dynamic situations not static. Continuing to think that the weight of the rode past a length of chain (biased weight as it was referred to) just doesn't make sense to me.