Ideal Catenary Angle

[Nomad Willy]

There is much talk about catenary and scope. Often, more often than it would seem necessary or appropriate.

But in many anchoring discussions I think it’s assumed that the anchor is in an ideal position somewhere north of 6-7 to 1 scope. And 10-1 talk sounds like “you can’t get better than that”. This post is saying you can.

Every anchor probably has an ideal angle of rode to fluke. At 10-1 scope w the rode at the anchor shank and laying flat on the seafloor one tends to think it’s an ideal arrangement. I think it may .... but it may not. Relative to a specific seafloor every anchor has an ideal angle to it’s fwd progress in the substrate relative to the angle of pull. The angle of pull is not the throat angle of the anchor.
If you discard the shank and put an eyebolt at various fore and aft positions along the fluke center the anchor fluke will assume a position based on a balance and when that balance positions the fluke at an ideal angle maximum resistance will give the anchor it’s maximum holding power.

But when all these positions, angles and balance come together it may not be at a 10-1 or even 7-1 scope. It may even happen at 5-1. No anchor test has ever addressed this issue or concept.
I read somewhere that one anchor in a test did better at a moderate scope than at a long scope ... like 10-1.
But I don’t see anchor tests turning into scope/holding power tests. First it would be too much work. But more importantly it would take down (IMO) the assumption that more scope is always better. I think that it’s possible 4-1 or 5-1 scope may be the best scope for performance.

Anyway I welcome any ideas that may shed light on the above. When I think about the ideal anchor I sometimes think of a joint in the shank that would allow the fluke to find it’s ideal angle. There are some big boat or ship anchors that articulate around a point in between the fluke tip and the crown. The Danforths articulate around and at the crown. There’s a lot to be gained by this (I think) and it’s related to ideal scope.

In this picture the articulation point is not at the crown like a Danforth. The articulation point is further fwd toward the fluke tip. This way there is not big pressure differences around the flukes but a more even fluke loading. With more even loading higher loading can be had overall.

 

[O C Diver]

IMO, if the first couple of feet of chain are laying on the bottom at maximum pull, you probably can't improve on that as far as holding and anchor penetration.

Now if you want to use 6' of 1" chain and 2:1 scope in rope on Willy to accomplish that, go for it.

Ted

 

[Ken E.]

Eric, I know from your past posts that you've observed claw anchors on short scope. My typical setup in 40-50 feet of water is 150' of 3/8" chain out with a 66 lb. Lewmar claw, assuming a typical mud bottom generally found in BC and SE AK. 3:1 scope works fine for me unless the wind freshens, at which time I'd let out more chain accordingly. My most vivid recent memory on scope was in Port Refugio, south of Craig AK, with a full gale blowing in Dixon Entrance and lots of west wind getting into the the anchorage. I went to 6:1 in 25 feet of water, mud bottom, and didn't move all night, surrounded by big white caps.

 

[O C Diver]

It's common for me to anchor in 8' to 12' of water. The lengths between 4:1, 7:1, and 10:1 (for a storm) for 10' are 40', 70', and 100' (surface to anchor). The 30' difference between 4:1 and 7:1 hardly seems worth not putting out the extra chain to play it safe.

Ted

 

[DDW]

I believe most anchors are designed to work best with a horizontal pull. The Danforths and big ship anchors articulate only to allow for the anchor hitting the bottom upside down. When they set they are against the stops. The Fortress anchor allows two positions on those stops, because in soft mud a larger angle helps.

Additionally, there is absolutely no way to control the angle of pull, other than to insure that it is horizontal. If you tuned your scope to the point that it was exactly some angle, the slightest change in wind or tide would upset this tuning.

Now there was an article published long ago (in PBO I think) in which the author calculated the critical scope for different weight chain, which resulted in the catenary just barely achieving horizontal pull at a given tension on the rode. The interesting thing was that this scope changes markedly with depth: less scope is required at deeper depths, to pull horizontal at a given tension. I thought I had saved the article somewhere but can't find it now. Rocna has a chart on their website showing that 7:1 at 10 ft and 4:1 at 90 ft depth are equivalent for the conditions chosen. You can construct your own chart using the catenary formula.

In the PNW, anchoring in many places at 10:1 or even 5:1 is not remotely practical on a small boat. A full drum of chain isn't enough.

 

[Nomad Willy]

WhatI’m saying is that a horizontal rode on the seafloor may not be optimal.

What needs to be optimal is the angle of attack of the fluke. And the position (vertically) of the shank tip does a lot to control the fluke angle. The shank tip may be in very different heights causing the performance of the anchor to vary considerably.

 

[O C Diver]

WhatI’m saying is that a horizontal rode on the seafloor may not be optimal.

What needs to be optimal is the angle of attack of the fluke. And the position (vertically) of the shank tip does a lot to control the fluke angle. The shank tip may be in very different heights causing the performance of the anchor to vary considerably.

Can you point to an anchor manufacturer that recommends that?

Ted

 

[psneeld]

Ted...I do agree with that comment...but despite what some might think...I am pretty sure manufactures base that angle on the shank being parallel to the bottom.....not some "theory" of amateur anchor theorists.

 

[Nomad Willy]

Of course not Ted.

This is theory and I think it may be valid.
But I could just as easily be wrong. Some anchors perform better at short scope so it stands to reason they may vary at long scope too.
I’m not just looking for a fight (50 against 1) me being one.

Best case as far as I’m concerned is to uncover some truths that that could show my thoughts are not applicable. Or to show that there are too many variables to sort out in any meaningful way. Or to show there’s much to learn and clearly beneficial knowledge to increase our ability to choose ... or dismiss old practices. Learning = change & better.

 

[O C Diver]

Of course not Ted.

This is theory and I think it may be valid.
But I could just as easily be wrong. Some anchors perform better at short scope so it stands to reason they may vary at long scope too.

Ok, what anchors perform better on short scope versus long scope? I can see an anchor performing adequately on shorter scope, but better?

Ted

 

[FF]

If there is a good distance between the bow roller and windlass quire heavy chain can be used.

It need not fit the windlass if the right length.

The bow roller will be bigger and the chain stopper way! more expensive , but only once.

 

[rslifkin]

I agree that chances are, no anchor will perform worse at a lower angle. But some designs are definitely more sensitive to angle than others. So one design may lose a lot more performance at 4:1 scope than another.

 

[psneeld]

Last time I read the Fortress instructions, they did mention because the anchor was so broad and light, it may sit on top of soft mud while the chain may sink into the mud. I see the same issue with grass/weed/etc for smaller models.

This might prevent good sets as the shank to fluke angle is negative.

Maybe others suffer the same, but scope wouldn't matter, just whether there is enough for the rode next to the anchor to be on the bottom.

This may only be a setting issue, not long term, but may affect resets.

This why I never trust danforth lookalikes or Fortresses to be a pulpit primary.

 

[rslifkin]

Fortress mentions that a work-around for that issue in very soft mud is to set it at short scope, then let out more after it's set. As far as a re-set, in a very soft bottom, it's more likely that you can set it deeply enough that it'll pivot on a reversal rather than tripping and resetting, so that somewhat mitigates the issue.

I'm not sure issues like that are specific to just the Fortress though. Any anchor allowed to settle slightly too long in soft mud could settle unevenly and then fail to properly orient when pulled on, as it's not just sitting on the bottom like it's designed to when first dropped.

 

[psneeld]

The observational aspects seem pretty clear to me.....but.....as far as I know, little or no data other than the Fortress warning supports either way......and pivoting reversals are all but a guess. People warn about betting on "best possible case scenarios" being an anchoring option.

Just pointing out that just any scope may or may not give a good set ( have had numerous danforth types fail to set in emergency like situations)...not so that much with other anchors.

On non pivoting flukes, I dont see it as an issue as the angle of attack is even better.

 

[Nomad Willy]

Ok, what anchors perform better on short scope versus long scope? I can see an anchor performing adequately on shorter scope, but better?

Ted

Ted that’s a good question but not one I’m going to answer. You know what happens when you tell a Ford man that his Ford has a feature that is inferior to what Chevys offer.

What if you had an anchor w a hinge at the shank to fluke join spot? And you had a lot of time to experiment w various settings. And over time you found the throat angle setting that resulted in the greatest holding power. Notice I didn’t say anything about chain, line o catenary.

One would think that optimum rode angle would be w the rode laying flat on the seafloor at the end of the rode at the anchor. Because it would seem only logical that w the rode even slightly off the seafloor the rode would be pulling the anchor UP ... to some small extent. So then the rode would be tending to pull the anchor OUT of the seafloor. It makes sense. And things that make sense are easy to accept or believe.

But what if our test anchor shank was welded to the fluke as most all anchors are, and what if it was a real anchor in the real world? The manufacturer wouldn’t know what the optimum throat angle would be. So now let’s assume they choose a throat angle (TA) for a rode/catenary angle that was w the rode well off the seafloor. By chance 50% of manufacturers would be making anchors w the TA such that best holding power would be achieved w the rode up above the seafloor. And we know that such an arrangement would only occur w a scope less than 10-1. So w some throat angles maximum holding power would be achieved w the rode NOT laying on the seafloor.
This example must exist as manufacturers don’t have the time or resources to accurately test all throat angles.

And if such an anchor was to be had so it’s max holding was achieved at a catenary angle less than horizontal then one could assume (rightfully or wrongfully) that an anchor that achieved max holding at less than 10-1 scope would out perform other anchors that didn’t do as well at short scope? It would “seem” so but things that just seem to be true may not be true.

So if my theory is correct there are probably a good lot to several anchor designs that perform better at less than long scopes. And the ideal rode angle and ideal TA frequently don’t match up. As the ideal rode angle and TA aren’t usually found together in the same anchoring situation in the real world.

The whole question revolves around the relationship between the rode angle where the rode and shank meet and the throat angle of the anchor.

 

[psneeld]

I was always taught that if you start with incorrect assumptions, you will probably wind up with incorrect conclusions.

Nowhere have I ever read that the throat angle is based on anything other than the shank parallel to the seabed and the throat angle then based on penetration and holding.

 

[O C Diver]

Ted that’s a good question but not one I’m going to answer. You know what happens when you tell a Ford man that his Ford has a feature that is inferior to what Chevys offer.

What if you had an anchor w a hinge at the shank to fluke join spot? And you had a lot of time to experiment w various settings. And over time you found the throat angle setting that resulted in the greatest holding power. Notice I didn’t say anything about chain, line o catenary.

One would think that optimum rode angle would be w the rode laying flat on the seafloor at the end of the rode at the anchor. Because it would seem only logical that w the rode even slightly off the seafloor the rode would be pulling the anchor UP ... to some small extent. So then the rode would be tending to pull the anchor OUT of the seafloor. It makes sense. And things that make sense are easy to accept or believe.

But what if our test anchor shank was welded to the fluke as most all anchors are, and what if it was a real anchor in the real world? The manufacturer wouldn’t know what the optimum throat angle would be. So now let’s assume they choose a throat angle (TA) for a rode/catenary angle that was w the rode well off the seafloor. By chance 50% of manufacturers would be making anchors w the TA such that best holding power would be achieved w the rode up above the seafloor. And we know that such an arrangement would only occur w a scope less than 10-1. So w some throat angles maximum holding power would be achieved w the rode NOT laying on the seafloor.
This example must exist as manufacturers don’t have the time or resources to accurately test all throat angles.

And if such an anchor was to be had so it’s max holding was achieved at a catenary angle less than horizontal then one could assume (rightfully or wrongfully) that an anchor that achieved max holding at less than 10-1 scope would out perform other anchors that didn’t do as well at short scope? It would “seem” so but things that just seem to be true may not be true.

So if my theory is correct there are probably a good lot to several anchor designs that perform better at less than long scopes. And the ideal rode angle and ideal TA frequently don’t match up. As the ideal rode angle and TA aren’t usually found together in the same anchoring situation in the real world.

The whole question revolves around the relationship between the rode angle where the rode and shank meet and the throat angle of the anchor.

If I may be allowed to poke a hole in your theory, the manufacturer has no control over whether the rode is lying on the seabed or some angle toward the surface. They have no control over the scope used, the angle of the shank to the seabed, or the speed it's being pulled at. It would seem much tougher to attain a desirable rode angle than the rode dragging across the seabed.

Ted

 

[Nomad Willy]

Sure,
Rode dragging across the seafloor is easily attained. Just let out a lot of rode.
And if one had an all nylon rode it could be easily estimated looking at the rode from the bow where everybody is while anchoring (haha) but w chain it would be basically impossible since the rode (above the seafloor) is a big long curve w an infinite number of angles.

But what exactly is the hole in my theory? Don’t see what you’re saying re my theory. But it may be interesting and revealing. But if design for max performance was 30 degrees then I would be correct. I’ll guess nobody does that though.
What does the fact that the manufacturer dos’t know all the variables you mentioned have to do w what?

 

[O C Diver]

Sure,
Rode dragging across the seafloor is easily attained. Just let out a lot of rode.
And if one had an all nylon rode it could be easily estimated looking at the rode from the bow where everybody is while anchoring (haha) but w chain it would be basically impossible since the rode (above the seafloor) is a big long curve w an infinite number of angles.

But what exactly is the hole in my theory? Don’t see what you’re saying re my theory. But it may be interesting and revealing. But if design for max performance was 30 degrees then I would be correct. I’ll guess nobody does that though.
What does the fact that the manufacturer dos’t know all the variables you mentioned have to do w what?

The hole in your theory is that a manufacturer would design an anchor whose optimal performance was at a specific rode angle AND have decreasing performance with both greater and lesser rode angle. It's easy for a manufacturer to say that holding failure was based on too steep an angle. To have to argue that a customer put out too much rode for the anchor to perform optimally, would get you laughed out of court and the boating business in a second.

I can see the warning label now:

CAUTION The use of too much rode will reduce the effectiveness of our anchor, possibly causing it to breakout or fail to reset on a tide reversal. Using this anchor during high wind conditions where prudent anchoring techniques would suggest increased rode and shallower scope, COULD INCREASE YOUR CHANCES OF DYING!

Ted

 

[DDW]

If I may be allowed to poke a hole in your theory, the manufacturer has no control over whether the rode is lying on the seabed or some angle toward the surface. They have no control over the scope used, the angle of the shank to the seabed, or the speed it's being pulled at. It would seem much tougher to attain a desirable rode angle than the rode dragging across the seabed.

Ted

I think more to the point, you have no control over the angle of the rode either - except in one case, horizontal. That can be achieved with chain and enough scope. Any other angle is fleeting and beyond your control unless firmly stern tied.

Now, if you are saying that the fluke to shank angle of some anchors may not be optimal for some conditions, then certainly that must be correct. They are a compromise, since you do not know with certainty what the bottom is, and generally don't have the time and equipment to perform a detailed holding test with several different angles, each time you anchor. There have been numerous tests on the Fortress (which has two angle settings) that show differences in different bottoms. I think Steve on Panope has shown pretty conclusively that short scope does not work better with any anchor, though some suffer more than others. In theory, the steeper angle on the Fortress that works better in mud could be used in sand if you could fix the rode angle to match the sand setting. How do you propose to do that?

Fortress provides "mud palms" with their anchors to float the shank head on the mud and allow the flukes to drop. But fouling of flukes with mud on reversal has been a problem with Danforth type anchors since their invention, and is not limited to those types either, as Steve's reversal tests on the Rocna have proven.

 

[O C Diver]

. But fouling of flukes with mud on reversal has been a problem with Danforth type anchors since their invention, and is not limited to those types either, as Steve's reversal tests on the Rocna have proven.

I think that may be misleading. If I'm thinking of the same tests as you, they were done with very short scope and excessive speed. While the standards were maintained uniformly though out the tests, they had no relationship to real world anchoring or manufacturer's recommendations.

Ted

 

[rslifkin]

I think those tests were basically the worst case for being anchored at fairly short scope and experiencing a sudden, large wind shift. Basically, however it does with that, it should do better in most realistic scenarios.

 

[Nomad Willy]

HaHa Ted I agree that would be awkward/funny. But just because the average guy would have a hard time learning. However the manufacturer could just say there was no advantage to scopes beyond 6-1. In the real world 6-1 is a lot of scope rarely used.

But Ted re your question about specific anchors I can say that the difference in performance of the Rocna and Supreme at short and long scopes are in line w my theory. The R is a bit better at long scope and thus max holding whereas the Supreme is better at short scope. Of the two anchors I did buy the Supreme and passed on the Rocna thinking “any good anchor holds well at long scope” but few really shine below 3-1. I anchor a lot at 3-1 and almost never over 4-1.

But it’s common knowledge that anchors vary in performance at short scope and it’s not much of a jump to say they vary at long scope too. And my theory says that unless the ideal rode angle is ridiculously long their max performance may fall off above about 6 or 7-1.

 

[psneeld]

I reject the theory anchor max holding "falls off" at longer scopes...I believe it doesn't get much or any better but "max holding" doesn't necessarily decrease.

I also would take most opinions based on an anchor test or two for what anchors have the highest holding power....just too many variables.

Real life experiences from active cruisers seem to suggest there are small differences in each class of anchor....

 

[O C Diver]

And my theory says that unless the ideal rode angle is ridiculously long their max performance may fall off above about 6 or 7-1.

Eric, that's an assumption that lacks any credible data points. I respectfully submit that you can't show me any information that shows holding power is diminished when any anchor's shank (draw bar) is continuously in contact with the seabed. Conversely, every anchor will eventually fail to hold when the angle of the anchor's shank to the seabed becomes to great. At what angle holding reaches it's maximum capacity may be open for debate.

Ted

 

[Nomad Willy]

Diver wrote;
“At what angle holding reaches it's maximum capacity may be open for debate.”

Indeed, that’s what I’m saying.
I’m not asking if it’s true. I’m looking for things that would show it could be true.
And my credibility is only my reputation here on TF and some of that may not support me. I’m thinking it could be possible and looking for discussion that shed light on it .. negative or positive. Everybody knows there’s at least hundreds or thousands of very credible people that think every or most all anchors never decrease in performance after extending scope.

But the earth was once flat ...... right?

 

[rslifkin]

Keep in mind, the difference in angle between 7:1 and 10:1 is a lot smaller than going from 4:1 to 7:1. That's probably why people don't worry about too low an angle. In practice, you end up having to put a ton more rode out for a small difference, so you'll never really run into it as a problem.

Plus, in deep water, if you've got, say, 400 feet of chain out, even if it's only 3:1 scope, you'll need quite a lot of wind to actually straighten the chain. It becomes a very different game than in more moderate depths. So an anchor would have to handle a very low angle pull in that situation.

 

[Nomad Willy]

rslifkin,
Very interesting.
I didn’t know about the degree of non-linearity in the scope/angle. If one dwelled on the shape of the complex catenary curve it would almost seem obvious. But I never dwelled on this matter. Probably mostly because I was much more interested in short scope dynamics.
So thanks for uncovering this.

Actually most probably also didn’t think much about long scope for the same reason. Why bother. Everyone knows ... go long will deliver.
I’ve only anchored at 10-1 once. In Calder Bay in SE Alaska. Look at your charts. It’s a big area only 20’ deep almost everywhere. I anchored at 10-1 just because I could and may have thought I may never get the chance again.
The weather was benign so I used my 13lb Danforth “S” type. The most interesting thing about the bay was it’s abundance of sea otters. I was seeing 12-15 together most of the time but the bays population of sea otters was probably 75or so. And the bottom of the bay was built by the sea otters over a period of several thousand years.
The night was very uneventful.
Pulling all that line in took me awhile and when I saw the anchor .. well I really didn’t see it because of all the dark grey mud. Took me a half-an-hour to work all that mud off the anchor. There were lots of clam shells in the mud that could have acted as rebar does in concrete.

In the two gales I have anchored in I was at about 5-1 scope. Once w a 13lb XYZ anchor and the other time w an 18lb XYZ.

That’s my long scope history. Nothing else to add.

 

[Peter B]

WhatI’m saying is that a horizontal rode on the seafloor may not be optimal.

What needs to be optimal is the angle of attack of the fluke. And the position (vertically) of the shank tip does a lot to control the fluke angle. The shank tip may be in very different heights causing the performance of the anchor to vary considerably.

I hear what you are saying Eric, and it may be so in a strictly geometrical sense, but you are overlooking the other huge factor in that statement. That is the shock protection effect of a decent length and weight of rode between the shank and the place where the pull lifts the rode off the bottom. It is generally recognised I think that it is when the pull is so strong that all the rode is lifted off the bottom, (effectively becoming a straight line, with no catenary), and the shocks/jerking from wind and wave action is being directly passed to the shank, that the anchor will most likely break loose, even if at that angle it gives the whole rode the perfect fluke to shank angle. Just sayin'...