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Old 08-12-2020, 04:58 PM   #1
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Ideal Catenary Angle

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.
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Old 08-12-2020, 05:18 PM   #2
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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.

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Old 08-12-2020, 05:26 PM   #3
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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.
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Old 08-12-2020, 05:40 PM   #4
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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.

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Old 08-12-2020, 07:29 PM   #5
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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.
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Old 08-12-2020, 07:38 PM   #6
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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.
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Old 08-12-2020, 08:16 PM   #7
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Quote:
Originally Posted by Nomad Willy View Post
WhatIím 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?

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Old 08-12-2020, 08:40 PM   #8
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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.
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Old 08-12-2020, 08:54 PM   #9
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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.
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Old 08-13-2020, 06:06 AM   #10
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Originally Posted by Nomad Willy View Post
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?

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Old 08-13-2020, 06:13 AM   #11
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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.
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Old 08-13-2020, 07:12 AM   #12
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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.
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Old 08-13-2020, 08:32 AM   #13
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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.
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Old 08-13-2020, 08:38 AM   #14
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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.
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Old 08-13-2020, 08:50 AM   #15
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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.
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Old 08-13-2020, 10:19 AM   #16
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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.
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Old 08-13-2020, 10:30 AM   #17
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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.
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Old 08-13-2020, 11:50 AM   #18
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Originally Posted by Nomad Willy View Post
Ted thatís a good question but not one Iím 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 didnít 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 wouldnít know what the optimum throat angle would be. So now letís 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 donít have the time or resources to accurately test all throat angles.

And if such an anchor was to be had so itís 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 didnít do as well at short scope? It would ďseemĒ 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 donít match up. As the ideal rode angle and TA arenít 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
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Old 08-13-2020, 12:15 PM   #19
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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?
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Old 08-13-2020, 12:36 PM   #20
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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? Donít see what youíre 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. Iíll guess nobody does that though.
What does the fact that the manufacturer dosít 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
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