Mantus rode

[chris mac]

The great thing about these type of debates, is the info it gives those of us who are building their knowledge on issues such as anchoring.
Single statements are hard to judge value from. You don't know how much experience is behind each statement.
Debates like this expose so much more. You have some people explaining their points with real world experiences, relating with other well respected individuals, and explaining the same point several different ways. It shows a deep level understanding of the forces at work.
Then there are other statements that seem knowledgeable on the surface, and in fact are in a controlled environment. But as the argument deepens, the statement remains the same with no added understanding.
It does kind of feel like a discussion between real world and hypothetical situations.
Both are important, so long as you keep an open mind always willing to expand your knowledge and ability.
I personally value listening in. It has taught me a lot that I look forward to testing.

[ramblinrod]

Originally Posted by skipmac

Since he claims to have anchored at least twice in 50' (although in one case he claimed on too short a scope so we will only count one) then he must carry at least 500' of rode.

Enough.

You have posted a handful of snarky comments aimed at me in this thread.

Why?

What have I ever done to you?

[Jim Cate]

OK Ethan, I'll put in my tuppence worth:

Tackle: 60 lb Supreme, yellow pin shackle, no swivel, ~80 m of 10 mm L grade chain, no nylon, ~10 feet of old double braid from bitter end to strong point. Spare 45 lb Manson plow, 20 H Danforth stern anchor on 200 ft of 12 mm three strand and ~30 ft of 10 mm chain.



Boat: 46 ft fairly light mono, estimated cruising displacement of ~12 tonnes.

Experience: 32+ years of full time distant cruising, with over 95% of that time under way or at anchor, from SF to Oz with a lot of wiggles in the wake. That works out to over 10,000 nights at anchor. Anchorages from nearly aground (rare) to 85 feet (also rare). Wind speeds from zero to well over 60 knots sustained.

Practice: In good shelter and good bottoms (places we know) we routinely anchor at between 3 and 4:1 scope. In unknown bottoms and/or in poorer shelter, we'll use 4 or 5:1 if no adverse wx is on the horizon. If there is, we'll go to 6 or 7:1. Have never used more than that. In deep anchorages we use around 3:1 without much worry. We always set the anchor with a fair amount of power in reverse.

Results: We have dragged a few times. IIRC, all events were in thin, soupy mud, mostly in rivers where fresh silt has been deposited, or in one case, where there was a huge pile of weed caught on the chain and we couldn't get it off... and 6 knots of current (flood on the Clarence river). The Supreme doesn't have enough fluke area to cope with very low shear strength soil, I think. We will sometimes add the Danforth on a second rode if we drag or suspect that we might drag due to such conditions. Additional scope does not help in such bottoms; the anchor isn't pulling out, it is "plowing a furrow" while dragging, buried.

Having good sense, we've never gone into Arctic waters, so can't comment on that, other than what DH has done (successfully) seems good practice to me!

Jim

[Pete7]

Originally Posted by Jim Cate

OK Ethan, I'll put in my tuppence worth:

Did Ethan want examples of what people have? OMG this thread is going to ramblin on

31ft yacht came with CQR copy and rusty chain 20 years old. On purchase the previous owner did say they have problems anchoring securely. Minor accident unloading saw 30m of chain disappear into Swanick Marina at a great rate of knots, oops. Anchor given away.

Replaced with 45m of 8mm chain and a 10Kg Delta. Worked well and never dragged but only used in sheltered areas. Replaced 8 years ago with a Rocna 10Kg. I wanted the maximum holding without going up a size so the anchor fits in the locker and is easy to deploy by hand. I consider this a working anchor as strongly recommended by Rod Anchor sits nicely in the middle of the bracket for this size of yacht as recommended by Peter Smith (Rocna designer) who does say don't over size, they have already done that for you.

First time out dragged in Studland Bay a well known great anchor area with good holding Weed on the anchor probably the cause. Since then no further problems. Second anchor is FX16 with 20m of chain and 50m of thick rope stored below decks.

I keep eyeing up the 32" long fisherman's anchor next to the garden pond at home

Pete

[Dockhead]

Skipping over all of the uninteresting verbal backing and filling and evading, let’s go straight to the objective questions, which really are interesting, especially since they are subject to objective proof. No one is interested in how much scope to use if you have unlimited swinging room. This is a trivial question. What we have been discussing is how much scope do you NEED, in case for whatever reason you can’t have what you might like. More is better – duh! That doesn’t answer the question. How much do you NEED?


In the next post I'll give the objective numbers on the effect of catenary, everything using all the data and calculators on Alain’s superb site here: Tuning an Anchor Rode. So anyone can play around with them to his heart’s content, try different assumptions from mine, etc. The calculations are based on a 40 foot yacht with light (10mm) chain, with a second set of numbers run with unrealistically heavy (14mm) chain just to see the influence of chain weight.


We assume this yacht has a large (30kg) and good (Spade) anchor, and it is anchoring in good (not excellent) bottom, where this anchor will produce 2573 daN of holding force at 0 angulation, 2315 at 10 degrees, 1801 at 15 degrees, and 1029 at 20 degrees.


How much scope is needed to hold it, assuming the ABYC design anchor rode forces of 508 kaN at 30 knots and 903 daN at 40 knots, and 1410 daN in 50 knots. These forces are about 3 times the typical forces, and if sheltered from wave action and rigged to prevent yawing, one might expect to encounter much lower forces than these.


The short and obvious answer based on these numbers is that we can skip the bloody catenary question -- as long as you have at least 3:1 scope, you have enough holding power for about 42 knots of wind with this anchor in a “good” (not “excellent”) bottom, even on rope rode, and more than 50 knots in an “excellent” bottom. 4:1 scope without any catenary and in a “good” bottom will give you holding exceeding the WLL of the light chain. 6:1 will do all of this, and in poor holding, without any catenary.


So let’s be clear that with what Dashew calls a “BFA”, well set in decent bottom, this whole conversation is irrelevant. That’s why Dashew often uses even 2:1 scope. So all this conversation about scope and catenary becomes relevant only when you are not sure about the holding, and the anchor needs all the help it can get. A good anchor like a Spade will lose its grip at 25 degrees, will have 40% of its holding power at 20 degrees, 70% at 15 degrees, and 90% at 10 degrees. Reducing angulation below 10 degrees is practically meaningless in terms of holding power – a rounding error in the context of even slight differences in bottom quality. But reducing angulation from 20 degrees to 10 degrees will more than double holding power – that can matter, in a poor bottom.


I have to say that working through the numbers has changed my mind. I can’t believe I’m saying this, but I now see that Dashew was absolutely right after all, and that catenary doesn’t affect holding much, and doesn’t affect it at all in storm conditions, even in deep water. Scope also has little effect, so long as you are using a good anchor which is more tolerant to angulation. You get 70% of the maximum holding force of a good anchor, on 4:1 with rope rode. Doing better than that is just playing around on the margins. What matters most of all, is having the biggest and best anchor you can manage. Going up two sizes in the anchor, will generally have more effect than going from 3:1 on rope, to an 8:1 scope on chain, see here:


25 kg Spade 1993 daN (797) in “good” bottom; 3171 daN (1269) in “excellent”.
37 kg Spade 3451 daN (1380) in “good”; 5490 daN (2196) in “excellent”
Values in parenthesis are holding power at 20 degrees, so 3:1 on rope.



Shocking, isn’t it? I didn’t expect this myself.


The other extremely important factor, more important still than the anchor, is the bottom. This is the most important factor of all. The difference in holding power from a “poor” bottom to an “excellent” one is about 700%, which overwhelms all other possible factors. So a sailor who chooses a worse bottom over a better one, just because he can get more scope, is a fool. He is a double fool if he sacrifices even a slight degree of bottom quality, to get more scope than 4:1, where good anchors have 70% of their maximum possible holding power even on rope.

My conclusions, which are in some respects different from what I believed previously:


1. Number one priority in anchoring is to select a good bottom, even if it requires accepting less scope.

2. Number one priority in ground tackle selection is the “BFA”, just like Dashew said. If you can carry and handle an anchor which will hold you in storm force winds with 40% of its maximum holding power in an average bottom, then you can just forget all this other crap and be secure anywhere on any type of rode so long as you can get at least 3:1 scope. Heavy chain has much less value than I thought (for holding; motion damping is a different question).

3. The RYA used to teach that 4:1 scope is plenty and 3:1 is ok in good conditions. Nowadays we tend to think that’s little. But by God, the numbers show that the RYA was right after all, at least for those using good anchors.



4. 5:1 or 6:1 is really plenty under almost any circumstances, and there are hardly any circumstances where using more will do anything useful at all. Going from 5:1 to 6:1 only improves the terminal angulation from 9.59 degrees to 7.18 degrees – that’s pretty meaningless. 8:1 to 10:1 is only 1.4 degrees more – totally meaningless, but we knew that.



5. “Carry the largest anchor you can reasonably handle” is even better advice than I imagined. These are, actually, words to live by.


6. Making a fetish out of scope is really stupid, even more so than I thought.

So I’m truly the “winner” of this debate – in the sense that I discovered that I was wrong about something. So all this work thinking through all this stuff again has concretely benefited me. It just so happens that my anchor chain is due to be regalvanized, and my anchor looks terrible after 8 years of intense use. So now I wonder if perhaps I shouldn’t change both of them, rather than just regalvanize them. I could go to a 55kg (121 pound) Spade from my current 100 pound one. I was not able to handle easily a Rocna anchor in that size, but I guess the better balanced Spade could be ok. I could go down a size with the chain to lighten up my bow a bit – go to 10mm G70, if I can find any.

[Dockhead]

Now the numbers:

[Q Xopa]

Originally Posted by ramblinrod

Well, again, I disagree.

I believe that the scope you described is very dangerous for storm conditions, regardless of depth.

You didn't identify the storm conditions, but if over 40 knots you were very, very lucky to hold.

One may get lucky with a bottom sloping steeply up toward shore to drag but reset, before landing on the rocks.

I have also been anchored in a fjord with 1000 ft depths and 1000 ft cliffs on either side, with 20 ft tides. Fortunately conditions anchored in 50 feet remained calm.

I have also been anchored short scope in a small bay of 50 ft depths near shore (not my first choice but nearest refuge).

We weren't so lucky when a microburst with 70 knot winds blew through and found ourselves, (with everyone else in the anchorage), pinned up against the rock cliff shore. Fortunately it only lasted about 10 minutes and we could wedge the cockpit cushions and inflatable dinghy between the hull and rocks, and didn't suffer any major damage.

Again you are entitled to your opinions, but your continued attempts to belittle mine are getting old and tired.

If you disagree with me, I don't care; but when you insult me; that's another matter. Knock it off.

Hmmm, crying victim, again. Honestly I cant see anything remotely insulting. Claiming this to me seems ridiculous.

Everyone here is entitled to their opinions. I especially like hearing ones being esposed by numerous experienced and respected posters that back up with their experience and well reasoned logic and references.

The fact that you so often seem to be at odds with so many other respected contributors offering well considered advice so often seems to be a remarkable coincidence.

[Dockhead]

Now, going back to RR’s “4 Theses”:


1. "The force required to pull the catenary from chain, increases proportionally with scope."


C’mon, this one is totally obvious. As long as chain is lying on the bottom, increasing scope does ZERO. So increasing scope from even 2:1 to 20:1, does ZERO if there is still chain on the bottom in the scenario you are analysing. The concept of OVERKILL is essential here. Chain lying on the bottom, which will never be pulled off the bottom in the possible range of conditions you are anchored in, does not increase any force and does ZERO except increase the risk of fouling.

Verdict: Busted.

2. "The greater the catenary, the more horizontal the force on the anchor is to the seabed."


See above. Catenary more than what is ENOUGH to keep the last link of the chain on the bottom, does not change the horizontalness of the force.


Verdict: Busted.


Note: Furthermore, you can’t even talk about this without knowing the actual effect of “horizontalness”, and how much angulation produces how much reduction in holding force. You cannot make a rational anchoring decision if you think you just can’t anchor if the there were even one degree of angulation. Proof of that: Rope rode NEVER produces 0 degrees of angulation, and we don’t say that it is impossible to anchor with rope, do we? In fact, 6:1 on rope produces 10 degrees of angulation – and it is obvious to everyone with any experience that 6:1 on rope is absolutely fine except in extreme conditions.

3. "The more horizontal the force on the anchor, the greater the holding power, and the less likely the dragging."


This one is true, but irrelevant. Good anchors have 90% of their holding power at 10 degrees of angulation, so “horizontalness” from 0 to 10 degrees is irrelevant. Proof of this – anchoring on rope. It is true that in a few cases (see numbers above), catenary CAN bring angulation out of a critical range and into an acceptable one, but this does NOT support the idea that “more scope is better in every single case”. It CAN be better in SOME LIMITED cases.

Verdict: Busted.

4. "Therefore 10:1 scope is vastly superior to 3:1 scope, in all cases, everything else being equal."


Not even one of the premises is correct (or at least, correct and relevant), therefore:

Verdict: Totally busted; a total misconception. 0 degrees of angulation is not "vastly superior" to 0 degrees of angulation, just to name one counterexample, based on one scenario -- 30 knots of wind, 50 foot boat, 30 meters depth, 90 meters vs 300 meters of chain. There is already 0 angulation on 3:1, therefore 10:1 is not superior at all to 3:1, in this case, and most other real life cases.


Note: The reality, proven by the numbers, is – 10:1 is not better than 8:1 at all, ever, and is better than 6:1 in non-zero but negligible circumstances. 10:1 is better than 3:1 only in cases where 40% of the maximum holding power of the anchor is insufficient. If the anchor is big enough, those cases will be very rare.

[Pete7]

DH, could you just clarify what the "Chain l", and "last link" measurements are telling us and what units?

Pete

[Dockhead]

Originally Posted by Pete7

DH, could you just clarify what the "Chain l", and "last link" measurements are telling us and what units?

Pete

Yes, sorry -- "Chain l" is chain length in meters.



"Last link" is the force in daN required to lift the last link of the chain off the seabed.


"10 deg" is the force in daN required to angulate the chain to 10 degrees


"15 deg" is the same, to 15 degrees (about 70% holding power).


There is no "20 deg" because that is geometrically impossible at 3:1

[ramblinrod]

Originally Posted by Dockhead

So I’m truly the “winner” of this debate – in the sense that I discovered that I was wrong about something.

Well, I am certainly glad to hear that you finally acknowledge this is a possibility.

Unfortunately, in my opinion, you simply haven't taken it far enough.

To suggest that chain laying on the seabed is no value, IS ABSOLUTELY FALSE.

This is what protects one from higher pull on the rode (typically from higher wind velocity) lifting chain off the bottom, increasing the draw angle on the anchor stock and pulling the anchor out.

The only thing, that will save you, should this be about to happen, is to add more scope.

If it is has already happened, and you are dragging, you are already screwed.

You will likely have to raise and reset the anchor, unless you get incredibly lucky and snag something, but a precarious hold at best.

What is the better solution?

Use more scope in the first place.

IT IS THIS SIMPLE.

BTW, I object to all of your offensive terms like "stupid" and" fetish" and what not.

I am neither stupid nor have a scope fetish.

I just know what works based on my experience.

If you want to do something different based on yours, be my guest.

If you drag onto the rocks and founder tomorrow, it will be no skin off my nose, because I have done my best to help you, and you chose to ignore it.

That is absolutely your right.

I just hope you don't lead others down the path to marine tragedy.

OK, so I don't have any more time for this thread; I have to go fix boats.

But let the record show, that me thinks you are oversensitive about anyone challenging your position on this subject.

If it helps you sleep better, to believe 3:1 scope in deep water is sufficient, and regardless what I or any other proponents of using more scope say (which a simple google search will show many "experts" about 10:1 in favour of more scope), so be it.

If you have learned anything from this thread, I hope that it is that if you drag using your preferred solution, one possible remedy is to reset, using more scope.

This very simple knowledge may save your bacon some day.

You're welcome.

So in closing, while I could correct your false contradictions, line by line, history dictates you will not listen, nor acknowledge any error I draw to your attention, so to try to do so would be fruitless, and I simply have better things to do, like consult real boaters on real boats about real ground tackle systems.

[Kelkara]

I still think there's more I can learn from this thread ... so I'm going to throw in one more example and question ... using a CQR in 10m of water and 5:1 all 6mm chain getting hit by 40 gusting 50 knots I started dragging, but increasing to 10:1 with 50% nylon rode and I stopped.


how much do people think that the extra scope helped, and how much help do you think I got from the extra snubbing in the gusts I got from having about 5x as much snubber length in the nylon rode?

[ramblinrod]

Originally Posted by Kelkara

I still think there's more I can learn from this thread ... so I'm going to throw in one more example and question ... using a CQR in 10m of water and 5:1 all 6mm chain getting hit by 40 gusting 50 knots I started dragging, but increasing to 10:1 with 50% nylon rode and I stopped.


how much do people think that the extra scope helped, and how much help do you think I got from the extra snubbing in the gusts I got from having about 5x as much snubber length in the nylon rode?

Well, on my way out the door...but my answer to the first question is that the additional rode "ultimately" helped you. The proof is that you were dragging and then you stopped, apparently as a result of paying out more rode.

To what degree the result was due to more scope or increased elasticity is anyone's guess, but my guess would be that likely both contributed.

Regardless, it saved your hiney.

Consider yourself lucky. It is not uncommon for the anchor to become fouled with weeds or debris after it has dragged and nothing but raising it, clearing it, and resetting it with longer scope will change the situation positively.

To the logical person, they would conclude that to handle those conditions, longer scope in the first place, so that you didn't drag in the first place, would have been beneficial.

Alas, others may try to justify their alternate position with whatever means they can, suggesting the boat was too big, or too little, or the water was too deep, or not deep enough, or the bottom was too soft, or too hard, or the wind was too strong, or not strong enough, or the anchor was too small, or too big, or the wrong type, etc., etc., etc.,

The real and abundantly obvious answer is, under the conditions you encountered, you did not have enough scope out, and paying out more saved the day.

[Dockhead]

Originally Posted by ramblinrod

. . . To suggest that chain laying on the seabed is no value, IS ABSOLUTELY FALSE.

This is what protects one from higher pull on the rode (typically from higher wind velocity) lifting chain off the bottom, increasing the draw angle on the anchor stock and pulling the anchor out.

OK, skipping over the irrelevant bits again -- to the extent that this means that extra chain, while lying on the seabed, is reserve catenary which comes into play when forces increase -- well, of course. That's why you should set up your ground tackle to deal with the MAXIMUM loads which there is a material risk from occurring.



However, this does not change the fact that increasing scope beyond what keeps the last link on the seabed according to the forces which actually occur -- does absolutely nothing except increase swing room and increase the risk of fouling the chain on something. Does not change the angle of pull, does not form catenary.


As to friction on the seabed of excess chain -- we never talked about that. I'm glad we've gotten beyond the Four Busted Theses -- this is something new. Yes, excess chain sliding on the seabed will be added to the holding force of the anchor. But this friction is irrelevant and useless, because (1) any force on the anchor which doesn't raise all the chain off the seabed, no matter how much scope is out, will not be enough to budget a well-set modern anchor even in poor holding; and (2) it is a small force, in the tens of kilos at best, in any case.

Originally Posted by ramblinrod

If it helps you sleep better, to believe 3:1 scope in deep water is sufficient, and regardless what I or any other proponents of using more scope say (which a simple google search will show many "experts" about 10:1 in favour of more scope), so be it.

I see I need to repeat yet again, that I did not say that 3:1 is sufficient.

Once again, I said that 3:1 can be sufficient, depending on the conditions and circumstances, and especially, on the size of your anchor. I have said many times, and I don't want for there to be any doubt in anyone reading this -- the amount of scope you need varies according to a lot of different factors. Every sailor needs to know specifically how much is needed in what set of conditions. It's great to have a reserve, but when conditions don't allow it, your life may depend on your knowing how much you can get away with, so that you can make correct choices between non-ideal variants.



I agree with real experts like Peter Smith:


"A 3:1 ratio of rode-length to water-depth should be considered minimal ("1" being the vertical distance from the seabed to the bow roller, not the waterline, at high tide). Generally speaking about 5:1 is appropriate. Typically you should set the anchor at the same scope it will be left at, in order to be sure the anchor can re-set itself again should it pull out. However, it may be necessary to set it at a higher scope and then pull in some of the rode.

"In bad conditions, the ratio can be increased up to around 8:1. There is little benefit in going beyond this point, and boaters using scopes of 10:1 and higher are gaining almost nothing from their efforts and huge swinging circles. At 8:1, the maximum angle of pull on the anchor is already capped at just over 7°, and of course the scope must be doubled in order to halve the angle (i.e, even very large increases in the scope result in angle reductions of only several degrees).. . "


Scope vs catenary (Rocna Knowledge Base)


And Morgan's Cloud:

1. Three to one scope is an absolute minimum, after setting, and five to one is a lot better.
2. If you have a decent anchor, seven to one scope is all you will ever need. Letting out more will just make your boat sail around more and result in higher loads on the system. Laying out too much scope is also inconsiderate, as it leaves less room for others.

https://www.morganscloud.com/2014/01...t-2-technique/

[Delfin]

Originally Posted by skipmac

Since he claims to have anchored at least twice in 50' (although in one case he claimed on too short a scope so we will only count one) then he must carry at least 500' of rode.

Rod may be onto something with the recommendation he gives his clients that you need to carry 10:1 worth of rode for your maximum anchor depth. For me, I think last season that would involve around 1200 feet of 1/2" chain, in the which case, I don't really need an anchor at all. Just dump 2,800 ponds of chain over the side and go to sleep. With a half mile of chain on the seabed, who needs an expensive anchor at all?