Anchor chain

[noelex]

Jonathan, you seem to have a bee in your bonnet about this subject.

It is not difficult to understand. A bigger anchor has more holding power than a smaller anchor of the same design. Conversely, an anchor deployed on a shorter scope has less holding power than than the same anchor deployed on a longer scope.

Thus, in general, for the same holding power you can use a shorter scope with a bigger anchor. Simple.

This advantage applies across all scopes not just to so called "short scopes". For example a larger anchor at 5:1 might have the same holding power as an identical design smaller anchor at 7:1.

This is just one advantage of larger anchor, there are many others, but it is not a miracle cure. You cannot use a short scope in very strong wind even with a large anchor. In the same way if using a very small anchor a medium scope may be inadequate in even moderatly strong wind.

Chain left in the locker does little good so generally it is better to deploy a longer scope if you have any doubts. It is difficult to know the nature of substrate and this is generally the largest variable. No one is advocating because you have a larger anchor you must use a shorter scope, but It does open up some options that would not othervise not be prudent especialy compared to those using a small anchor. Long scopes are sadly not always practical.

You have long been an advocate of very small anchors, it seems to work for you but it does have some obvious limitations. By using smaller anchors than most cruisers you are forced to use longer scopes for the same holding power. You have indicated before that you always deploy two anchors if the forecast is above 25 knots and you now indicate that you never anchor at less than 5:1. Both of these limitations are unnecessary with reasonable anchor gear, and personally I would find them restrictive.

Snubbers are a seperate issue. I agree completely that a good snubber is an important component of the anchoring system. I think snubbers have been discussed at length on this forum and doubt any regular reader of anchor threads is unaware of their use.

 

[Stemar]

I don't suppose I'm the only one bemused by the calculations and disagreements on the delightful subject of anchoring!

Most of this seems more relevant to bigger boats and deeper water, rather than the 24 footer I sail, generally anchoring in shallow water with limited swinging room. I have 17m of 8mm chain plus a good length of octoplait which came with the boat when I bought her 15 years ago. Since it's still perfectly flexible, I'm guessing it's polyester rather than nylon, but a few metres of rope seems to do a good job of absorbing shocks.

I try and set out at least a 5:1 scope, more of it's blowy. If I haven't got room for enough rope, I use a snubber of 10mm nylon, an off-cut from when making mooring lines. It's about 2'5m long, with a chain hook one end and an eye splice at the other to drop over the samson post. In between is a rubber mooring snubber, and there's a bit of water pipe to protect from chafe where it goes over the bow roller. Obviously, I don't rely on it for ultimate strength, but it does a good job of damping shock loads and allowing a decent night's sleep. Aided by an oversized anchor (10kg Delta for a boat weighing less than 3 tonnes), it's kept me as comfortable as can be expected when anchored in far worse conditions than I would choose.

Without having measured anything, I reckon my snubber gives me a good couple of inches of movement to absorb shock loads. If an all chain rode of a particular length allows 1/4" of elastic movement to absorb a shock to give a load of 0.25 g - a shock load of about 700kg for my little boat, then 2" will reduce that load to 1/16 G, bringing the shock load down to about 171 kg.

Those sums are strictly back of fag packet and the figures are probably wrong, but I think they're good enough to illustrate the importance of snubbers.

On the subject of anchor size, someone once asked, "How can I know if my anchor is big enough". A rather facetious reply was, "When everyone else in the marina is laughing at how big it is". For cruising boats that spend the night at anchor, I rather tend to agree. If you can manage it - especially pulling it up with whatever equipment you have - it isn't too big. Following a serious illness before Christmas, I may need to review my arrangements as 17m of chain plus 10kg of anchor plus however much mud is probably more than I will be able to lift, but I'd rather get a winch than reduce anchor size

 

[GHA]

I must have misread something.

But you seem to suggest you think the idea, that I obviously dreamt up, that short scope can be safe if you use a big anchor - is a load of nonsense.

Not at all, haven't suggested anything.just wondering who these Proponents of big, monster, anchors and their justification for the recommendation to use tham is that they are 'safe' at short scope." actually are?
Does seem a bit over the top - who are these people?

Enjoy the wine

 

[Poignard]

I don't suppose I'm the only one bemused by the calculations and disagreements on the delightful subject of anchoring!
(...)

You are not.

Not knowing any better, I simply followed Eric Hiscock's recommendations in his 'Cruising Under Sail' on the basis that he seemed to be a thinking man with a lot of practical experience who knew what he was talking about. Not a very scientific way of going about choosing ground tackle, I admit, but I have never once regretted following his advice (except when my back aches a bit ).

 

[Neeves]

Noelex,

I think if you check back I would use 2 anchors if the winds are 30 knots or above, not for the extra holding power - but to reduce veering. It is commonly and erroneously thought 2 anchors increases hold, it might - never measured it. But it reduces veering and making the movement of the vessel less aggressive (which is what snubbers also do) will reduce stress on the anchor.

The ultimate holding power of an anchor is determined by a big winch. The holding power of a small anchor, well set, by the yacht is exactly the same as the holding power of the same design of large anchor in the same conditions, no more no less. So if I set a small anchor with a 40hp engine, which will develop about 400kg of tension the developed hold will be about 400kg, for both big and small anchors. If the wind builds and the rode tension develops to 500kg - both anchors will have a hold of 500kg - no more no less.

There is no magic - the hold developed is determined by the yacht (and conditions) - whether the anchor is large or small (as long as it is not too small) - the hold is identical.

If I measure snatch loads, in my previously described test of 700kg - as long as the anchor holds - it does not matter if its big or small but the developed hold for a big or small anchor is 700kg. If we reach the ultimate hold of the small anchor, look at the 2006 SAIL and YM article for potential holds, then there is an issue but as most small anchors will achieve 2,000kg in decent sand (and one twice the size around 4,000kg) I remain to be convinced of the bigger anchors - and have seen no evidence to support the claims.

It might be said that in difficult bottoms a bigger anchor is better - they both are subject to the same rode tension - why is 400kg applied to a large anchor somehow better than the same 400kg applied to small anchor. In fact in a harder substrate the smaller anchor will bury more easily - in the same way a small garden spade is easier to dig deeply compared to a large one.

I raised a thread some, many, months ago on the frequency of dragging of new gen anchors - it did not matter if anchors were big or small - no-one admitted that their anchor had dragged.

Noelex said I quote:


This advantage applies across all scopes not just to so called "short scopes". For example a larger anchor at 5:1 might have the same holding power as an identical design smaller anchor at 7:1.'

unquote

Any evidence, at all? These 2 anchors used by the same yacht will have the same hold at 5:1 and might have slightly more, but the same hold at 7:1.

How, precisely, does two anchors under the same tension develop different holds? Its impossible!

A big anchor does not make its own hold - its 'imposed' by the yacht and that does not vary.

I think I'll go and have another glass of shiraz

Jonathan

 

[noelex]

Jonathan, you seem to continuously argue a small anchor has the same maximium holding power as a larger anchor of the same design. I am not sure if you really believe this or you are just trying to justify the case for using small anchors.

Much of what you have written is technically correct, but completely ignores the fact that in all but very unusual substrates such as solid rock, the ultimate holding capacity of a small anchor is less than a large anchor of identical design. This principal is well established.

It seems rather absurd to argue otherwise.

 

[Neeves]

The small anchor has a lower ULTIMATE hold, for a 15kg anchor, say, about 2,000kg. One twice the weight will be higher 3,000kg maybe 3,500kg (but it will not be 2 times - no anchor scaled up is scaled perfectly - as data shows). These ULTIMATE holds are irrelevant - no yacht of the sizes we are talking about will ever have that tension in the chain - its well beyond WLL of the chain, even if nothing else. Looking at the ultimate hold of the large and small anchors and using that difference to justify use of a large anchor is missing reality. There may be reason to use a larger anchor - but the ultimate hold is irrelevant - it must be some other factor that has not been mentioned.

But the hold developed is that of the tension in the chain - it has nothing whatsoever to do with the anchor (assuming the anchor holds). If the chain tension is 400kg, the sort of tension you could develop under max revs with a 40hp motor - the hold at the anchor, as long as it is holding, is 400kg, whether big or small. If you think the big anchor develops more hold - you are in cloud cuckoo land - and you need to come down to earth. You need to consider why the big anchor magically plucks extra hold from the sea or sand when the tension is fixed.

If the rode tension then increases due to wind, to say 500kg then the large and small anchor have a hold of 500kg - exactly the same. There is no magic that allows the large anchor to develop more hold - this is true of most seabeds and almost any sensible scope - the hold will be the same.

As I mention you will never reach the ultimate hold of the small anchor with the vessel for which it is intended - 2,000kg tension is huge and I doubt the bow roller will take the loads and the chain will start to stretch, assuming an 8mm G30 chain.

It does not matter the scope - the 400kg or 500kg tension will both allow the anchor at the end to develop 400kg or 500kg at 5:1 or 7:1 and whether the anchor is 15kg or 30kg. At 2:1 scope neither anchor may hold at 400kg nor 500kg - it will depend on the anchor - but 2;1 seems a bit low to expect hold - and I have not tried it (nor see any reason to try it).

Yes the ULTIMATE hold of the large anchor is higher - but I'm not sure why that is relevant - you will never, ever get there. You will never ever get to the ULTIMATE hold of the small anchor. If you do exceed ULTIMATE hold the anchor will drag, through the seabed - and commonly it will surface and drag.

its about the hold the anchors develop at the time in that seabed - and that is the same for both anchors as its the same tension.

Differences

The large anchor will be set more shallow and will be in the upper layers, the less consolidated layers, of the seabed. The smaller anchor will be more deeply set - and shear strength increases with the square of depth. It depends on design but the small anchor will have buried more chain - and buried chain resists any movement of the yacht being imposed on the anchor. If you dive on an anchor and the chain is off the seabed then any movement of the yacht is transferred down the chain and if you touch the anchor you will find it twitches, every yaw every up and down. Every twitch of the anchor reduces shear strength of the seabed. An anchor with buried chain twitches less, an anchor buried deep twitches less. If the anchor is fully buried you can use a screw driver to find a bit of the anchor, top of shank, roll bar - and twitches are felt through the 'probe'.

Because the larger anchor is more shallow set it MAY rotate more easily, in a change of wind - but there is no evidence either way.

In thin mud or very soft sand a large anchor will perform better - as you may reach the ULTIMATE hold of the small anchor and maybe the big anchor - in that substrate. But in mud you would be better using an anchor optimised for mud - such as a Fortress as most anchors will not self right, in a soft substrate, and will develop no hold at all.

To elaborate: most anchors fall, when deployed, and land on their side, sit in on the top of the shank or on the roll bar. If the shank or roll bar sink into the surface the righting movement that normally allows the anchor to address the seabed at the designed angle is never achieved - in a soft seabed some anchors roll upside down - and never self right.

In a hard substrate and weed the small anchor may, I stress the may, set more easily - because it is smaller and better able to penetrate. However I have never found a substrate so hard where any of 'our' anchors are defeated - Rocna, Supreme, Mantus, Excel, Spade, nor have I heard it reported - so I don't know of the relevance and am unable to confirm. I am not sure in weed - but the benefit of the fishermans is the small fluke better able to penetrate weed??? We try to avoid weed, poor hold if you do penetrate and not very friendly to the environment.

So far the large anchor mantra appears to be based on the idea that the difference in ULTIMATE hold, which will be bigger for the larger anchor, can somehow be extrapolated back to lower tensions. This is simply a dream and has no foundation, at all, in practice. There maybe other reasons for using a large anchor, thin mud or very soft sand being examples. But there is no other evidence to support the assertions that a large anchor will have a higher, 'intermediate', hold - just because its bigger. Its gut feel, nothing more. If there are other reasons - no-one has yet articulated those reasons - they are guessing (or clairvoyant with no basis to substantiate their visions).

You are correct, and we agree (I'm going to run out of shiraz!) a large anchor has a higher ULTIMATE hold than a smaller one - so what?

Please advise any technical reason why a big anchor is better - the tensions will always be the same, so the hold is the same. What makes a bigger one better?

If you want better - get a better design, do not waste money with a bigger anchor. No anchor maker will argue with you - you are increasing their pension fund! No retailer will argue with you, you are increasing their pension fund.

I am sure, because everyone says so - a bigger anchor allows you to sleep better. Now that is a good reason to buy a bigger anchor - but it lacks a technical basis. People like the idea - they want re-assurance, they want to be able to say to their wife, 'don't worry our anchor is bigger than recommended'. Its currently based on the marine equivalent of the urban myth, perpetuated on the internet.

Strarzinger, Novak - both suggest bigger, one uses a CQR and the other used Bruce - so I question relevance. Dashew recommends big anchors - he can set his anchors deeply because his smallest 60' motor yacht has 250hp engines vs a typical 60" sailboat engine of say 70hp.

Let's get some reasoning behind it. Something on which everyone can agree - and I can have an almost perpetual need to open a new bottle

Jonathan

 

[thinwater]

Doug,

I think you are correct, your final paragraph, and it not only applies to Europe. I think if bad weather is forecast most people will head for the best shelter available and being subject, directly to 30 knots would be avoided (by not going out that weekend or going somewhere with good shelter).

It would be cavalier and irresponsible to suggest that, for whatever reason, people only made recommendation based on 'best case scenario' so stressing that the concept of catenary has been undermined merits mention and individuals can make up their own mind. I for one had confidence in catenary for years and never used a snubber, now I'd never anchor without a snubber (or bridle)- as I've experienced strong winds with a bit of chop and found catenary 'wanting'. Because of this latter the idea of anchoring safely at short scope, less than 3:1 in less than, say 6m depth (maybe much more), is a piece of romantic nonsense.

Jonathan

I disagree. 50 kt summer squalls are commonplace here (US, Chesapeake Bay)--I expect to experience one or several each season. Nearly every day there is some risk, but you never know. I've seen it go from 5 KT to 60 KT in less than a minute, and yes, that was in shelter. If you can't bear that, sell the boat. It won't last more than 15 minutes, but it's like a hurricane with stuff blowing horizontal. Micro bursts over 100 KT have been experienced on the Bay.

And yes, when those come through, a lot of folks drag. I take then seriously, often adding a second anchor (Fortress) in a hurry if it is clear I am in the track. Lots of scope and always a snubber. Once I was caught by surprise and only had time to secure my tender on-deck by the painter. I watched it though a window, floating horizontal to the deck for 10 seconds at a time. The boat was only 27 feet, but it took some time to recover the anchors, since they were dug in deep (Danforth + Fortress).

I the summer I always assume 50 knots is possible, and add a second anchor if I really expect more.

The rest of the year I anchor more as you suggest.

 

[thinwater]

Neeves makes an interesting point, but I would like to pose the question a different way; why does the anchor drag? Some swimming will be required.

Soft mud etc. Obviously it is simply overpowered... or is it? I've done a lot of diving on test sets, and the actual reason is often (70%?) that the anchor chain rode across something (stick) that guided the anchor to the surface. This is a main reason why setting immediately fails; if you start dragging the anchor around, before it is ready, you plow farther and hit more stuff. Better to let it soak and set it in a shorter distance.

Wind shift. Specific to pivoting fluke anchors, a small anchor deep in the mud is safer than a big one with only the nose caught, and most of us lack the power to set a large one. I know from both experience and testing that recovering a set Fortress is easier the larger the anchor is. I've set a 2-pound Guardian so well that the windlass wouldn't touch it. A 12-pound Fortress set with the same amount of power came up easily. Interesting.

Rocks and other. Chance may matter as much as size. Better dive.

Size helps, but understand helps even more. Dive on your anchor.

 

[Neeves]

For those with time on their hands this thread looked to answer, with a lack of success, the frequency of new gen anchors dragging and whether there was any relationship to size.

http://www.ybw.com/forums/showthread.php?440143-Dragging-of-anchors&highlight=Dragging+of+anchors

And one factor, not described in the above thread, of anchor dragging issues is alluded to here, Sadly I am not a contributor and the meat of the article is this not available - but you identify the conclusion.

https://www.morganscloud.com/2016/0...lures-with-rocna-and-some-thoughts-on-vulcan/

Morgan's Cloud, not mentioned so far are also advocates of large anchors, specifically Spade. You can link to some of their thoughts from the above link.

Jonathan

 

[GHA]

For those with time on their hands this thread looked to answer,

Love to , but far to busy on google trying to answer who all those "Proponents of big, monster, anchors and their justification for the recommendation to use tham is that they are 'safe' at short scope" are you were banging on about

 

[Neeves]

GHA,

I'd not worry about it - everyone seems to agree with you that the idea of short scope, with a big or small anchor, is a nonsense - so we will hear no more about it (thank goodness). At least that's one coffin nailed closed.

Having said that I vaguely recall Dashew anchored up, somewhere, don't recall where but if I had to make a punt - New Zealand - with a Rocna (which I think he has since moved away from and now favours Supreme). I read the 'test' on his website. I think he purposefully anchored to test the concept, big anchor, short scope, strong wind and he did document the successful outcome. Again from memory it was a big Rocna, monster in my book, and his usual rode is one size down G70. He did have a picture of the Supreme to be used on one of his motor yachts and it was easily bigger than the man standing beside it. I do not recall use of snubbers - but it was some years ago when I read it. In fact I don't recall snubbers being emphasised at all in any of his writings - like you I'm busy, in my case tilting at windmills.

Jonathan

 

[Neeves]

Jonathan, you seem to have a bee in your bonnet about this subject.

It is not difficult to understand. A bigger anchor has more holding power than a smaller anchor of the same design. Conversely, an anchor deployed on a shorter scope has less holding power than than the same anchor deployed on a longer scope.

Thus, in general, for the same holding power you can use a shorter scope with a bigger anchor. Simple.

This advantage applies across all scopes not just to so called "short scopes". For example a larger anchor at 5:1 might have the same holding power as an identical design smaller anchor at 7:1.

This is just one advantage of larger anchor, there are many others, but it is not a miracle cure. /QUOTE]

Basically - if this is the justification for a bigger anchor - its rubbish - which is why I have a bee in my bonnet

As said repeatedly

For a given tension the hold are identical, big or small.

So if this is rubbish - what exactly is the reason for the bigger anchor?

We have put to bed the idea that short scopes are a panacea - lets get to grips with big anchors!

If there are good, supportable, reasons (other than gut feel) I'm with you.

Jonathan

 

[noelex]

The small anchor has a lower ULTIMATE hold, for a 15kg anchor, say, about 2,000kg. One twice the weight will be higher 3,000kg maybe 3,500kg (but it will not be 2 times - no anchor scaled up is scaled perfectly - as data shows). These ULTIMATE holds are irrelevant - no yacht of the sizes we are talking about will ever have that tension in the chain - its well beyond WLL of the chain, even if nothing else.

So we seem to agree that a larger anchor will hold a higher force before it begins to drag. That is at least a sensible starting point.

You frequently state that:

For a given tension the hold are identical, big or small.

This is true, but it just another way of stating if stating if the anchor is not dragging its holding ability equals the force applied by the boat. This applies if the anchor is big or small if it is a CQR or if it is best new generation anchor.

The important question is what happens if the force rises. I believe a better anchor design and/or a larger anchor will generally hold a higher force. This seems obvious but you are reluctant admit this.

Your argument for using smaller anchors seems to be summed up that the small anchor has plenty of reserve capacity. If you believe this why do you feel the need to deploy two anchors in 25-30 knots? You have stated two anchors are deployed to reduce the stress on the anchor. Why is it necessary to reduce the stress if you truely believe your anchor has a holding capacity greater than the WLL of the chain? That is a lot of force.

Then we have the question of scope. Once again, if you believe your anchor has plenty of reserve capacity. But you will never anchor at less than 5:1. Shorter scopes will reduce the potential holding power of the anchor, but with the large reserve that you claim why are scopes of 4:1 or 3:1 such a concern that you would never consider using these options? Surely these sort of scopes could be used in settled weather given your faith in the performance of your anchoring gear?

Don't misunderstand me. I think your precautions are entirely appropriate given your anchoring equipment. I would take similar steps if anchoring with the small anchors and light chain that you use. So I guess in one way we are in agreement.

It seems to me you accept some rather severe limitations when using such lightweight anchoring gear. There are no perfect solutions and sailing a lightweight catamaran your choices are naturally different to someone like myself on a cruising monohull anchoring almost full time.

 

[RupertW]

So we seem to agree that a larger anchor will hold a higher force before it begins to drag. That is at least a sensible starting point.

Your argument for using smaller anchors seems to be summed up that the small anchor has plenty of reserve capacity. If you believe this why do you feel the need to deploy two anchors in 25-30 knots? You have stated two anchors are deployed to reduce the stress on the anchor. Why is it necessary to reduce the stress if you truely believe your anchor has a holding capacity greater than the WLL of the chain? That is a lot of force.

Then we have the question of scope. Once again, if you believe your anchor has plenty of reserve capacity. But you will never anchor at less than 5:1 Shorter scopes will reduce the potential holding power of the anchor, but with the large reserve that you claim why are scopes of 4:1 or 3:1 such a concern that you woyld never consider using these options? Surely these sort of scopes could be used in settled weather given your faith in the performance of your anchoring gear?

Don't misunderstand me. I think your precautions are entirely appropriate given your anchoring equipment. I would take similar steps if anchoring with the small anchors and light chain that you use. So I guess in one way we are in agreement.

It seems to me you accept some rather severe limitations when using such lightweight anchoring gear. There are no perfect solutions and sailing a lightweight catamaran your choices are naturally different to someone like myself on a cruising monohull anchoring almost full time.

Even though I have disagreed with many of the conclusions of various posters this has got to be the best fact based thread on anchor chains of all that I have seen over the years with some real maths behind them too - even if the maths is used inevitably to prove ones on point of view.

I very much agree with your post above - particularly the contention that different combinations of light and heavy anchors and short and long scope produce different results with different weaknesses. I may be simplifying it too much (and certainly am still holding only slightly modified views to those I had a view days ago) but I think the following good points have come up (and a few more):

1) An anchor holds best at 0 degrees pull but is designed to work gradually less well at much higher angles
2) A big anchor of the same design as a small one will hold in stronger conditions or a higher angle of pull
3) Catenary is great for low to moderate conditions for reducing snatch loads and reducing anchor pull angle
4) We all disagree from deep experience whether catenery has any importance when chain taut in strong conditions. The maths proves things both ways round depending on whether you happily accept a higher pull angle or not, and whether you have an infinite amount of infinitely heavy chain in a bigger boat in a 50 knot gust
5) 1 person believes that the anchor is just for burying the chain which provides the real grip (and has been challenged many times to detach their anchor and try this even in 10 knots breeze
6) We all like chain for it's anti chafe properties and the way it never gets caught around rudders or keels
7) People who sail in cold waters don't snorkel every time they anchor and feel much more reassured than us warm water softees if they have big gear that looks like it would hold a battleship.
8) And this is the crucial point - nearly everybody here has found a way of anchoring in the worst conditions we each experience and it works almost every time for us - even if we don't understand why - so somebody like me trusts the 15kg Delta that has held my 42 foot boat every time in 50 or even 60 knot gusts and nasty chop and somebody else needs a 25kg Rocna for the same experience because they have been in the situation where a Delta let them down and I haven't yet.

 

[vyv_cox]

Strarzinger, Novak - both suggest bigger, one uses a CQR and the other used Bruce - so I question relevance. Dashew recommends big anchors - he can set his anchors deeply because his smallest 60' motor yacht has 250hp engines vs a typical 60" sailboat engine of say 70hp.

Jonathan

Some background to the first two. On Evans Starzinger's website he describes (described?) his reasoning. In Chile he carried out various trials using the Manson copy of a Bruce and a Rocna. the holding is very poor where he did this work, quite deliberately. I forget all the detail but he was convinced that the Manson Bruce held better than the Rocna in those conditions, AFAIR even a lighter Bruce was better than the heavier Rocna. These were highly oversized anchors for a 38 ft boat, 35 and 44 kg I think.

In recent feedback on a Facebook anchoring page moderated by Andy Marsh, Skip Novak said that he now uses a Rocna and has retired his CQR.

Dashew also used a very large Manson Bruce - reputedly with wider tips than the beam of his boat! It seems he now uses Rocnas, but specially made in higher than normal commercial weight.

 

[thinwater]

The problem i have with this idea of "properly sized" according to a chart is that boats are infinitely variable in weight and windage (the two main factors that contribute to loads on an anchor, but chain sizes are not. We only effectively have 4 chain sizes covering boats from around 30-50' and weights between say 3 and 15 tons. So, how can you have properly sized chain for all these boats.

You state that strength is proportional to weight in your second sentence which is simply not true, and you can get chain of the same size (and therefore weight) with significantly difference strengths. As weight is often a negative from both the chain handling point of view and from trim of the boat there has been a move in recent years toward smaller, stronger chain. So your "chart" may say 10mm but you can get equal strength with a better 8mm chain at a substantial weight saving, or an increase in length for the same weight.

I think it is obvious that I am well aware of these variables. G30 vs. G70. I was responding to a single post that implied that catenary somehow works differently for larger boats. In fact, it always works the same UNLESS something else is different.

I was out on the water this morning measuring rode tension and wind load on a trimaran anchored with a variety of bridles, snubbers, rodes, and foils (CB and rudders up or down). Yes, there are a LOT of variables. Best thing to do is experiment.

 

[thinwater]

I was out playing with anchors today, measuring loads and slaying with bridles. I've been doing this for several weeks. Much is said about the variability of load with "style" of the boat. The boats I was testing were a PDQ 32 (cruising catamaran, 9000 pounds) and Coraisr F-24 (folding trimaran, 1600 pounds) The front profile of these boats is WAY different, as is the weight.

Over a range of 10 - 28 knots, in the same place and same ind direction, the ratio of load was 0.54-0.57 (F-24 vs, PDQ). If we assume windage varies with the square of length, (24/32)^0.5 = 0.56. To relate multihull windloads to monohulls you apply a correction factor.

Coincidence, I'm sure. But my point is the ABYC tables are going to be far more accurate for most of you than guessing.

---

What I was actually testing was the effect of certain changes on yawing. Wind load was just for baseline data.
I also went for a swim to look at the anchor. At 10:1 scope, 10' to the roller, the chain was lifting at 20 knots, as the calculations suggest. But the angle remained very low. However, it was not buried (firm sand and was moving side to side). I then reduce the chain to 20 feet. And guess what; the tension dropped and the chain did NOT lift any sooner than it did with all chain. The boat yawed just as much. In light winds there would be a difference, but once it hit 24 knots, not much friends. The things that reduce yawing were changes to rigging, windage and the like, not chain. And this is on a 1600-pound boat with 1/4-inch chain, which is arguably far too heavy. The chain stayed off the bottom with the PDQ.

---

Anyone want to guess what the 25 knot wind load is on a 32-foot catamaran? Best guess gets bragging rights (if you subscribe to Practical Sailor and look at old articles, that is cheating).

Can you guess what the working load is on your 35- to 40-foot mono, with the chain and snubber you have? How do you know?

 

[Neeves]

Noelex,

The answer is simple.

You carry an anchor on your bow roller twice the size recommended. It has a large surface area and is heavy. It contributes to bow weight and will impact performance of your yacht when sailing upwind in seas. You need the big anchor for severe conditions - yet most of the time the conditions are not severe.

I carry a light anchor, undersize for the spreadsheets. it is perfectly adequate upto 30 knots. It does not impact performance of the yacht it is not catching seas when they break over the bow. I listen to forecast - when winds are forecast to be more than 30 knots - in an anchorage - I deploy a second anchor. I maybe deploy a second anchor one time in 10 maybe more so 1:15.

Deploying 2 anchors is easy - deploy main anchor, mark location on GPS, power set with adequate scope, motor to assumed position of second anchor, drop and deploy rode by hand. Allow yacht to drop back, , motor in second anchor - simple. if this is too complicated I'd suggest you will find it impossible to moor in the Baltic or Patagonia where you will possible need 2 anchors and 3 shore lines.

Many people on this forum have AWBs which have all the weight squeezed out of them. having an anchor twice the size and twice the area will impact sailing performance - specifically upwind in breaking seas.

Most people sail in breaking seas quite frequently but anchor in winds over 30knots seldom (most of us seek shelter). To carry a heavy anchor that impact sailing performance and is only required very occasionally - to me lacks logic. But then I sail in heavy seas and big swell, find suitable anchorages and religiously listen to forecasts,

Finally - I don't think you really accept that the holding capacity of a small anchor is factorially higher than anything a yacht is likely to develop. As I have mentioned holding capacity tests on small anchors return holds or 2,000kgs. Secondly reports of new gen anchors dragging, of any size, are like hens teeth

I take a quote from your post:

The important question is what happens if the force rises. I believe a better anchor design and/or a larger anchor will generally hold a higher force. This seems obvious but you are reluctant admit this.

unquote

I think you need to polish up your comprehension - or I need to repeat.

A larger anchor has a higher holding capacity but the hold of the small anchor is so high a yacht, for which that anchor is recommended, will not reach that tension in the rode.

Of course a bigger anchor has more potential - and as you are willing to admit - so does one of a better design Possibly my choice is dictated by using anchors of better design? and have less experience of designs I would reject.

I have measured the tensions on my rode with my yacht. I have measured the holding capacity in a variety of seabeds with the anchors I use - as Thinwater asks - have you? As far as I am aware there is no data on the Ultimate hold of a Mantus - you and anyone else who has one has no idea (and I'd guess a decent dinner) no idea of the tension your yacht develops from windage coupled with no idea of the hold of the anchor you use - no wonder you use an anchor so large.

Finally - chain does impact performance of the anchor, if it is on or in the seabed. But if you think your chain is the key - then take the chain off and replace with rope - and you will find the performance of your ground tackle similar to all chain (except you lose abrasion resistance). Now replace the chain and take the anchor off - and guess what happens. If you have a decent snubber you could use super strong piano wire (chain helps but there are alternatives) anchors have been found to be critical.

Jonathan

 

[thinwater]

I would fit and had all chain, my view is the weight of the catenary holds the boat and the anchor is the back up when the wind picks up.

This is similar to believing that the pulley holds the rope and what it is tied to is not very important. In fact, the catenary only deflects the chain, like a zero friction pulley. If you measure the friction (I've done this) of the chain (that is actually still on the bottom--only small fraction of the total) you'll find it is 20-50 pounds at most, probably less. Once it lifts friction is zero.

Have you tried anchoring with just chain?