JW's Upgrades for 2016 - Anchoring

[Neeves]

I never said you used the full 40%. I said that nylon had the ability to stretch 40%, to failure. it depends how the nylon is made but most nylon cordage, Marlow or Liros will stretch 40% to failure. We actually use climbing rope that has greater stretch (its made that way) and we have a 12m, each side, bridle. The 40% total stretch (its roughly linear) means that it will work through the whole range of windspeeds. The chain catenary slowly disappears (it never disappears) but as the sag is removed then the chain has less and less ability to soften any snatch loads. A straight, looks straight, chain acts like simple piece of steel rod - any snatch is absorbed by the anchor (or seabed) and your yacht. As the chain starts to look horizontal - that is when the wind is up and your anchor needs all the help it can get, the chain has given up - but nylon is there continuing to work on your behalf. You obviously size the nylon, diameter of cordage, to suit your yacht, skinny for a little one fat and overfed for a big one, and anything in between. You obviously do not want to be approaching breaking point - so you are never 'testing' that 40%. Some carry 2 bridles, one for, say, upto 35 knots and another storm set for beyond. We carry spares, having broken 2 arms so far.

Something else to consider - you want to take the load off the windlass and are adding chain locks - your anchor takes the same load as a chain lock. I'd not want constant snatch loads on any part of my yacht if I can possibly remove them. The lock might take the load, 20mm steel plate might take the load - but my guess is most boats are built to be in marina?? not to take the full ooad of an anchor chain at 35/40knots. The snatch load on a, or from a, chain is much higher than you are suggesting. There are 2 snatch loads, that from yawing (sailing at anchor) and that from waves.

We find, using a bridle, that first one side takes the load then the other - the 2 arms only share the load when the yacht moves though the centre 'point'. But a bridle helps to steady the yacht - it reduces the swing and veering.

But if you guys all bow out and head to a marina when it gets breezy then the question or debate is academic.

One day that option might not be possible - maybe then you will wonder if you might have advantageously used a decent long snubber or bridle. You might also wonder how it works and how you can size for your yacht - too thick a snubber - there is insufficient elasticity, too short there is insufficient elasticity. As I mentioned those rubber snubbers, like dog bones - each is equivalent to about 2m of nylon (basically an expensive waste of time). Try to get more out of them by adding more wraps - they snap.

We had a gale through Sydney early this morning, all the coal ships (there can be a queue of 40 of them) sitting to load outside Newcastle, will have been ordered off the anchorage (its simply the bit of coast off the harbour and no shelter at all, about 40/50m deep). These ships take 100,000t/200,000t (maybe more) of coal to China, Japan etc - we have had too many ships on beaches - they no longer leave it to chance and the inadequacy of commercial vessel ground tackle. HMS Ark Royal when at anchor had one officer specifically tasked to watch drag. The ship slowly dragged back and at a specific point engines were started, anchor lifted, she moved forward, dropped anchor and the sequence re-started. Common practice - but does not engender a decent nights sleep (unless you employ crew). The other problem is that the anchors of commercial vessels break, but that is another story.

And for us there are possibly better ways and as elasticity and dampening loads is not something to be considered we can agree to disagree.

Good luck

Jonathan

 

[jfm]

Neeves you are running entirely on gut feeling and ignoring science. Sure, an uber taught chain is like a steel rod. But I've shown you with maths that a typical anchor chain isn't taught even in very high winds. There is still stretch left in the catenary. In hurricane's pictured example, which I think is typical, there is much more stretch in the catenary then in the nylon. Your gut feel is telling you otherwise but the maths gives the correct answer. You wish me luck I wish you some science

And I repeat that you're in a tiny minority. Zillions of professionally crewed yachts do not use snubbers. No classification society in the world requires snubbers. They're all wrong? The benefit of a snubber is reduced noise on the roller when sleeping plus relief of winch load if the boat has no other stopper, plus as you say it might tend to reduce swing. But that's it IMHO; it doesn't add meaningful elasticity enough to "pay for" the 12m ( in your case) reduction in catenary length

I'm happy to agree to disagree though. We've done it to death! Best wishes

 

[Neeves]

What on earth gives you the idea it reduces catenary length by 12m where on earth did you get that idea?!

On the rest I'm happy to disagree, but I'll not be quite as dogmatic, nor if I may so - rude.. I get paid by people whom I suspect have a greater marine and boating background than you for explaining in detail what i have briefly summarised.

Sorry to anyone else who might have wanted to explore the idea - I don't need replies like the one above.

 

[MapisM]

Been offline for a while, and I only just read the whole elasticity debate.
And while I see the point of having a somewhat elastic component if/when the chain is perfectly straight, I can't actually see that happening.
I mean, if that happens, it's either because there isn't enough chain in the water (easy to fix) or the conditions are so extreme that the anchorage is untenable anyway.
In fact, if/when there isn't at least a last bit of chain laying horizontally on the seabed, it means that the anchor begins to be pulled also vertically, not just horizontally. Which is obviously a recipe for dragging PDQ, and no snubber can solve that problem, no matter how long.
Conversely, if the last bit of chain IS horizontal, it means that the catenary is doing its job.
Btw, Neeves, the fact that a 12m snubber reduces the catenary length by 12m isn't implicit in the fact that if you wouldn't have such long snubber you would have the same length of chain added to the catenary effect?
That's what I guess jfm assumed, anyway.

 

[Neeves]

Deleted - life is too short

 

[Deleted User YDKXO]

Btw, Neeves, the fact that a 12m snubber reduces the catenary length by 12m isn't implicit in the fact that if you wouldn't have such long snubber you would have the same length of chain added to the catenary effect?
That's what I guess jfm assumed, anyway.

My thoughts too. If you need a 12m length of rope to aid the catenary effect of the chain, why not just let out more chain to achieve the same thing? I'm sure I haven't anchored in 48kts of wind but I have anchored in some pretty strong winds and sometimes I have snorkeled out to see how the anchor/chain is behaving. It is often a struggle if the wind is strong even with flippers! However I have never seen the chain completely lift off the seabed which tends to support your point in that if the chain is bar straight, that is going to cause the anchor to lift anyway so either you shouldn't be there or you need more chain. Either way, 12m of rope isn't going to help much in those circumstances

 

[Neeves]

Some of you are re-equiping with modern anchors, some are even re-equiping with a specific modern anchor - maybe read what the inventor of the anchor says of chain, you can say he is also just basing his comment on gut feeling and no science - I wonder if that goes for his anchor on which you are going to base your faith.

Catenary is very 20th Century - some of us have discarded old science and moved on:

http://www.petersmith.net.nz/boat-anchors/catenary.php

Later look at the motor yachts being designed by Dashew, another one possibly working with gut feel and no science.

Have a nice day

 

[petem]

Some of you are re-equiping with modern anchors, some are even re-equiping with a specific modern anchor - maybe read what the inventor of the anchor says of chain, you can say he is also just basing his comment on gut feeling and no science - I wonder if that goes for his anchor on which you are going to base your faith.

Catenary is very 20th Century - some of us have discarded old science and moved on:

http://www.petersmith.net.nz/boat-anchors/catenary.php

Later look at the motor yachts being designed by Dashew, another one possibly working with gut feel and no science.

Have a nice day

I enjoyed your link, made a very boring conference call far more interesting.

 

[jfm]

Hi Neeves
I apologise for any offence and it was not intended. I was being robust (on which score you opened proceedings, but no complaints from me!) but nevertheless I apologise. It's forums and the internet - this would be a better discussion in a pub. I am interested to hear the counter argument and will happily stand corrected upon seeing the maths.

What on earth gives you the idea it reduces catenary length by 12m where on earth did you get that idea?!

There was some shorthand in what I wrote there but I thought you'd get it. Inserting 12m of rope snubber keeps the same catenary cord length but it replace 12m of heavy chain with 12m of lightweight rope. The elasticity of the catenary requires weight, and swapping 12m of chain in the catenary for 12m of rope is pretty similar to shortening the catenary by 12m, because chain is useful in a catenary while rope largely isn't. There is advanced maths that I'm too rusty for now where you analyse catenaries with non-constant weight-per-metre, but let's not go there other than to remember that if you tie 38m of heavy chain to 12m of almost zero-weight thin steel wire and droop that into a 50m catenary, it will have very similar properties to a 38m chain-only catenary (whose ends are at different heights, but the effect of that isn't large here). Hence my comment that you've shortened the catenary by adding the snubber line.

 

[Nick_H]

Btw, Neeves, the fact that a 12m snubber reduces the catenary length by 12m isn't implicit in the fact that if you wouldn't have such long snubber you would have the same length of chain added to the catenary effect?
That's what I guess jfm assumed, anyway.

Neeves mentioned in an earlier post that he runs each side of the snubber through a bow fairlead and down the side deck to the aft cleat.

 

[jfm]

Neeves mentioned in an earlier post that he runs each side of the snubber through a bow fairlead and down the side deck to the aft cleat.

Ah ok, thanks Nick. I missed that (forums eh?). OK, then the catenary remains of the same length, so the elasticity (tiny though it is) of the nylon line (net of fairlead friction) is an addition to the elasticity in the system, without any "price paid" in form of lost chain catenary. Still...

 

[MapisM]

Neeves mentioned in an earlier post that he runs each side of the snubber through a bow fairlead and down the side deck to the aft cleat.

Doh! I suppose you're talking of his last sentence in post #45, that I must admit to have not understood, the first time I quickly read it.
Thanks for clarifying!

 

[MapisM]

Deleted - life is too short

Well, thanks for your further explanations anyway.
In fact, when I had to check out earlier, I had just seen your reply that you now deleted, and I left it open in a browser page, so I could now read it.
And found it interesting, btw. I'm not reposting it because if you choose to deleted it, who am I to argue?
But in case you would change your mind and you didn't keep a copy, just ask and I can avoid you the hassle of re-writing it.

From my part, I would only add that I have indeed been anchored in some 40+kts (albeit in wave-sheltered bays), and even with my above stone age anchor, I never dragged one inch.
And never had the impression that the chain was completely raised from the seabed, either - though obviously I wasn't snorkeling around to check.
Otoh, I did use an 8+ scope, as you also mentioned in your reply, and I'm rather skeptic that some kind of 1m further elasticity, on top of say 80m of heavy chain, would have made any difference.

Thanks also for your link, that was an interesting reading - though it's not so surprising to read that the Rocna anchor designer believes to have built something better than my stockless bricks...
But even assuming that there is a significantly higher holding power with modern anchors (something I'm not arguing with), I would never ever trade the convenience of hawsepipe anchors for that. But that's me, of course.

At the end of the day, I suppose it also comes down to the fact that I'm not seeing myself anchored in 60kts wind at Deception Island anytime soon.
With all due respect for those who do, imho life's way too short also for that.

 

[benjenbav]

Isn't the basic premise of Mr Smith's argument that the advent of anchors which grip like a limpet means that catenary is no longer particularly useful in extreme conditions owing to the fact that the anchor will stay put long enough for the chain to be vanishingly close to being straight and so its ability to give will have been removed? If that is so, then surely any nylon snubber (with greater stretch than the flexibility offered by the catenary) will already be stretched to the max also and its added advantage will lie in spreading the load rather than in having some stretch in reserve?

 

[BruceK]

Can I diverge here slightly? I have a standard Bruce anchor and having read this thread and the links am rather concerned about it's poor holding power in tests, albeit I anchor frequently, in good weather it must be said, I have never drifted. Nonetheless the subsequent research into Rocna and more pertinent due to my pulpit destign, Vulcan anchors do show excellent results. But here is the rub. Once all the slack has been taken up from the windlass on retrieve I need to often give the Bruce anchor a bit of a heave to get it out of the sand as the windlass struggles a bit if it has set correctly. This can be done by hand if not set too deep or a dab of throttle to cause it to skip if it is. If the Rocna/Vulcan bury themselves so deeply and so quickly, and can set within their own length, how the dickens am I going to retrieve them? I dont think there is anything wrong with my windlass, if anything it's a bit overkill with 10mm chain on a 7 ton boat and it is well capable of retrieving the chain, just not digging deeply set anchors out.

 

[Deleted User YDKXO]

Isn't the basic premise of Mr Smith's argument that the advent of anchors which grip like a limpet

Yup no prizes for guessing that the author sells Rocna anchors. I want to raise one practical issue regarding a mixed rope/chain rode. Yes I can accept that when the proportion of rope to chain gets below say 20/80 (ie the upper sections of the curves) the max tension is not significantly different compared to a 100% chain rode. Thats sort of intuitively correct. The problem is this though. If you carry a mixed rope/chain rode, how do you always ensure that the proportion of rope to chain is at the upper end of the curve for all depths and scopes? In fact the author specifically states that there is now relatively more benefit from a higher portion of chain in deeper water. So given that if you carry a mixed rope/chain rode you can't easily vary the proportion of rope to chain, I would still contend that it is just simpler to carry a 100% chain rode because its always going to give you the maximum tension under all depth/scope conditions compared to a mixed rode. This is particularly the case for a motorboat which usually has more than enough power to carry a 100% chain rode and a powered winch to handle it

 

[Neeves]

Neeves mentioned in an earlier post that he runs each side of the snubber through a bow fairlead and down the side deck to the aft cleat.

Correct - its nice that someone is observant.

The decks are 10m long with cleats at stern, bow and amidships. Thew snubbers are run through the stanchion bases down each sidedeck and protrude about 2m to the join in a common shackle and chain hook. The chain hook is stored, when not in use on, or just above, the bow roller. Consequently they replace no chain, or only 2m. The snubbers are left permanently and because they run through the stanchions are not obtrusive.

The joke is that instead of buying 100m of 12mm chain (and adding 100kg on the bow) the previous 10mm chain could have been used to better effect with 10m of a correctly sized snubber. This would also have saved buying a new, gypsy, wildcat, chainwheel and negated the need for a chain lock. The snubber would have provided all the elasticity the chain offers but would offer the same and additional elasticity through the complete wind range and at higher wind speeds the catenary benefit disappears geometrically (the force of the wind increases as square of windspeed) and there is less catenary (sag) to have any impact.

To introduce even more controversy the chain, assuming 10mm is the correct size (actually strength) the 10mm chain could have been replaced with 8mm G70 chain (and a new chain wheel) saving over 200kg in weight. The 8mm G70 should cost about the same as 10mm G30/40. The 8mm could have been manufactured in America, Peerless, or Italy, Maggi, and should have been supplied with certification confirming strength of that batch and that the length had been Proof Tested (documentation sadly lacking from most Chinese sourced chain)..

jfm, apology accepted. A problem I see is that people who are looking for advise will read your posts and because you shout louder think you offer the only solution. There are usually alternatives to most problems and people use those alternatives because they commonly work and sometimes solve other issues - in this case save lots of money and weight but also offer a technical advantage.

 

[Neeves]

Anchor makers have a tough life, we complain when they drag and now we complain that they set too deeply! If you try to lift a modern well set anchor aggressively there is a chance you will damage something. The only answer is to be patient and tolerant (a bit like contributing to a thread on the internet) and allow the gentle hobbyhorsing of the yacht and the power that hobbyhorsing produces to slowly break the anchor out - the seabed will 'give' but slowly.

Your yacht does not simply sit there in strong wind, it moves side to side and back and forth (and up and down in any waves) the nylon is stretched, say 20% - but it still has another 20% of stretch left (I pluck figures out of the air as illustration). As the yacht moves forward (pulled by the potential energy stored in the snubber (and chain)) it (the yacht) develops kinetic energy (momentum) that works against the next gust etc.

I appreciate no-one intentionally anchors in the path of a forecast 50 knot storm. Anyone with any intelligence when offered a questionable forecast ups anchor and moves to somewhere that the effects of a 50 knot wind are factorially reduced. However some storms are simply not forecast (they are simply forecasts, not guarantees, usually covering too larger an area), you cannot move somewhere else (or when you get there you find the anchorage is filled with like minded people and your space is on the 'outside'. Snubbers offer an advantage in these worst case scenario situations. Most of the time you do not 'need' a snubber - catenary is fine. But beyond, say, 30 knots for most people catenary has disappeared (they do not want to carry the weight of an overfed gentleman permanently standing on the bow) and snubbers are the only thing to ensure there are no shock loads. However when you go to sleep at night, in a 20 knot breeze, you do not know what will happen overnight and its good seamanship to attach the snubber - every time.

You are rightly concerned that Mr Smith is selling anchors (so sort out the hype from the 'gut feel') - but Dashew does not - he sells motor yachts and big ones. Smith is simply 'copying' Dashew's ideas. Dashew's yachts have been proven in some tough places - he is not recommending lightweight strong chain because he sells chain - he does it because he thinks and has found it to be right (along with increasing numbers of others). Chain is considered old fashioned, or old technology, the use of lightweight High Tensile chain shows this idea is old fashioned and the ideas of a Luddite - chain technology is developing and even Dashew's idea of using Hot Dipped Galvanised HT (G70) chain is starting to look dated - there are further developments.

 

[jfm]

Neeves, hang on a minute. If you base your case on an unusual set up, namely the snubber line running along the deck, you have a duty to emphasise that and draw attention to it, not bury it in a long post and then sort of think everyone has a duty to find it and smugly sort of chastise everyone for not noticing. We're mostly busy and can only read stuff fast, so you need a sentence along the lines of "I need to emphasis my somewhat unusual snubber line layout so that no-one inadvertently thinks I've shortened my chain catenary...". If Nick H hadn't point it out I think we'd still be in the dark on this. All that is just imho of course.

I'm pretty astonished by the linked-to anchor ramblings of Mr Smith. I need to write a longer post and don't have time now, but as a starter-for-ten can you or anyone see anything at all that is correct in the 6 paragraphs under the heading "Mathematical modelling" [sic]? The guy utterly fails to understand his own graph doesn't he? It only becomes pointless to increase scope once the graph flatlines; as long as the graph has meaningful gradient, extra scope is worthwhile. Surely that's obvious, isn't it? Then he says there is no point increasing scope beyond 8:1, yet his own data says if you increase from 8:1 to 10:1 (hardly a lot of effort!) you go from 600 to 1000 lbf (pounds force) of anchor lift-point tension, which is a massive 1.67x benefit, and is highly meaningful. In the para beginning "If one studies...", is anything at all correct? Is he all there, or am I going badly wrong in analysing his data? Somebody please say, and shoot him or me please!

Neeves, with your 12m snubber lines, you've already said you go nowhere near 40% stretch. What percentage are you truly getting, in wind speeds of say 20-40 knots? (below that wind speed no one cares). Alternatively, what is your boat and what diameter of nylon rope are you using? I'm thinking you are getting stretch measured in a small number of centimetres, but if you're actually getting something in the order of or more than the 0.8m I calculated for my snubberless catenary then the stretch becomes meaningful. So what figures are you actually getting? And I need to throw in at this juncture, because it is highly relevant, that your comment about only one of the two lines stretching (except when at top dead centre so to speak) doesn't hold water, because as soon as the first loaded line stretches a few cm then as a matter of geometry the second one has to kick in and be stretched too, even when not at top dead centre, so halving the system's elasticity (ie there is a kink in the graph at that point)

 

[jfm]

I still haven't read it in full but when I saw his wind drag numbers on a quick skim yesterday I was suspicious.

Just under the 7th graph he says a 10m boat has 500daN of wind drag in 40knots of wind. That's 500kg of force, so to speak

Beside the second graph he says a 15m boat in 50 knots of wind has 2000kg of wind drag. The wind velocity is a square law at these speeds, so the 10m boat would have 1.25x1.25 x 500kg of wind drag if it were in 50 not 40 knots of wind. That's 780kgf. The other relevant factor is frontal area (because I'm assuming the same drag coefficient for the two boats, which is sensible) so he is saying a 15m boat has 2.6 x the frontal area of the 10m boat. Now, for the same type of boat, area is a squared relationship to length or width, so the frontal area of the 15m boat is only 2.25x that of the 10m boat, not 2.6x.

Then let's keep going. The 8th graph (the last one) shows a near linear relationship between wind drag and boat LOA, which is plainly wrong because it is a square law AOTBE. And look at the value for a 15m boat in 50 knots of wind - the graph shows 1100kgf not the 2000kgf shown in the second graph; a discrepancy of nearly x2. FWIW, I'm confident the 1100 is closer to correct, despite the dodgy graph, but the 2000 number suited the point he was trying to make under the second graph

I could go on but you get the idea. Some of his conclusions are of course correct, those being the self evident ones. but the rest is garbage and his numbers and analysis get an E grade, though they suit the conclusion he is trying to justify. BTW I have nothing bad to say about Rocna anchors - they are widely accepted as outstandingly good and I'd :encouragement:anyone thinking of specifying one. My comments on Mr Smith's paper are all in relation to his analysis that catenary = useless