Displacement vs Chain MBL

[BabaYaga]

With a tension in the rode of about 70kgs a 30m x 8mm deployed rode in 6m depth (from bow roller to seabed) will have the very last link lifted off the seabed. I know this as I deployed 30m x 8mm chain at the defined scope and measured the tension, in air, and made a crude allowance for seawater, to arrive at the 70kg.

8mm chain weighs about 1.45kg/m and 6mm chain about 0.8kg/m, total weight of chain deployed @ 30m is 43kg and 24kg, respectively. Now assuming we have this 30m hanging vertically from the bow roller in deep water and no wind - which is easier to lift, 43kg or 24kg? If we add back the windage (of 27kg) which is easier to retrieve 70kg or 51kg. Would anyone notice the difference?

(Sorry about the absence of a few days...)
There is something in your reasoning on these matters, exemplified by the quotes above, that goes against my intuition. I think in this calculation you grossly over estimate the importance of chain weight and, consequently, under estimate the importance of the windage. The fact that the last (or first) link is off the seabed does not mean, in my view, that the total weight of all that chain but be lifted by the man hauling on the foredeck. It is in fact the windage and/or current force on the boat that does the lifting of the chain.
Imagine a yacht in the same situation with a rode consisting of dyneema rope (virtually weightless in water). With the weight factor gone, the person hauling would only have to overcome 27 kgs (instead of 70 kgs) to retrieve the rode?
Surely that cannot be the case?

 

[Pete7]

When you down size chain, in our case from 8mm to 6mm you need to ensure that the new rode has the same strength as the old -

Rubbish, just because something has been done a particular way in the past is no reason to continue to do it. Instead go back and work out what is actually needed. In this case what is the load your 35ft cat is likely to see in say 50 knots? Then look at the maximum holding you could expect to achieve from the anchor and think about the attachment to the boat, is it capable of holding that force? Good decisions should be based on facts, not comparisons a previous owner may have made.

So what is the likely maximum load on your 35ft cat in say 50 knots of wind. 42 knots will do if its easier or data to hand.

 

[TernVI]

Rubbish, just because something has been done a particular way in the past is no reason to continue to do it. Instead go back and work out what is actually needed. In this case what is the load your 35ft cat is likely to see in say 50 knots? Then look at the maximum holding you could expect to achieve from the anchor and think about the attachment to the boat, is it capable of holding that force? Good decisions should be based on facts, not comparisons a previous owner may have made.

So what is the likely maximum load on your 35ft cat in say 50 knots of wind. 42 knots will do if its easier or data to hand.

I don't disagree, but:
How would you propose calculating the maximum instantaneous force in a real world anchorage in 50 knots?

Once you realise that the sea doesn't always stay millpond flat, the sums get hard!

Bigger chain will give more damping due to moving through the water and its distributed mass.
The lighter the chain, the more heavily you rely on rope snubbers.

It's not for nothing that many people stick to using what's been proven to be a reasonable compromise over decades of collective experience.

 

[Neeves]

Rubbish, just because something has been done a particular way in the past is no reason to continue to do it. Instead go back and work out what is actually needed. In this case what is the load your 35ft cat is likely to see in say 50 knots? Then look at the maximum holding you could expect to achieve from the anchor and think about the attachment to the boat, is it capable of holding that force? Good decisions should be based on facts, not comparisons a previous owner may have made.

So what is the likely maximum load on your 35ft cat in say 50 knots of wind. 42 knots will do if its easier or data to hand.

Whereas the idea that choosing chain strength from first principals might be correct in theory - in practice it is impossible and a very unrealistic idea. Sounds good - but not feasible.

Most chain sizing has been chosen from chain weight vs vessel size by Classification Societies and these spread sheets, though crude, have stood the test of time. Indicative of this is the fact that chain failure is simply not reported, other than a very few exceptional cases, (usually unknown chain of dubious origin that was well past its used by date). These spread sheets are therefore correct though may include too high a safety factor.

Very few people have any idea of the tension in their rode and to expect an owner to be able to calculate the possible tension in a 55knot/65knot wind with any accompanying chop and swell is frankly, a joke. I think the idea of basing a calculation on 50 knots is incorrect, in any case, as we have been subject to higher.

As an example please advise the tension in your rode under the conditions you think is likely to occur - and advise how you made the calculation. I do note that some members make outrageous suggestions (like using a soft shackle to secure an anchor to the chain) but have never actually acted on the recommendation they suggest. This at best can be dangerous and is very misleading. So back up your comments with some data.

I happen to be one of the few who has made actual measurements and I am happy that for our catamaran the 8mm G30 chain originally specified by the designer of the yacht is as near correct as possible. I have never seen a yacht with chain undersized - but it is common to see oversized chain supplied - chosen because whoever made the choice prioritised catenary over strength. I don't need to calculate, I've made the actual measurements and though to identify the tension atl 65knots I need to extrapolate I have a fairly accurate view of the, actual, information.

I have heard of chain stretching - but this has not been accompanied by a test being conducted on the chain to define whether the chain was badly made or incorrectly sized. I have tested many chains from a cross section of US and European suppliers and a few Chinese suppliers - and have found one chain with a stretch characteristic well outside specification. So chain is badly made (Western supplier) - but this has nothing to do with incorrectly specified for the yacht on which is might be used. Badly made chain has been the reason chain has failed - in the distant past. Recent chain failure is now noticeable by its absence - which reflects possibly on better quality chain being made today, from China and the West.

Anchor performance is another phurphy you include in your post. Modern anchor holding capacity in a decent sand seabed for a 15kg anchor for our yacht yacht (chosen based on a similar set of spread sheets developed by Classification Societies) has a hold of 2,000kg - well beyond the Proof Test load of the chain recommended. Based on the anchor spread sheets a 15kg anchor is far too big for our yacht - yet you and many others would quote the old mantra - buy bigger. When you reply with the tension likely for your yacht you can perhaps include reasoning on why you use a bigger anchor than recommended. It is interesting that the mantra to buy a bigger anchor is never accompanied by the comment 'buy a bigger chain'?

I look forward in anticipation, but little optimism, for the data you will provide on your yacht. I will be interested to see how you calculate windage - which you will need to make any sensible attempt at defining tensions. Having rubbished the idea of using spread sheets derived by Classification Societies I assume you will have the date at your finger tips - or are you one of those who rubbish established practices and make outrageous suggestions that you do not actually follow yourself, a bit like soft shackles to join anchor to chain.

Jonathan

 

[Neeves]

I lifted this spread sheet, which is I think fairly typical, from West Marine. In view of threads title and Pete7's comments it does seem relevant and might be food for thought.

I am unaware of 3/16th" being used as an anchor chain - I wonder what quality of attachments are available in the US to join anchor to chain and whether you can have an electric retrieval device. 4mm G120 chain is available and I think it might be possible to galvanise it to produce a product with the same strength of 8mm G30. But as there are no matching shackles, no chain hooks, no hammerlocks, no omega links it would be impossible to thread a dyneema short shackle through the link and no-one makes a windlass with a 4mm gypsy. It would thus be difficult to attach an anchor and you would need to retrieve by hand or treat it like wire and retrieve with a drum winch. I'm not sure even why G120 4mm chain is made - as with no attachments - I cannot see or imagine how it is used.


1/4" we would recognise as 6mm; 5/16th" as 8mm; 3/8th" as10mm and 1/2" as 12/13mm.

Light Boat	Medium Boat	Heavy Boat	3-Strand Nylon Dia.	Chain Dia.
26'–30'	23'–27'	21'–24'	3/8"	3/16" PC
31'–35'	28'–32'	25'–28'	7/16"	1/4" PC
36'–40'	32'–36'	29'–32'	1/2"	1/4" PC
41'–45'	37'–40'	33'–36'	9/16"	5/16" PC/BBB or 1/4" HT
46'–50'	41'–45'	37'–40'	5/8"	5/16" PC/BBB/HT
51'–60'	46'–54'	41'–48'	3/4"	3/8" PC/BBB or 5/16" HT
61'–70'	55'–63'	49'–56'	7/8"	1/2" PC or 3/8" HT
71'–80'	64'–72'	57'–64'	1"	5/8" PC or 1/2" HT

Basically for a 28'-36' yacht WM are suggesting 6mm chain, 8mm for a 36' - 45' yacht (and our cat is equivalent in windage to a lightweight 45' yacht), 10mm for 46'-54' yacht and 12/13mm for a yacht 55'-63'. I note that WM are suggesting a 6mm x G40 chain for a 37'-40' medium displacement yacht.


As few on this forum have a yacht larger than 55' the spread sheet relevant to YBW is actually very small and covers 6mm, 8mm and 10mm chain.

I am assuming that WM would be advocating use of a snubber - but I have not checked what views WM have on snubbers. Their suggestion that a 6mm chain would be suitable for a 37'-40' yacht implies to me they are discounting catenary and the rode would need a decent, sensible, long snubber.

Pete7 seems to imply he sails a 31' Moody - perfect (according to WM) for 6mm chain. it will be interesting to hear his views and what he calculates is the correct size of chain for his yacht. As WM are discounting catenary (or that is my assessment) their choice of chain is based on chain strength and it appears our chain is perfectly matched (the old 8mm and the new 6mm).

I have to assume WM know what they are talking about as they are the biggest chandler chain in the world (and sponsored the oft quoted 2006 anchor holding capacity tests).

Over to you Pete (and anyone else who cares to comment)

Take care, stay safe

Jonathan

 

[PhillM]

Ok so I think that clearly makes the case for 6mm chain on little Paean. Interesting that Jimmy Green go with boat weight not just size an by their sizing I would need 8, chain. Although to be fair to them there are few 24 footers with 3T displacement. That I guess is the function of being wood and with a decent sized keel.

 

[Neeves]

If you do run with 6mm - take your anchor to your chandler and find a shackle that fits the anchor and chain, what seems to match might not - been there, done that. To fit the chain the shackle needs to be small - and it might be too small too physically fit the anchor (and not lock up in the slot).

Jonathan

 

[Pete7]

Whereas the idea that choosing chain strength from first principals might be correct in theory - in practice it is impossible and a very unrealistic idea. Sounds good - but not feasible. Jonathan

Well the good news is you just have to watch a video as Steve has done the hard work for you. Indeed you have probably seen it were he is talking about holding power and state "that is the equivalent of a tropical storm for yachts that would be using anchor of this size range". Its been a while but I think the Rocna Knowledge pages used to show the force on yachts in 42 knots of wind to assist in matching an anchor to size of yacht.

yet you and many others would quote the old mantra - buy bigger

Not true, I size my anchors in accordance with the manufacturers recommendations. Did you get loads of 2000kgs dragging an anchor down the beach as shown in your photo in Practical Sailor? 2000kgs seems very high given the recent results Steve has shown in less than idea conditions which you could argue are more realistic.

The idea of 6mm chain on a 31ft yacht came from another owner who suggested a while back. A couple of minutes with a calculator suggested this had some merit in enabling either a reduction in weight at the bow or increasing the length. The only downside I have is I can't find a 6mm Gypsy for a SL Anchorman, so it's a new windlass plus chain and next thing we are in for £1000. Perhaps when the existing chain wears out would be the right time to change.

Bigger chain will give more damping due to moving through the water and its distributed mass. The lighter the chain, the more heavily you rely on rope snubbers.

There was a good thread a few months ago on Cruisers forums which showed that the chain size had little effect just when you needed it most, eg when its blowing a hooley.

It's not for nothing that many people stick to using what's been proven to be a reasonable compromise over decades of collective experience.

Like using a CQR? Strange world, anyway we are running out of time, it will be April shortly so another anchor thread by Johnathan is imminent.

Pete

 

[Neeves]

(Sorry about the absence of a few days...)
There is something in your reasoning on these matters, exemplified by the quotes above, that goes against my intuition. I think in this calculation you grossly over estimate the importance of chain weight and, consequently, under estimate the importance of the windage. The fact that the last (or first) link is off the seabed does not mean, in my view, that the total weight of all that chain but be lifted by the man hauling on the foredeck. It is in fact the windage and/or current force on the boat that does the lifting of the chain.
Imagine a yacht in the same situation with a rode consisting of dyneema rope (virtually weightless in water). With the weight factor gone, the person hauling would only have to overcome 27 kgs (instead of 70 kgs) to retrieve the rode?
Surely that cannot be the case?

It is the wind or current (more likely the wind) that develops the tension that lifts the chain off the seabed. In my example that is 70kg. In order to retrieve the chain you need to over come that tension developed by the wind - 70kg. If you can only develop 50kg it will be insufficient to move the yacht. 70kg is a lot of tension considering you will be doing this hand over hand and need to keep the retrieved chain out of the way. Obviously as you retrieve the chain the tension needed will reduce.

The tension in the chain, the 70kg is part the development of catenary.

Jonathan

 

[Neeves]

Well the good news is you just have to watch a video as Steve has done the hard work for you. Indeed you have probably seen it were he is talking about holding power and state "that is the equivalent of a tropical storm for yachts that would be using anchor of this size range". Its been a while but I think the Rocna Knowledge pages used to show the force on yachts in 42 knots of wind to assist in matching an anchor to size of yacht.

Not true, I size my anchors in accordance with the manufacturers recommendations. Did you get loads of 2000kgs dragging an anchor down the beach as shown in your photo in Practical Sailor? 2000kgs seems very high given the recent results Steve has shown in less than idea conditions which you could argue are more realistic.

The idea of 6mm chain on a 31ft yacht came from another owner who suggested a while back. A couple of minutes with a calculator suggested this had some merit in enabling either a reduction in weight at the bow or increasing the length. The only downside I have is I can't find a 6mm Gypsy for a SL Anchorman, so it's a new windlass plus chain and next thing we are in for £1000. Perhaps when the existing chain wears out would be the right time to change.


There was a good thread a few months ago on Cruisers forums which showed that the chain size had little effect just when you needed it most, eg when its blowing a hooley.

Like using a CQR? Strange world, anyway we are running out of time, it will be April shortly so another anchor thread by Johnathan is imminent.

Pete

I have never dragged anchors down a beach. All the anchors are set under water. Maybe you have not heard of tide .

If you read the West Marine tests you will find anchors developing hold of near 2,000kg. Steve is unable to set anchors of 15kgs to their ultimate hold - his engine is too small. I'm not sure his load cell is large enough either.

There are a number of spreadsheets where vessel size vs tension in the rode are quoted. You were suggesting this should be done from 1st principals - in fact you rubbished the idea that chain size recommended by yacht designers or builders should be followed and the owner should work it out for themselves. Like you ideas on soft shackles - all talk and no trousers.

Its easy to criticise, very difficult (for you ) to support your criticisms and even more difficult for you to follow your own advise.

I note you are championing Steve aka Panope's work - it is a significant contribution. I also note some sarcasm in your mention of the CQR - which Steve rates more highly than a Rocna. Possibly YOU can explain the contradictions.

I think you will find that this thread was initiated by PhilM, if you bother to look. You will also find that the another thread on anchors/anchoring was initiated by Cornishwestely. In common with many of your comments - you appear to lack accuracy..

Jonathan

 

[Pete7]

Johnathan, indeed other forum members have started threads, but not as many as you. I can't quite work out the frequency of yours though, perhaps its tied to the moons orbit.

A few posts back you took Steve to task for his spreadsheet using characteristics that didn't alter the holding power, like engineering and tip weights. Now you say look at the results of the CQR using these characteristics. That was a sharp u turn wasn't it?

So these anchor tests you did on the beach with the tide in, what was used to generate the pull? after all you have recently criticised anchor tests carried out on a beach. Is this another huge u turn?

You seem fixated with anchors, have you thought about investigating something else for a change?

 

[Neeves]

Pete much though I think you are a lovely guy offering fair criticism I have plenty to keep me busy without commenting on topics about which I know nothing, or very little..

As is common with anchor testing a winch was used to generate the pull, using wire on the winch and then appropriately size chain and Crosby shackles. A winch gives a controlled pull at a fixed rate and with a load cell in line it is possible to record the exact tension in the rode. It is thus possible to repeat tests and to ensure each anchor is tested in an adjacent seabed. The usual protocol is to conduct three tests on each anchor and test A then B then C then A again etc. Same day, adjacent seabeds. Ideally you need 3 anchors of each design and have at least one anchor that you have tested previously to engender consistency. Buried anchors can be left with a pole as marker for location - so that you know where to dig.

A problem with setting anchors to their ultimate hold is retrieving them as the retrieval can demand a lot of digging, which is surprisingly difficult - under water. The advantage of digging the anchor out is that you can determine the angle of the fluke - as it is well known that a shallow setting anchor takes a long time to lock up and a shallow setting anchors develops a low hold. Maybe I should refresh an old thread or start a new one on a shallow setting anchor - so as to emphasise the problem.

The other advantage of working in the intertidal zone is you can actually watch the anchor setting and know exactly when it has stopped moving (and has thus reached ultimate hold). You will find in many of the Panope videos that the anchor is still moving and has not actually set - and an anchor that is still moving and then tested for a veer test - lacks some reproducibility.

Not a sharp 'U' turn at all - you are the one championing the work done by Steve and simultaneously you are being critical of CQR - but Steve says CQR is better than a Rocna (I wonder what anchor you use?)

But please keep checking my anchor threads. I take it as a compliment you take such a close interest and are so keen to read every thread. You obviously find them informative and educational. I confess I tend not to bother to read threads in which I have no interest, so I cherry pick. As you take such interest in my activity (I think you really need to get a life) I'll try to increase the frequency.

But thank you for your contributions very illuminating.

Take care, stay safe

Jonathan

 

[Pete7]

- but Steve says CQR is better than a Rocna (I wonder what anchor you use?)

But please keep checking my anchor threads. I take it as a compliment you take such a close interest and are so keen to read every thread. You obviously find them informative and educational. I confess I tend not to bother to read threads in which I have no interest, so I cherry pick. As you take such interest in my activity (I think you really need to get a life) I'll try to increase the frequency.

But thank you for your contributions very illuminating.

Take care, stay safe

Jonathan

Johnathan the answer to your question is easy, I ditched the CQR and replaced it with a Delta which was upgraded to a Rocna when funds allowed. Can't remember the CQR size, but similar to the 10kg Delta and Rocna, which sit nicely in the manufacturers recommendations. Told you I don't oversize.

Can't get a life at the moment as its illegal to enter or leave the UK without a really good excuse. However, I can go down to the boat on Monday. Sadly, I suspect my new anchor is currently sat on a sand bank in the middle of the Suez, so I will have to use the old Rocna for a while yet.

You take care too, looks a bit wet down under, make sure you have some good wellies as winter is coming.

Pete

 

[Neeves]

Johnathan the answer to your question is easy, I ditched the CQR and replaced it with a Delta which was upgraded to a Rocna when funds allowed. Can't remember the CQR size, but similar to the 10kg Delta and Rocna, which sit nicely in the manufacturers recommendations. Told you I don't oversize.

Can't get a life at the moment as its illegal to enter or leave the UK without a really good excuse. However, I can go down to the boat on Monday. Sadly, I suspect my new anchor is currently sat on a sand bank in the middle of the Suez, so I will have to use the old Rocna for a while yet.

You take care too, looks a bit wet down under, make sure you have some good wellies as winter is coming.

Pete

Thanks Pete,

The big wet was simply an inconvenience to us. The Hawkesbury River flows to the sea at Broken Bay (a couple of miles north) and the whole area is now the colour of milk tea. Now we have had 3 days of brilliant sunshine and tops of upper 20s. Big change. We will be on the boat next week - basically all Covid restrictions ended.

Pleasure and refreshing to hear you did not adopt the religous cult of buy bigger.

Why are you waiting for a new Rocna, or what is wrong with the old one. Despite the longevity of CQR (and Delta) they do need replaced, due to lack of gal and Rocna have been around for 15 years now - many must need to be re-galvanised. Other than galvanising they should last for ever. If your anchor is on a sand bank in Suez - it looks like a long wait.

Just wondered.

Take care, stay safe - Covid is hiding round every corner (they have just had an outbreak in Brisbane, one person, caused by the return of a UK traveller and a failure in our quarantine system).

Jonathan

 

[geem]

Johnathan, indeed other forum members have started threads, but not as many as you. I can't quite work out the frequency of yours though, perhaps its tied to the moons orbit.

A few posts back you took Steve to task for his spreadsheet using characteristics that didn't alter the holding power, like engineering and tip weights. Now you say look at the results of the CQR using these characteristics. That was a sharp u turn wasn't it?

So these anchor tests you did on the beach with the tide in, what was used to generate the pull? after all you have recently criticised anchor tests carried out on a beach. Is this another huge u turn?

You seem fixated with anchors, have you thought about investigating something else for a change?

If you strip out the peripheral columns such as tip weight, galvanising and engineering and concentrate on purely the holding aspects of Steves spreadsheet the CQR out performs the Rocna by some margin. I am sure there are thousands of satisfied Rocna owners who would take issue with this. I am with Johnathan on this one. No u turn. Just attention to detail