Catenary?

[Trident]

On my boat I use all chain, but use a stretchy nylon bridle if the conditions look like they might warrant it. I attach this to the chain and then let out the chain until the bridle is taking the load, and then a couple of more metres, so the bridle can stretch before jerking on the chain.
I am not sure how a cat would react to having 10m of bridle. May be that is too long and it would tack from one side to the other the whole time.

I can alter the bridle length to conditions so in lighter weather use about 4m a side to the front cleats and in heavy weather probably about 10m - always sits fine to the bridle

 

[MikeBz]

From the forces on a catenary

The cable follows the shape of a parable and the horizontal support forces can be calculated as

I knew anchors were a religious topic...

 

[geem]

So can you advise, from all your experience what combination of chain to rode you think best. I have 60m of 10mm chain on a 50 foot cat (9 tonnes loaded) with a 32kg Rocna which so far has never moved an inch in any conditions. However, I have just spent a bit of time and money removing a 60 kg anchor platform from the front of the boat to replace with a 12kg aluminium ladder frame I built to save weight, so having over 100kg of chain up front of the boat does seem to leave room for further weight savings but I have always believed it to be safer than rode due to the extra weight. Reading some of the detailed posts on here seem to disprove that. So given there must be some chain at the deep end, what do you suggest for chain to rode out of my 60M? And given I always use a bridle on a cat should I go strong dyneema rode or stretchy polypropylene etc ?

If I was you, I would use 8mm chain. Plenty strong enough but a good weight saving over 10mm chain. Keep to 60m. Use an elastic bridle but don't oversize it. You need a lot of stretch to absorb shock loads. Something like 10mm to 12mm nylon. Keep a spare and a spare chain hook. We have an additional 50m of anchorplait fixed to our 60m of chain but very rarely deploy it.

 

[Tranona]

OK. Accepting all this & the NZ article, and being in the market for a new anchor and chain in due course, can the panel advise on the following.

1. Should I up size my anchor (currently a 25kg CQR on 60m of 10mm chain holding a 36' 10 tonne yacht)? I won't ask which anchor type to buy but let's assume Rocna.

2. If I use a part rope rode, how do you take the load off the bow roller (which is a mini bowsprit)? I always use a V bridle at present, with a long V to midship cleats in reserve for strong winds.

3. How do you lift a rope/chain rode? I find the rope drum angle and swap to chain a little fraught.

Currently I was inclined to current rode and a 25kg Rocna.

Pleased you have asked this question as I have just gone through exactly this process with my latest boat, a Golden Hind 31 - old style heavy displacement (5.3 tonnes on a 26' waterline). its a (good) project boat and therefore comes with gear that is an accumulation of near 50 years and several owners' use. One of the reasons I bought it was I needed a means of filling my last few years with intellectual and practical tasks similar to those that filled my working years. I had a thorough survey on the boat which formed the basis of my work plan and for the benefit of my insurer, whose cover is dependent on following the survey recommendations.

The anchor fitted was an old 35lb copy CQR with 50m of 6mm chain and a newly installed Lofrans Kobra windlass. The chain is in excellent condition but may not have been new. One of the recommendations was obviously "consider replacing chain with 8mm". Being a regular contributor here I have followed most of the anchor debate, but now I had to make a decision "for real". So like a good academic I treated it like a mini research project, essentially carrying out a literature review complemented by accounts of others' experiences. Part of the driver for doing all this time consuming work was that 50m of new chain and a gypsy would cost over £500 - had budgeted for a new anchor anyway.

I won't bore with details of the sources I used as most of them have been posted here over the years, so will limit to just the key findings and my conclusions.

First thing to say is that in general the advice on anchor and chain sizes from manufacturers and others like specialist retailers is misleading and even useless. Essentially recommendations are almost all in relation to size of boat and weight of anchor and have barely changed over the years, with some exceptions.. This is in spite of the enormous changes in the holding power of anchors, big variations in types of boats of a given size and improvements in chain quality, strength and reliability. So you get the situation where Lewmar use essentially the same guide for their 3 different styles of plough type anchors (CQR, Delta and Epsilon) even though their own tests show that they have widely different holding powers. They cover this lack of precision by using wide ranges with shading and (like most manufacturers) have a caveat that it is only a guide so you should consider your own individual boat's characteristics and if in doubt go the next size up.

To do that rather than follow blindly the size based recommendations one has to go back to the data available on the factors that impact on anchor performance and therefore choice of size and type in relation to your boat. What are they?

Ultimate holding power,
Ease and reliability of setting so that the holding power can be achieved
Resetting after disturbance

Nothing clever about this and essentially what the empirical tests cover

Ignoring the nature of the seabed holding power is a function of the load applied primarily by the wind. Two things become clear, that is not linearly relate to boat length or displacement, but mainly to area exposed to the wind. Whatever is applying the load however it is always less than than the theoretical load a good anchor can withstand, which in turn is well within the limits of the typical size chain that might be used. That is for a boat like mine the maximum load in storm conditions is less than half the safe load of 6mm and around 30% of 8mm. That is why you never hear of chains breaking, and rarely even stretching which typically occurs at between 40-50% of maximum.

Setting is primarily a function of design and application of load, not weight and from every test and video observed, most NG anchors set quickly with minimal load. There is a belief that weight of chain as in the catenary it creates can aid setting because the angle of pull may be lower. However this does not show up on most of the videos and I can find no quantification of the effect. The angle of pull is primarily a function of scope. 3:1 is 30 degrees to horizontal, 5:1 is 20, 7:1 is just over 10. This is considered the practical maximum as no advantage can be shown over that. If the use of chain in setting is advantageous (and most sources consider it marginal) then what is of interest to me is whether the difference between 6 and 8mm of the same length is significant. I intend keeping all chain anyway - the choice is between 6 or 8mm. If it is just weight (rather than weight/length ratio) then just let out more of the lighter chain, or use a chum/angel!

The various tests and videos indicate that most of the NG anchors are good at resetting and no one brand stands out as being clearly better than another.

I have therefore come to the conclusion there is no benefit in changing to 8mm, which is a relief not least on the pocket, but also because the current chain runs very cleanly down the chute into the anchor locker with no bunching up. This is because it is a good design with a long drop, but a bulkier chain may cause problems.

That is the chain sorted. Now the anchor. My first Bavaria had a 35lb copy CQR which was hopeless when the boat was in Greece so replaced with a 16kg Delta that was better but still had difficulty in setting in the hard shale in the Ionian. When I got the boat back to UK this problem essentially disappeared and no problems in our muddy and sandy seabeds around Poole. The next Bavaria came with a 10kg Delta and 8mm. This is perverse as the 33 had the same waterline length, displacement and windage as the 37. However the Lewmar recommendation shows 33' at the top end of 10kg and 37 at the upper end of 16kg! Anyway no problems with the 10kg in the 6 years I had it. So the decision is another Delta or NG. It is clear from the data on setting and holding power there is no advantage in using an anchor bigger than your analysis of the data for your boat indicate, primarily because you can never generate the forces required to access the additional potential holding power. This is exactly the advice from Pete Smith - concentrate on the design, not on the weight. I am convinced of the benefits of NG designs, particularly in terms of setting in a range of seabeds although do not see how I would ever need their ultimate holding power - and most are 2-3 times the price of a Delta. You can probably guess the choice I made. A 10kg Epsilon. Hit it just right last November with a special offer on the Epsilon (as well as on an autopilot) at Marine Superstore. Add in a few other items and got an extra 10%. so my Epsilon cost little more than the current price of a Delta.

I have only tried it once in benign conditions in the lovely mud off Brownsea Island. 5m water 3:1 scope and set instantly I applied a touch of reverse. Not much to get excited about and hopefully i can try it in a wider range of settings over the summer.

So, based on my research and reasoning, my advice for your boat is a 20kg (or if you are really brave a 16kg - at the top of the Rocna recommendations for that size) and 8mm chain. Personally I would not spend the extra money on a Rocna as I think based on the data it offers little over an Epsilon. Of course there is far more user experience with Rocna (and many other established NG designs) and there is comfort in buying a proven product. For me, though I am not a demanding user and in the absence of an Epsilon would have continued with another Delta.

Hope this helps - sorry for the length but you did ask.

 

[lustyd]

Never mind the catenary, you've either ploughed into someone's mooring or you're one of those unsociable anchor buoy people who reduce the capacity of the anchorage for no good reason!

 

[Tranona]

Thanks gents. I'll have a look for these articles.

250~300 kgs is a pretty hefty load. My boat has a 50hp engine and a fixed 3-blade prop. Still learning with this boat and so far I've been a bit leary of digging in the anchor with all the horsepower at my disposal for fear of just ploughing up the seabed (Confession: I have a 35lb CQR).
To date I've gone to @2,500 rpm, which according to the graphs is @15hp using the propellor power curve. Only dragged once, and that was in heavy weed. Maybe I should be a bit more ballsy and give 'er some welly!

Don't think that will help. The CQR is not setting because it is not good at setting. I have in front of me the 2006 YM test with a lovely description of a 35lb CQR failing to set 3 times in a row even with a 7:1 scope. all the other achors had no problems. Of course it may have just been that type of seabed, but poor setting performance is a consistent finding in just about all the tests with a CQR.

Answer to your problem is to change at least to a Delta, but better to an NG anchor. See my long post for the reasoning.

 

[Supertramp]

Pleased you have asked this question as I have just gone through exactly this process with my latest boat, a Golden Hind 31 - old style heavy displacement (5.3 tonnes on a 26' waterline). its a (good) project boat and therefore comes with gear that is an accumulation of near 50 years and several owners' use. One of the reasons I bought it was I needed a means of filling my last few years with intellectual and practical tasks similar to those that filled my working years. I had a thorough survey on the boat which formed the basis of my work plan and for the benefit of my insurer, whose cover is dependent on following the survey recommendations.

The anchor fitted was an old 35lb copy CQR with 50m of 6mm chain and a newly installed Lofrans Kobra windlass. The chain is in excellent condition but may not have been new. One of the recommendations was obviously "consider replacing chain with 8mm". Being a regular contributor here I have followed most of the anchor debate, but now I had to make a decision "for real". So like a good academic I treated it like a mini research project, essentially carrying out a literature review complemented by accounts of others' experiences. Part of the driver for doing all this time consuming work was that 50m of new chain and a gypsy would cost over £500 - had budgeted for a new anchor anyway.

I won't bore with details of the sources I used as most of them have been posted here over the years, so will limit to just the key findings and my conclusions.

First thing to say is that in general the advice on anchor and chain sizes from manufacturers and others like specialist retailers is misleading and even useless. Essentially recommendations are almost all in relation to size of boat and weight of anchor and have barely changed over the years, with some exceptions.. This is in spite of the enormous changes in the holding power of anchors, big variations in types of boats of a given size and improvements in chain quality, strength and reliability. So you get the situation where Lewmar use essentially the same guide for their 3 different styles of plough type anchors (CQR, Delta and Epsilon) even though their own tests show that they have widely different holding powers. They cover this lack of precision by using wide ranges with shading and (like most manufacturers) have a caveat that it is only a guide so you should consider your own individual boat's characteristics and if in doubt go the next size up.

To do that rather than follow blindly the size based recommendations one has to go back to the data available on the factors that impact on anchor performance and therefore choice of size and type in relation to your boat. What are they?

Ultimate holding power,
Ease and reliability of setting so that the holding power can be achieved
Resetting after disturbance

Nothing clever about this and essentially what the empirical tests cover

Ignoring the nature of the seabed holding power is a function of the load applied primarily by the wind. Two things become clear, that is not linearly relate to boat length or displacement, but mainly to area exposed to the wind. Whatever is applying the load however it is always less than than the theoretical load a good anchor can withstand, which in turn is well within the limits of the typical size chain that might be used. That is for a boat like mine the maximum load in storm conditions is less than half the safe load of 6mm and around 30% of 8mm. That is why you never hear of chains breaking, and rarely even stretching which typically occurs at between 40-50% of maximum.

Setting is primarily a function of design and application of load, not weight and from every test and video observed, most NG anchors set quickly with minimal load. There is a belief that weight of chain as in the catenary it creates can aid setting because the angle of pull may be lower. However this does not show up on most of the videos and I can find no quantification of the effect. The angle of pull is primarily a function of scope. 3:1 is 30 degrees to horizontal, 5:1 is 20, 7:1 is just over 10. This is considered the practical maximum as no advantage can be shown over that. If the use of chain in setting is advantageous (and most sources consider it marginal) then what is of interest to me is whether the difference between 6 and 8mm of the same length is significant. I intend keeping all chain anyway - the choice is between 6 or 8mm. If it is just weight (rather than weight/length ratio) then just let out more of the lighter chain, or use a chum/angel!

The various tests and videos indicate that most of the NG anchors are good at resetting and no one brand stands out as being clearly better than another.

I have therefore come to the conclusion there is no benefit in changing to 8mm, which is a relief not least on the pocket, but also because the current chain runs very cleanly down the chute into the anchor locker with no bunching up. This is because it is a good design with a long drop, but a bulkier chain may cause problems.

That is the chain sorted. Now the anchor. My first Bavaria had a 35lb copy CQR which was hopeless when the boat was in Greece so replaced with a 16kg Delta that was better but still had difficulty in setting in the hard shale in the Ionian. When I got the boat back to UK this problem essentially disappeared and no problems in our muddy and sandy seabeds around Poole. The next Bavaria came with a 10kg Delta and 8mm. This is perverse as the 33 had the same waterline length, displacement and windage as the 37. However the Lewmar recommendation shows 33' at the top end of 10kg and 37 at the upper end of 16kg! Anyway no problems with the 10kg in the 6 years I had it. So the decision is another Delta or NG. It is clear from the data on setting and holding power there is no advantage in using an anchor bigger than your analysis of the data for your boat indicate, primarily because you can never generate the forces required to access the additional potential holding power. This is exactly the advice from Pete Smith - concentrate on the design, not on the weight. I am convinced of the benefits of NG designs, particularly in terms of setting in a range of seabeds although do not see how I would ever need their ultimate holding power - and most are 2-3 times the price of a Delta. You can probably guess the choice I made. A 10kg Epsilon. Hit it just right last November with a special offer on the Epsilon (as well as on an autopilot) at Marine Superstore. Add in a few other items and got an extra 10%. so my Epsilon cost little more than the current price of a Delta.

I have only tried it once in benign conditions in the lovely mud off Brownsea Island. 5m water 3:1 scope and set instantly I applied a touch of reverse. Not much to get excited about and hopefully i can try it in a wider range of settings over the summer.

So, based on my research and reasoning, my advice for your boat is a 20kg (or if you are really brave a 16kg - at the top of the Rocna recommendations for that size) and 8mm chain. Personally I would not spend the extra money on a Rocna as I think based on the data it offers little over an Epsilon. Of course there is far more user experience with Rocna (and many other established NG designs) and there is comfort in buying a proven product. For me, though I am not a demanding user and in the absence of an Epsilon would have continued with another Delta.

Hope this helps - sorry for the length but you did ask.

Thank you. Clear and valuable reply. I think 20kg NG type is right, staying with original chain or downsizing to 8mm if necessary and cost effective. I carry two secondary anchors and chain/rope rodes and I suspect my 20kg CQR kedge with 10m chain and rope would double as a main anchor should I choose or need to use it.

I do feel happier with all chain rode and bridle, partly superstition, partly seeing the chafe that can occur in gales.

 

[Tranona]

Not worth downsizing chain. Only sensible to go for lighter if you have nothing or your chain and gypsy need replacing anyway. Fortress would make a better kedge than a CQR, but you have what you have and it will rarely get used! Chafe and abrasion are key things in favour of all chain but for me the main thing is smooth retrieval having never got on with the rope/chain on my first Bavaria. We added 20m rope because sometimes in Greece you need a bit extra when backed up to a quay to get your anchor clear of others. Always used the warping drum for the rope then transferred chain to gypsy.

 

[Bilgediver]

I use a nylon bridle about 20m in total length (i.e. ~10m each side). This provides enough elasticity in the conditions where the rode is almost straight. And in light conditions it is then purely the chain order which provides for the "elasticity" in the system.

We used to use a length of nylon 8 strand with a further length of poly in our supply ship moorings fitted to our rigs , The nylon certainly absorbed the shock loads and reduced breakage .

 

[Porthandbuoy]

Don't think that will help. The CQR is not setting because it is not good at setting. I have in front of me the 2006 YM test with a lovely description of a 35lb CQR failing to set 3 times in a row even with a 7:1 scope. all the other achors had no problems. Of course it may have just been that type of seabed, but poor setting performance is a consistent finding in just about all the tests with a CQR.

Answer to your problem is to change at least to a Delta, but better to an NG anchor. See my long post for the reasoning.

Thanks again Tranona.
I found that YM 2006 report. Pretty damning for CQR and Bruce anchors. I intend to swap Sheolin's CQR, probably for a 13kg Knox. I know changing to a Knox on my previous boat, a Nicholson 26, made a huge difference. Until then I'm tempted to use my aluminium Fortress as my primary anchor for cruise up the Western Isles next month. The only problem being it doesn't stow well on the bow.

 

[sarabande]

A bit of a fred drift but still on 'loaded' anchor chains.

I am trying to visualise the effects of a twisted chain on breaking strain and 'elasticity'.

The OP's pic shows a few turns in the chain which have distributed themselves evenly along the chain under tension.

If one sat at anchor through several tides or wind changes, it's possible to end up with several/many twists.

In agriculture, getting twists in chains is strictly verboten as they weaken the chain ( so I was told).

What happens in sailing, with longer chains ? Will the torsion in a twisted chain weaken the chain as links do not pull in a linear fashion ?

And as tension in a twisted chain increases, will the twists provide some form of elasticity as the links try to re-form themselves in a 90 degree relationship with the adjacent links ?

 

[Snowgoose-1]

We used to use a length of nylon 8 strand with a further length of poly in our supply ship moorings fitted to our rigs , The nylon certainly absorbed the shock loads and reduced breakage .

I think that's important whatever system you have. A bight of nylon in the chain work wonders.

 

[boomerangben]

Sarabande makes an interesting observation re chain twist. Is there a swivel in that system Vyv?

 

[Stemar]

There are good arguments for a mixed rode - lighter weight, greater elasticity, cost - unless you're anchoring in coral that will eat the rope. Retrieval is an issue, as the OP suggests, but the gypsy on my Quick windlass works for both chain and rope, with the shape to take the chain and a slot in the bottom that grips the rope. It deals fine with a rope-chain splice

 

[Tranona]

Mixed rodes are far more common in the US. Short (20' or so) of chain for abrasion resistance then rope. Part of the obsession with heavy all chain here is as a means of getting round the difficulties of setting old style anchors. The heavy is best brigade's claim is that it works for them and has been proven over many years is defensible at one level BUT it does not mean that lighter chain or less chain in conjunction with NG anchors cannot be equally, if not more effective. This is exactly what all the tests show.

 

[vyv_cox]

Sarabande makes an interesting observation re chain twist. Is there a swivel in that system Vyv?

Yes there is. I have observed that most of the chain twist occurs due to rotation of the anchor when raising and lowering. Our windlass is a Maxwell, the fastest tested by YM.

 

[Roberto]

A bit of a fred drift but still on 'loaded' anchor chains.

I am trying to visualise the effects of a twisted chain on breaking strain and 'elasticity'.

The OP's pic shows a few turns in the chain which have distributed themselves evenly along the chain under tension.

If one sat at anchor through several tides or wind changes, it's possible to end up with several/many twists.

In agriculture, getting twists in chains is strictly verboten as they weaken the chain ( so I was told).

What happens in sailing, with longer chains ? Will the torsion in a twisted chain weaken the chain as links do not pull in a linear fashion ?

And as tension in a twisted chain increases, will the twists provide some form of elasticity as the links try to re-form themselves in a 90 degree relationship with the adjacent links ?

Random thoughts with round figures, 30m of chain has about 1000 links, anchor for one month --> max 30 full twists if the boat turns always the same way --> 30 x 360 / 1000 gives about 10° from one link to the adjacent one. Maybe Does not seem much, and that would need several coinciding factors.
A problem would arise when lifting the anchor: the initial chain meters will be kept untwisted by the gypsy and all the twists will accumulate towards the end of the chain, where it will begin jumping out of it.
More annoyingly, it might come up covered by small barnacles (this after about 1 month in warm waters)

 

[noelex]

I am trying to visualise the effects of a twisted chain on breaking strain and 'elasticity'.

The OP's pic shows a few turns in the chain which have distributed themselves evenly along the chain under tension.

If one sat at anchor through several tides or wind changes, it's possible to end up with several/many twists.

In agriculture, getting twists in chains is strictly verboten as they weaken the chain ( so I was told).

Every time the boat spins through 360° it will put a twist in the chain.The boat does not have to swing around the anchor, it only has to spin itself through 360°. The most common situation where I have encountered this problem is in weather situations where there are two opposing wind systems. For example, a southerly breeze with a local northerly sea breeze. In these situations the boat can do multiple spins in a short space of time as one or the other wind predominates.

At anchor the twists in the chain are not noticeable because they are distributed over the whole length of the rode. However, when the chain is retrieved the gypsy will not pass the twisted chain so the twists become concentrated in the ever shortening length of chain between the gypsy and the anchor. Chain will only accept a certain number of twists per meter before it starts to hockle.

The torque can be very impressive. On one occasion I was concerned the chain links would be permanently deformed. Once the anchor has broken free the twists will spin out even without a swivel (sometimes very dramatically). But to break out a good quality modern anchor the scope has to be close to 1:1. If the twists become excessive before the rode can be shortened to this scope, the anchor cannot be broken out.

Fortunately, this is very rare problem. With no swivel this has only occured three times in over 5,000 nights at anchor.

The photo below is not very good but it was chain so twisted is started to hockle. This is the time when you want a swivel (I do not use one).

 

[[2574]]

Interesting picture. My first thought was how the roll bar must impede anchor setting in a thick muddy seabed, dragging that pipe through such a substrate would take some force. I used a Rocna for a decade and thought it excellent. The new boat couldn’t accommodate a roll bar so have chosen a Rocna Vulcan. Performance has impressed.

 

[ylop]

View attachment 157020
This photo was taken as I was setting my Rocna with the engine reversing at 2500 rpm. I now know that this is matched almost exactly by wind strength force 6.

Although I am well aware that the chain is not exactly straight, it is visibly very close to it. This was in a depth of 4-5 metres with a scope of around 4:1. Catenary is just about non-existent.

Photo taken by noelex's 'mermaid'.

Whilst the point you think you are making may be valid - I'd add a couple of observations about your "evidence":
- there is clearly a catenary in the image, its not as pronounced as the text books show, but at 4:1 with 2500 rpm I wouldn't expect it to be
- the image is taken from an oblique angle, and so determining the shape of the curve is affected by parallax error, its likely that the true catenary would be more obvious if viewed orthogonal to the anchor chain

What would be interesting would be to see carefully taken images with different scopes and different engine RPM. What surely matters though is that the force on the anchor is applied as close to the horizontal as possible. If you drew the straight line in the above image my estimate is that it is something like 20-30 deg above the horizontal whereas the anchor chain is perhaps 5 deg as it exits the stock (all very rough due to the angle of the pic and not actually seeing where your boat is).