How long / how slack should the slack chain segment between bow and chain hook be?

[MathiasW]

The other day, harbour police forced me to relocate into a crowed anchorage field, where I could not deploy as much chain as I would like. And as a storm was heading towards us -- the very reason the harbour police forced me to leave the previous anchorage, which I consider very safe with the anchor gear I have -- I was a bit uneasy and started to think how to get more elasticity into my anchor gear without increasing my swinging radius. I had more than enough chain in the locker, but I could not make any use of it...

The solution I found was to massively increase the length of the slack chain segment between bow and chain hook. In the past, I had always kept this segment short enough so that it would limit the maximal stretch of the (long and elastic) snubber / bridle that I deploy. The idea had been that this approach will prevent the snubber / bridle from being overloaded in very severe gusts.

Well, but if overload is not the issue, then one can figure out what happens when increasing the length of this segment.

After some mathematical analysis, I found that the potential energy stored in this slack chain segment is quadratic in its length, and this can be tapped into when the snubber / bridle starts to stretch (and thereby stretch and lift the slack chain segment). Because of the quadratic dependency, it pays off to make this slack chain segment as long as possible, then the gain is the largest. One should not expect a miracle here, though, and it will only work if the snubber / bridle is very elastic to begin with, but it does help the snubber and the rest of the chain to buffer strong gusts.

In some sense, this approach is similar to deploying a kellet closer to the bow - yet without the hassle the latter brings in its deployment.

One does need to pay attention to the nature of the seabed, though. If it is all sand like here, there is no risk of this very long slack chain segment getting caught somewhere and thereby killing the entire setup. In other cases, one has to make sure it will never touch the seabed.

A bit more detail can be found here, including an update to the long LaTeX document:

Catenary Anchor Chain Length - Die Kettenkurve - (In English) (in English, despite the German in this link... )

My anchor chain calculator at

www.anchorchaincalculator.com

does not include this effect yet, but it may be useful to test out other setups.

And yes, no problems with the storm that has just passed us.

Cheers, Mathias

 

[boomerangben]

I would be interested in how significant the benefit would be. Quite complicated to calculate given the position of one end of the catenary is always different as the tension varies. I would be surprised if it is significant but interested to know how much benefit you’d get

 

[Refueler]

Not wanting to upset anyone - just posting my opinion ..

I would be surprised if it really had any use TBH. Rough weather is not such that it follows a nice mathematical form .. it becomes violent - snatching - jerking - such that the catenary (OP is obviously using chain as there's no point having a snubber on a rope rode !!) .. would provide any dampening effect ...

The time frame of each jerk / snatch would be insufficient to allow real benefit of the snubber and the 'slack' chain.

Forgive my skepticism - but I wonder how much of the benefit is in minds eye of OP ??

 

[Biggles Wader]

It would all depend on what anchor you were using----------

 

[thinwater]

The other day, harbour police forced me to relocate into a crowed anchorage field, where I could not deploy as much chain as I would like. And as a storm was heading towards us -- the very reason the harbour police forced me to leave the previous anchorage, which I consider very safe with the anchor gear I have -- I was a bit uneasy and started to think how to get more elasticity into my anchor gear without increasing my swinging radius. I had more than enough chain in the locker, but I could not make any use of it...

The solution I found was to massively increase the length of the slack chain segment between bow and chain hook. In the past, I had always kept this segment short enough so that it would limit the maximal stretch of the (long and elastic) snubber / bridle that I deploy. The idea had been that this approach will prevent the snubber / bridle from being overloaded in very severe gusts.

Well, but if overload is not the issue, then one can figure out what happens when increasing the length of this segment.

After some mathematical analysis, I found that the potential energy stored in this slack chain segment is quadratic in its length, and this can be tapped into when the snubber / bridle starts to stretch (and thereby stretch and lift the slack chain segment). Because of the quadratic dependency, it pays off to make this slack chain segment as long as possible, then the gain is the largest. One should not expect a miracle here, though, and it will only work if the snubber / bridle is very elastic to begin with, but it does help the snubber and the rest of the chain to buffer strong gusts.

In some sense, this approach is similar to deploying a kellet closer to the bow - yet without the hassle the latter brings in its deployment.

One does need to pay attention to the nature of the seabed, though. If it is all sand like here, there is no risk of this very long slack chain segment getting caught somewhere and thereby killing the entire setup. In other cases, one has to make sure it will never touch the seabed.

A bit more detail can be found here, including an update to the long LaTeX document:

Catenary Anchor Chain Length - Die Kettenkurve - (In English) (in English, despite the German in this link... )

My anchor chain calculator at

www.anchorchaincalculator.com

does not include this effect yet, but it may be useful to test out other setups.

And yes, no problems with the storm that has just passed us.

Cheers, Mathias

Just to clarify my understanding. You mean that there will be a lot of chain hanging between the hook and the bow, acting something like a kellet attached at the location of the hook. You give up the stretch-limiting backup of the chain and you introduce some (none on sand) risk of the chain loop snagging.

I've never done it for that reason, but I see your logic.

A simpler kellet to deploy (not simpler than what you suggest), and one I sometimes use on my rope rode F-24, is to take a loop of extra chain (15-30 feet), secure it in a coil with a soft shackle, and attached it to the chain with a Prusik. The advantage over a traditional kellet is that it can come over the rollers., avoiding the bow dance and struggle.

 

[Roberto]

IMHO the most relevant contribution of a long stretch of hanging chain comes from the friction with water: it's visually very apparent, it slows down the bow movement, moves forward the hydro center of effort and ultimately reduces the boat swinging lateral movement. A bit like a stern riding sail will shift aft the aero center of effort, to give more balance/reduce unbalance, same final effect. I personally always let out the longest possible amount of chain, subject to depth etc.

 

[MathiasW]

Just to clarify my understanding. You mean that there will be a lot of chain hanging between the hook and the bow, acting something like a kellet attached at the location of the hook. You give up the stretch-limiting backup of the chain and you introduce some (none on sand) risk of the chain loop snagging.

I've never done it for that reason, but I see your logic.

Yes, exactly, that is what I did. It is river sand here, and so the risk of snagging is really minimal. And one can reduce that risk to some extent by starting with not too much chain in this slack chain segment and then start feeding more as the wind picks up.

 

[MathiasW]

I would be interested in how significant the benefit would be. Quite complicated to calculate given the position of one end of the catenary is always different as the tension varies. I would be surprised if it is significant but interested to know how much benefit you’d get

Yes, I would be too... I think it will be similar to a kellet of comparable weight, and so I reckon it may be a 10%ish effect, give or take, for the elasticity, if this segment is made long enough.

 

[MathiasW]

Not wanting to upset anyone - just posting my opinion ..

I would be surprised if it really had any use TBH. Rough weather is not such that it follows a nice mathematical form .. it becomes violent - snatching - jerking - such that the catenary (OP is obviously using chain as there's no point having a snubber on a rope rode !!) .. would provide any dampening effect ...

The time frame of each jerk / snatch would be insufficient to allow real benefit of the snubber and the 'slack' chain.

Forgive my skepticism - but I wonder how much of the benefit is in minds eye of OP ??

Yep, agree, a precise calculation of all the effects is tough, when the vessel jerks around, swell is hitting from all sides, as are the gusts. But that does not mean that the catenary and its dampening effect does not exist. Just hard to calculate precisely. In deep water, it will exist much more likely than in shallow water.

I think it is a proven fact that long, elastic snubbers do help - also, and in particular in rough weather.

As a matter of fact, the vessel next to me dragged. No snubber, only 40 metres of chain, and a massive 60 kg anchor... I have just a 32 kg anchor, could use only about 60 metres of chain in order not to get into conflict with my neighbours, and I have a very good snubber / bridle installed. So, I think all this did help.

 

[MathiasW]

IMHO the most relevant contribution of a long stretch of hanging chain comes from the friction with water: it's visually very apparent, it slows down the bow movement, moves forward the hydro center of effort and ultimately reduces the boat swinging lateral movement. A bit like a stern riding sail will shift aft the aero center of effort, to give more balance/reduce unbalance, same final effect. I personally always let out the longest possible amount of chain, subject to depth etc.

Not really! The friction is certainly beneficial for dampening the system so that one does not keep swinging back and forth, but the main mechanism is the charging and discharging of potential energy of the chain as it gets raised or lowered. The energy transferred by a gust gets stored in this potential energy (or the stretching of an elastic snubber), and when it is over, it gets released again. And yes, the friction will prevent this system from oscillating forever.

 

[Refueler]

As a matter of fact, the vessel next to me dragged. No snubber, only 40 metres of chain, and a massive 60 kg anchor... I have just a 32 kg anchor, could use only about 60 metres of chain in order not to get into conflict with my neighbours, and I have a very good snubber / bridle installed. So, I think all this did help.

Without knowing boat etc .. plus you used 20m more scope ...

If you used 60m - and you say you could not use what you would normally - that says to me - deep water ... and other guy only used 40m ????

 

[MathiasW]

Without knowing boat etc .. plus you used 20m more scope ...

If you used 60m - and you say you could not use what you would normally - that says to me - deep water ... and other guy only used 40m ????

Fair enough - I am a 16 m tri and the other was a slightly more bulky monohull of similar length and two masts... So, if anything, I have more windage area. I would not call it deep water, though. About 6 metres under the keel. But why are you suggesting 20 metres of chain more would make any difference, if catenary and all that does not work according to your views... The scope is not that different, is it.

 

[Neeves]

The practice you are suggesting supports the idea of heavy, or big, chain as the effect will be greatest with a large chain.

I'm sorry but I would be advocating deployment of more snubber, but without the need to deploy more chain. You still have the snubbing effect of more chain, or more catenary, but its elasticity not more chain.

As long as you have a long enough snubber or bridle you can set up a series of blocks at each end of the deck and the 'purchase system' or better the extra snubber length will give you all the snubbing of more chain. You can, of course, deploy more of a lazy loop - but more elasticity does mean you can rely on a short rode.

This means you need a really long snubber or bridle to start with - but the long snubber/bridle is cheaper and lighter than lots of chain. Its easy to set up a long snubber - provided you have planned ahead and/or use snatch blocks. It would be unusual for a 'storm' to develop out of blue sky so quickly that you cannot set up a long snubber. You can also introduce a second independent snubber, using heavier duty nylon, to keep the elasticity imposed on the everyday snubber to within 10% of overall length.

For us to deploy a short rode, of small linked chain, but use a 30m bridle was commonplace. It means, if nothing else, you can use tight anchorages unavailable to those without a snubber or only a short snubber.

Refueler​

You should use a snubber with a mixed rode. The textile portion of a mixed rode should have, roughly, the same strength as the chain, unless the chain is overly big. The rope is the ultimate fall back and if it is beefy it will not be elastic. Its light, cheap - but does not have the desired elasticity of a snubber. A snubber is all about the amount of stretch, a mixed rode is all about ensuring the rope does not fail. To provide effective or comfortable snubbing you thus still need the elasticity. Your problem is how to attach the lighter snubber to the inelastic textile - and Prussik comes to mind.

If you use a snubber, or textile in the rode you are not discarding the catenary - but you have two, or 3 with a mixed rode, mechanisms all working together catenary and elasticity. In theory elasticity and catenary offer the self same benefits - they offer snubbing.

Jonathan

 

[Neeves]

Fair enough - I am a 16 m tri and the other was a slightly more bulky monohull of similar length and two masts... So, if anything, I have more windage area. I would not call it deep water, though. About 6 metres under the keel. But why are you suggesting 20 metres of chain more would make any difference, if catenary and all that does not work according to your views... The scope is not that different, is it.

It depends - if he is yawing then with the yaws and a broadside, yawing, wind he has, much, greater windage than you. It depends on the stability of the wind, which is usually unstable in many anchorages.

Jonathan

 

[Roberto]

Not really! The friction is certainly beneficial for dampening the system so that one does not keep swinging back and forth, but the main mechanism is the charging and discharging of potential energy of the chain as it gets raised or lowered. The energy transferred by a gust gets stored in this potential energy (or the stretching of an elastic snubber), and when it is over, it gets released again. And yes, the friction will prevent this system from oscillating forever.

Yes really
Suppress the lateral oscillation, you suppress the biggest percentage of snatch loads; a boat moving back and forth along a straight line exerts a lot less peak energy than one that veers 20-30-40° to port, is stopped by the rode, bounces back through the straight line pull and then is pushed to starboard 20-40°, until it is stopped again... and so on.

 

[Neeves]

Yes really
Suppress the lateral oscillation, you suppress the biggest percentage of snatch loads; a boat moving back and forth along a straight line exerts a lot less peak energy than one that veers 20-30-40° to port, is stopped by the rode, bounces back through the straight line pull and then is pushed to starboard 20-40°, until it is stopped again... and so on.

Not negating anything posted by Mathias nor Roberto

Another way to minimise veering is to use a bridle or 2 anchors set in a 'V' or fork. Often veering is caused not by the yacht but by the wind, though the dominant wind direction might be, say due south, the local geography can cause wild variations and the variants, because of funneling, might be seriously stronger (and called a variety of names, bullets being but one) than the average wind speed. These bullets being both strong and from different directions 'make' the yacht sail at anchor - causing those increased snatches when the yacht gets to the 'end of its tether'

Lateral Thinking & Anchoring - Practical Sailor

Jonathan

 

[thinwater]

Sidestepping the rabbit hole of chain and friction and all that....

A snubber helps with the snatch load but does not help with short scope (other than reducing the peak force, which is important) or yawing.

A kellet nearish the anchor reduces the angle of the chain to the bottom in all but the strongest conditions. In the middle, not so much, but nearer the anchor the effect is greater. It does not benefit snubbing as much in that location, but you have a snubber for that. If it was anchor lift I was concerned about, I would put the kellet within ~ 5-10 m of the anchor.

A kellet nearish the anchor will drag on the bottom, reducing yawing. Again, if your concern is yawing in a blow, it should be nearish the anchor so that drags on the bottom. There are many ways to reduce yawing, but if we are contemplating putting something over the side for this, the most efficient tool is probably a second anchor at about 1.5:1 scope. In the US we call this a hammerlock mooring, but it has other names. The anchor does not truly set, but it is not too easy to drag side-to-side either. It really reduces yawing, and is probably the best use for twin anchor rollers. Unlike a kellet, it will not be lifted off the bottom. If you want to swing with changes in the wind direction, the scope can be less, so long as it touches the bottom. It will not create tangles and does not complicate anchor recovery, since it is under the bow, away from the other rode, and just barely on the bottom.

Yawing is a big deal. It can easily double the peak load on the anchor (I did a lot of tests), and loosen the anchor through repeated twitching, rendering the micro-arguments about which anchor is best or whether another 5 kg is vital a little silly in most cases. Yawing is a demon.

 

[MathiasW]

Yawing is a big deal. It can easily double the peak load on the anchor (I did a lot of tests), and loosen the anchor through repeated twitching, rendering the micro-arguments about which anchor is best or whether another 5 kg is vital a little silly in most cases. Yawing is a demon.

Absolutely! The storm before the last one swept a large motor vessel onto the beach. I had watched it for a while, and it was yawing and veering massively.

 

[MathiasW]

Yes really
Suppress the lateral oscillation, you suppress the biggest percentage of snatch loads; a boat moving back and forth along a straight line exerts a lot less peak energy than one that veers 20-30-40° to port, is stopped by the rode, bounces back through the straight line pull and then is pushed to starboard 20-40°, until it is stopped again... and so on.

Well, but it is not the friction of the chain in the water that causes this. Just compare the surface area of the chain with that of the boat. The latter is orders of magnitude larger than the former, and so it is mostly the friction of the boat that will dampen the system, not the friction of the chain.

And as a Gedankenexperiment replace the chain with a strong dyneema line and even in the absence of lateral movements the load on the anchor will be excessive.

 

[GHA]

Absolutely! The storm before the last one swept a large motor vessel onto the beach. I had watched it for a while, and it was yawing and veering massively.

Though not always.... I've watched mine for hours and hours in gusting up over 30Kts, the boat tacks each side of the the wind but only moves a few metres left & right, sog rarely above 0.6Kt and next to 0 when the boat actually tacks. Chain is straightest after the tack when the wind get to closer to beam on. In those conditions a snubber does nothing, no energy to move away from a moving boat, force is from the wind only.
Still wouldn't ever argue about a good elastic snubber being a good thing though!