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

Roberto

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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.
Of course not, I said the friction of the chain in the water dampens the lateral yawing, which is given by the combined effect of aero and hydro resultant forces and their respective points of application.
Bring the hydro forward and the the aero aft and the system stability increases, leaving a long loop of chain under water at the bow contributes to bring forward the point of application of hydrodynamic forces hence contributes to yaw dampening.

Just to be sure we are talking about the same thing: I am referring to lazy chain going over the davit roller, into the water, then upwards towards the immersed claw/hook/whatever of the snubber, then being under tension forward to the anchor.
An amount of chain all amassed and tied to the snubber claw with nothing going to the boat would have a much less relevant hydrodynamic effect and would mostly act as a kellet.
 

Neeves

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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.
I don't quite understand this.

A snubber reduces the snatch loads and reduces the peak loads. A better scope reduces the peak loads and reduces the snatch loads - so in effect a snubber offers a mechanism to overcome the inadequacies of a poor scope - which is not only important but critical.

Reduce the peak loads, remove the snatch loads and this will make your anchor more secure. Use a bridle or a fork anchor arrangement and you will manage the yawing, even with a poor scope.

There are have been many occasions when owners are forced into inadequate scope, or use it by mistake, and modern anchors are very forgiving of poor scope - its not the death sentence of the older styles or shallow set anchors.

But adding to Mathias suggestion:

When you are comfortably at anchor your major concern moves from you own situation to that of your neighbours. If the ground tackle of the neighbour upwind of you 'fails' then your concern might be an ability to get out of the way as quickly as possible. Having your chain hook as close to the bow as possible is the ideal - as then you don't need to retrieve much chain (which will need to be done under motor - in winds strong enough to force your neighbour to drag). Having the chain hook at bow would commonly be thought to mean - a short snubber. But returning to Mathias post - it also means the 2 lengths of the lazy loop, between bow roller and hook will be close together - .....

does this matter?

Once you have retrieved the short length of chain to the hook, you can remove the hook and retreive only having to manage the retrieval not a long length of snubber as well.

Having a long snubber and chain hook at the bow are not mutually exclusive.

I'd be cautious of having the lazy loop dragging on the seabed - if considered as a common practice, the increased abrasion will make short work of the galvanising. I made some tests on the abrasion resistance of different form of galvanising and galvanising from different chain suppliers and simply hung short lengths of chain between the transoms and ensured that the chain samples were on the seabed 24 hrs a day. I was effectively using the movement of the cat on its swing mooring to abrade the chain samples, a row of them. I was appalled at how quickly the abrasion wore the chain to bare metal ( and led me to conclude you need as thick and as hard a gal coating as possible - if you focus on gal life. Chain size, 6mm vs 12mm had no effect, neither did G30, G40 or G100. Sadly you cannot make a robust gal coating much thicker than 100 microns (industry standards are about 80 microns). I also concluded expecting a gal life (or chain life) of more than, about, 1,200 nights at anchor with a 80 micron coating is totally unrealistic.

IMG_7585.jpeg

Armorgalv vs. Hot-Dipped - Practical Sailor

Jonathan
 
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Neeves

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All I meant was that a snubber does not change the angle with the bottom when the chain is straight. A little bit, by damping strain, but that is not the main function.

I just wanted to move the conversation in the direction of yawing. But it is all connected.
There are a number of excellent articles on yawing, this is but one of them:

Deep Anchors Stay Put in Moderate Yawing - Practical Sailor

Make a search using Google, or the PS search function, and there will be an answer.

One factor that does come through, in general modern anchors resist, or hold, during yawing.

Jonathan
 

boomerangben

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If you went to extremes and put your snubber hook close to the anchor, the catenary effect would be similar to that for the chain without the snubber in light conditions. To get the maximum effect, you would need to tune the length of snubber, the location of the attachment and length of chain very carefully so that the chain just approaches taught at the maximum scope (and therefore expected tension in combines rode). You then run the risk of twisting your chain and snubber together in a real 3d dynamic situation. In that case the maths for a basic estimate would be fairly straight forward if you’ve already got catenary software. If you could get that tuning right, the chain could add a significant benefit. The down side is as Neeves shows from experience damaging your chain through abrasion when the bight lies on the seabed, which it would do much of the time. You then have the 2nd and third order effects of chain drag as it moves through the water and acceleration of the mass of the chain, not to mention vortex shedding which would require a significant amount of computing power and expertise and may be quite significant.

So if you have some software (I’ve found mine but it’s in Fortran and written on paper - the electronic version long since gone with the demise of the floppy disc…..one of these days……..). You could work out the benefit by seeing the tension difference in an all chain rode between two scopes, one relating to a nominal low tension in the snubber l/short chain rode and then again for the scope of the snubber when stretched to say 10% elongation.
I not sure that makes sense, perhaps I need to draw it out.

But I think if you have the snubber attached mid way in shallow water, the effect is going to be tiny. To test, hang a bight of chain between your bow roller and a line attached to the dinghy and see how far you can row away from the bow roller.

The effect of a suspended bight on yawing and swaying might be interesting though, but then again I would have thought the hammerhead(?) technique mentioned above might be more effective. Before I read that I was thinking of a stern anchor on a short scope would be more effective.
 

thinwater

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... The effect of a suspended bight on yawing and swaying might be interesting though, but then again I would have thought the hammerhead(?) technique mentioned above might be more effective. Before I read that I was thinking of a stern anchor on a short scope would be more effective.
The problems with a stern anchor, in most cases, are two fold:
  • The wind will shift and you will be taking the wind and chop on the beam. High strain on the anchor and a bad ride.
  • The OP's initial concern was that boats would swing into him. If you place a stern anchor, they WILL swing into you, because they rotate with the shift and you don't. There will be an argument made that it is your fault, because what you did is non-customary in the area.
(Case law say the boat that arrived first has the right to a clear berth, but case law also says that you had a duty to take action and to maintain an anchor watch, and that you could have released the stern anchor. Yes, COLREGS requires a watch at all times when the boat is not secured in a marina--no exceptions.)
 

MathiasW

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I'd be cautious of having the lazy loop dragging on the seabed - if considered as a common practice, the increased abrasion will make short work of the galvanising. I made some tests on the abrasion resistance of different form of galvanising and galvanising from different chain suppliers and simply hung short lengths of chain between the transoms and ensured that the chain samples were on the seabed 24 hrs a day. I was effectively using the movement of the cat on its swing mooring to abrade the chain samples, a row of them. I was appalled at how quickly the abrasion wore the chain to bare metal ( and led me to conclude you need as thick and as hard a gal coating as possible - if you focus on gal life. Chain size, 6mm vs 12mm had no effect, neither did G30, G40 or G100. Sadly you cannot make a robust gal coating much thicker than 100 microns (industry standards are about 80 microns). I also concluded expecting a gal life (or chain life) of more than, about, 1,200 nights at anchor with a 80 micron coating is totally unrealistic.

No worry for me, as I have a duplex chain, so no galvanising that can come off. But otherwise, you are right, of course.
 

MathiasW

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If you went to extremes and put your snubber hook close to the anchor, the catenary effect would be similar to that for the chain without the snubber in light conditions. To get the maximum effect, you would need to tune the length of snubber, the location of the attachment and length of chain very carefully so that the chain just approaches taught at the maximum scope (and therefore expected tension in combines rode). You then run the risk of twisting your chain and snubber together in a real 3d dynamic situation. In that case the maths for a basic estimate would be fairly straight forward if you’ve already got catenary software. If you could get that tuning right, the chain could add a significant benefit. The down side is as Neeves shows from experience damaging your chain through abrasion when the bight lies on the seabed, which it would do much of the time. You then have the 2nd and third order effects of chain drag as it moves through the water and acceleration of the mass of the chain, not to mention vortex shedding which would require a significant amount of computing power and expertise and may be quite significant.

So if you have some software (I’ve found mine but it’s in Fortran and written on paper - the electronic version long since gone with the demise of the floppy disc…..one of these days……..). You could work out the benefit by seeing the tension difference in an all chain rode between two scopes, one relating to a nominal low tension in the snubber l/short chain rode and then again for the scope of the snubber when stretched to say 10% elongation.
I not sure that makes sense, perhaps I need to draw it out.

But I think if you have the snubber attached mid way in shallow water, the effect is going to be tiny. To test, hang a bight of chain between your bow roller and a line attached to the dinghy and see how far you can row away from the bow roller.

The effect of a suspended bight on yawing and swaying might be interesting though, but then again I would have thought the hammerhead(?) technique mentioned above might be more effective. Before I read that I was thinking of a stern anchor on a short scope would be more effective.

Hmm, I am afraid this went over my head. If you can provide a drawing or two, I may be able to understand it. I am sure my software can address this scenario.

I had to smile when I read Fortran... It has been ages when I last used it. For the app I need to work with languages at an abstraction level unheard of in the times of Fortran... Took me a while to get used to... ;)
 

boomerangben

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I hope the diagrams make sense. A) is the standard chain rode, b) is I think what you are suggesting with a snubber attached mid water to rode. I guess it would extend to the seabed. Either way you could estimate a shape of the catenary and determine the horizontal components of the tension in the slack bight.

To investigate the maximum effect, case c) is an average load with the snubber taking most of the load and the chain taking a minimal load. Determine the horizontal load in the chain for this case. The in d) the boat has moved aft due to wind, stretching the snubber by say 10 or 20%, changing the geometry of the chain catenary, increasing the horizontal load of both the snubber and the chain. Calculate the difference between c) and d) and you will get an idea of how effective the chain bight is.Anchoring.jpg

Snubber is in pink, chain in orange. Apologies for the hand drawn sketches
 
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