How Chain Size Impacts the Catenary

[MathiasW]

Originally Posted by SVHarmonie

Mathias,

The only place I might quibble is that in strong winds you are better off in deep water. If I let out 10:1 scope in shallow water, my pull on the anchor is so close to horizontal that it matters not past that. Assuming a good snubber... which most boats do not use, or understand.

Yes, of course, a good long snubber, perhaps with a rubber spring, will go a very long way to help you in that situation. I was perhaps a bit too short in my reasoning. I had only wanted to point to an extreme case. If you do NOT have a snubber, then in shallow water the chain cannot absorb enough energy and the load at the anchor will become too high. If you then relocate to a little deeper water, use a little more chain, the chain can cope much better.

But of course, not using a snubber / bridle is just not a responsible approach to anchoring. I would not mind being a minority, if it were not for being impacted and put at risk by this via neighbours at anchor...

[MathiasW]

Another thought: The British Admiralty Manual of Seamanship has this guidance for the required minimal length of anchor chain: 1.5 * square_root(Anchor depth in meters), where the result is given in shackles. One shackle is 90 ft and thus 27.45 meter.

Interestingly, such a square root dependence on the water depth is an approximation to the catenary-curve results for shallow water and / or strong winds.

The only odd thing is that this formula by the Admiralty does not include any wind strength at all...

[BjarneK]

You may find my anchor calculator useful. It supports the view that catenary often has a beneficial effect.

[MathiasW]

Originally Posted by BjarneK

You may find my anchor calculator useful. It supports the view that catenary often has a beneficial effect.

Hi Bjarne,

Greetings to Denmark from a Northern German guy stuck in Panama right now...

Very impressive, I had been thinking of adding such a calculator to my web page, but you have done it already.

From your description I would think that we are using the same model for wind, but also for absorbing temporary energy intakes. I ignored the snubber for that calculation and assumed the chain has to do the entire job. To this end I derived a formula for the potential energy of a general catenary and then I can always work out how much does a given catenary have to change to be able to absorb so and so much more energy. I was even able to get some analytical formulas for that, which seem to work well for not too large energy intakes and not too low wind strengths.

The snubber / bridle could be easily added as a model with a spring constant. But I did not do that for now.

[BjarneK]

Originally Posted by MathiasW

Hi Bjarne,

Greetings to Denmark from a Northern German guy stuck in Panama right now...

Very impressive, I had been thinking of adding such a calculator to my web page, but you have done it already.

From your description I would think that we are using the same model for wind, but also for absorbing temporary energy intakes. I ignored the snubber for that calculation and assumed the chain has to do the entire job. To this end I derived a formula for the potential energy of a general catenary and then I can always work out how much does a given catenary have to change to be able to absorb so and so much more energy. I was even able to get some analytical formulas for that, which seem to work well for not too large energy intakes and not too low wind strengths.

The snubber / bridle could be easily added as a model with a spring constant. But I did not do that for now.

Thanks, it is nice to know that you are thinking along the same lines. Then there is probably something to it

My wife and I are stuck in Mexico now (Yucatan). We were on our way to Panama, but had to change plans. I am interested in knowing about the situation in Panama, so I will send you a PM.

[MathiasW]

Originally Posted by BjarneK

Thanks, it is nice to know that you are thinking along the same lines. Then there is probably something to it

Absolutely!

[Boatfreak]

Mathias

Thank you for making this wealth of information available, however having read the whole thread I can only conclude that you are a very brave man to expose yourself to the vitriol that may be coming your way.

I have just skimmed through one of your digests at the moment and can't really claim enough understanding of it to comment yet but I had been planning something similar myself. Last year I lost my entire 90m of 12mm chain and anchor in a dragging situation and I was wondering whether I would be better advised to replace the 12mm chain with 10mm. The 12mm chain is much too heavy for my 5.5 tonne catamaran and does not stow well.

Just as an aside, although I have had a catamaran for 17 years we had anchored very little and certainly never been forced to remain at anchor in 45knot winds. A change of boat and sailing location and becoming summer liveaboards resulted in an experience gap. I had not realised that when you may have to to jettison your anchor and chain in a hurry to accept a tow, it is wise on a catamaran to have the anchor bouy line pre threaded under the trampoline handy for attaching to the chain!

[MathiasW]

Boatfreak,

I am very sorry to hear that! Must have been traumatic. At what depth was your anchor at that time? Wind strength? And how much chain had you put out? All the 90 m of your 12 mm chain?

Would be interesting to know what type of catamaran you have. 5.5t means it is slightly under a third in weight compared to our trimaran. And we use a 10 mm chain of high quality (duplex). So yes, it does seem that 12 mm is rather oversized for your boat. 10 mm or perhaps even 8 mm might be more appropriate. The latter possibly also of duplex material for extra strength.

If you sail around the UK then you have very strong tides and this suggests a thinner but longer chain to use. Of course, working loads must be heeded. It goes without saying that a good bridle is an important part of the anchor gear. I often see it at catamarans, but sometimes it is rather short. Each leg should be at least 10 m long. For my bridle I have added two rubber springs in each leg.

Also, I should be grateful if you can share with me, perhaps as a PM, where you had problems understanding the web page content or the digest. I am sure there is room to improve the clarity of my pitch.

As to vitriol coming my way - I sure hope not... But then, I deemed it more important to bring as much analysis as possible to the table. For sure I will not have the full picture yet, but I am happy to expand and adapt the work if there is any more insight. I am doing this out of pure self-interest - if all my neighbours anchor safely, I can sleep better.

The essence of this work will also soon be published in the German Die Yacht sailing magazine. And I am breathing down the neck of the German Sailing Association for improving on their 3:1 scope recommendation, where only in the small print you see it is only meant for very calm conditions...