How Chain Size Impacts the Catenary

[Jolly Roger]

What's a kelet?

[TaoJones]

Originally Posted by Jolly Roger

What's a kelet?

Here's a list of threads here at Cruisers Forum that deal in some way with kellets.

kellet - Google Search

TaoJones

[Therapy]

Originally Posted by Jolly Roger

What's a kelet?

Near as I can figure out it is an extra weight to store that is of little or no use.

[MarkJ]

Originally Posted by Jolly Roger

What's a kelet?

I filled a cheap plastic bucket with concrete and set in a galvanised U bolt so I could shackle it onto the anchor chain 1 meter from the sea bottom in a storm.

But in a storm it was like a ping pong ball... and the extra ropes/ shackles etc meant it was unsafe if we needed to move in an emergency.
I bought an extra 50 meters of chain instead.

[Therapy]

Originally Posted by Maine Sail

BTW the video I made of my Rocna anchor setting in hard sand vs. the CQR has sold a fair number of your anchors, I get the emails thanking me for showing it. While selling more of your anchors was certainly not my intent, people apparently like "real world" visual data....

I sure did.

Amazing how a plow is plow.

Thanks.

I think Craig owes you lot.

[Jolly Roger]

I apologize that this is not following this exact thread, which is way above my head anyway, but it is about anchoring.
I’ve never used a Kellet, but I do always use a tandem anchor system every single time I anchor at night, without fail! The reason for this is because I nearly lost my boat and family once, when a very strong Mistral sprang up in Majorca and I vowed to find a better way to anchor peacefully at night. This was long before the internet and forums, so I devised my own set-up, which I have used since in sailboats up to 35 tons and in hurricane force winds.
Your main bower anchor is snubbed in and set with whatever scope is determined depending on depth, a snatch block with a pulley large enough to run down the chain is then attached. This has a second anchor bent on by a strong line long enough to be able to man-handle the anchor over the pulpit. A rope line is attached to the snatch block so the anchor can be rolled down the chain until it hits the ground. More chain and rope is then paid out and the second anchor snubbed in using the rope. The boat now lies to both chain and rope, and the second anchor keeps the chain flat so the pull on the first anchor is always horizontal.
If the wind shifts the second anchor might indeed drag, but must also pull the chain along the sea bed, always eventually stopping long before the whole thing has dragged to the new wind direction. I have never dragged this set-up and the last time I used it was a month ago in 45 knots of wind, just round Cape St Vincent, Portugal. Anchoring in 45 knots of wind is great fun.
When it comes time to go, pull the first anchor up the chain using the line, disengage the snatch block and stow the anchor. The boat remains anchored while you do this. Then weigh the bower as normal and off you go.

[hummingway]

I think I was able to picture this. I'm guessing you only do this when you are able to anchor some distance for others? Otherwise you risk having someone swing into do you not?

[Hiracer]

Originally Posted by Jolly Roger

but I do always use a tandem anchor system every single time I anchor at night, without fail!

What two anchors do you use with this system?

Thanks in advance.

[Jolly Roger]

Reply to Hummingway: Fair point sir, so in crowded anchorages you just let out your normal scope after bedding the second anchor. That way you swing to a circle expected by everyone else. But they don’t know your first anchor is a mile further up the chain. In this case it is prudent to buoy the first anchor.
Reply to Hiracer: The answer to that is, “whatever you’ve got.” Most people’s bower is the heaviest with the most chain, and then you can more or less sling any anchor over as the second. The main thing is that it be an actual anchor, so it can be snubbed in and holds the boat in it’s own right, unlike a simple weight.
It’s not such a tedious business if you get it sorted properly beforehand, but definitely a superior feeling knowing you are pretty much drag proof when you go to bed. One time I had two boats handing off my stern when a midnight blow came up in the Meddi’. People who are there know exactly what I mean, and these flew past so fast we could hardly heave a line over. A third was too far away and lost within minutes on the rocks—one person drowned. We never budged, but I confess we didn't get much sleep that night.

[READY2GO]

Jolly Roger,
Sorry I am so slow at picturing this. Is there anyway you could post a drawing of this?

[Jolly Roger]

Sorry, don't have that. What don't you understand?

[Mark Zarley]

I like to look at the angle of my chain in heavy winds. If my attachment point is 3 ft above the water and the chain enters the water 10 ft in front of that point , it tells me that in the 12 ft of water I am anchored in I need a min of 50 ft at that angle just to reach the bottom. 3ft of rise for 10 ft of length, total depth 15 ft divide by 3 times 10 . Just my way of trying to figure out how much chain is still pulling horizonal. If a ronca digs in deeper even with a vertical (upward)pull, how the heck do you get it back onboard? No answer required, just find it curious.

[thinwater]

There is one sepecfic, dynamic case, where I have found all-chain is trouble, and Alain does talk about it I believe; strong conditions in shallow water, where waves are trying to break. The boat will fall back from a gust, drop off a wave, and then snatch up against the chain when the combination of wave, gust, and backward momentum meet; the forces can easily be several time the static forces. The answer is (other than not to get caught is such a place) to have enough rope out to absorb the wave portion of the surges, or to have very long and properly sized snubbers. However, this behavior is very situation and boat specific, so modeling is a battle. Only once (caught by a thunderstorm off a beach) have I found myself in this possition, and adding 75' of nylon made all the difference.

However, if there was one correct answer, I would always use chain or always use line; I have both choices and vary my selection. Generally all chain works great... but not always.

[MathiasW]

Now, it would appear I am 10 years late to this thread...

Still, I find the discussions somewhat irritating. Yes, scope is a good way of keeping track that your anchor will not be pulled too vertically.

And I do like catenary. Yes, it is a model and not exact, but that doesn't mean it is no use. It helps to know when there is lift off at the anchor shank. You may want to ignore it, but it is a useful information to have.

Also, it is simply wrong that the catenary always seizes to exist in strong winds. If you are anchoring in very deep water, like 20+ m or so, then the scope formula becomes impossible to fulfil. But when you look at the catenary results for such a situation, you see that with a reasonable amount of chain you are still in the position to offer the anchor shank a horizontal pull, only. It may not look like it when looking at the straight chain at the bow, but a lot can happen as it submerges for 20 meters or more.

I did a lot of calculations of all this, and yes, my profession of old does end with 'cist', but that does not mean it is wrong.

Ignoring bridles and snubbers for a moment (which I believe are vital) I found that in shallow waters you need to account for the fact that the standard catenary form of the chain becomes less and less capable of absorbing energy. This is simply due to the fact that an almost horizontal chain cannot increase its potential energy anymore, which is the only way a chain can temporarily store energy.

As a result, the load on the anchor can become unbearable in shallow water and the only solution is to relocate to deeper water and use a little more chain, or improve the snubber.

All this can be calculated using a modified catenary.

When you are in deep water, the standard catenary is dominant. And here it often suggests less chain than a scope approach would dictate. This should be proof enough that the catenary is still effective in this regime.

And as I said, I do not trust pictures of a straight chain at the bow. There is no knowing how it will look like at the anchor, unless you show pictures of the anchor itself.

So, my main conclusion of all this is that a fixed scope approach is not good - and may not even be safe. In shallow water it often underestimates the required amount of chain, whilst in deep water it overestimates it.

Anyway, if you are interested in my work, I have created a series of short digests for various boat sizes (of different windage areas and chain thicknesses), which you can download the one closest to your needs. It covers not only wind (and gusts), but also strong swell.

Here is the link:

https://trimaran-san.de/die-kettenku...atiker-ankert/

And no worries, despite this link being German, the digests are in English. Only the long paper is in German.

[SVHarmonie]

Originally Posted by MathiasW

Now, it would appear I am 10 years late to this thread...

Still, I find the discussions somewhat irritating. Yes, scope is a good way of keeping track that your anchor will not be pulled too vertically.

And I do like catenary. Yes, it is a model and not exact, but that doesn't mean it is no use. It helps to know when there is lift off at the anchor shank. You may want to ignore it, but it is a useful information to have.

Also, it is simply wrong that the catenary always seizes to exist in strong winds. If you are anchoring in very deep water, like 20+ m or so, then the scope formula becomes impossible to fulfil. But when you look at the catenary results for such a situation, you see that with a reasonable amount of chain you are still in the position to offer the anchor shank a horizontal pull, only. It may not look like it when looking at the straight chain at the bow, but a lot can happen as it submerges for 20 meters or more.

I did a lot of calculations of all this, and yes, my profession of old does end with 'cist', but that does not mean it is wrong.

Ignoring bridles and snubbers for a moment (which I believe are vital) I found that in shallow waters you need to account for the fact that the standard catenary form of the chain becomes less and less capable of absorbing energy. This is simply due to the fact that an almost horizontal chain cannot increase its potential energy anymore, which is the only way a chain can temporarily store energy.

As a result, the load on the anchor can become unbearable in shallow water and the only solution is to relocate to deeper water and use a little more chain, or improve the snubber.

All this can be calculated using a modified catenary.

When you are in deep water, the standard catenary is dominant. And here it often suggests less chain than a scope approach would dictate. This should be proof enough that the catenary is still effective in this regime.

And as I said, I do not trust pictures of a straight chain at the bow. There is no knowing how it will look like at the anchor, unless you show pictures of the anchor itself.

So, my main conclusion of all this is that a fixed scope approach is not good - and may not even be safe. In shallow water it often underestimates the required amount of chain, whilst in deep water it overestimates it.

Anyway, if you are interested in my work, I have created a series of short digests for various boat sizes (of different windage areas and chain thicknesses), which you can download the one closest to your needs. It covers not only wind (and gusts), but also strong swell.

Here is the link:

https://trimaran-san.de/die-kettenku...atiker-ankert/

And no worries, despite this link being German, the digests are in English. Only the long paper is in German.

Mathias,

You are 100% right, including the part about the discussions being irritating and full of half truths. Scope is a guide, not an absolute. Anybody with any real experience with anchors knows that very deep anchors do not need the same scope as the same setup in shallower water.

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.