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Old 23-02-2023, 07:39   #1
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Animation of anchor loads in shallow and deep water - the impact of gusts and swell

Hi again,

I have created two short animated videos to show the massive difference in anchor loads when anchoring in very shallow versus in deep water, when there is substantial gusts or swell to deal with.

shallow water: The chain does not work well and needs an elastic snubber to absorb gusts and swell

https://trimaran-san.de/wp-content/u...6-x-2160-6.mp4


deep water: The chain is working much better now. A snubber still helps, though

https://trimaran-san.de/wp-content/u...6-x-2160-6.mp4

In a severe gust, an elastic snubber will reduce the anchor peak load substantially. Particularly so in shallow water, where in this example the load is almost halved! In all these 4 cases the chain length is kept the same! Interestingly, in this example, the anchor peak load in deeper water is less than in shallow water, despite a substantial amount of chain still lying on the seabed in the latter case. The chain is simply not used in that case and is wasted! See the red and green arrows labelled ‘last link’ indicating how much chain is still lying on the seabed. (Contrary to popular belief, chain lying on the seabed is not reducing the anchor load by a lot.) The old sailor’s wisdom of “a lot of chain helps a lot” is thus not quite correct. More accurately it should say “a lot of chain helps a lot – provided it is given enough anchor depth to work with”.

The input to the videos comes from Anchor Chain Calculator, so it is not just an artists impression...

What I mean by elastic snubber is the sort of thing that Jonathan Neeves is advocating. Not these short 1 metre "snubblets" one only too often sees. A snubber needs to be long to work well.

Precise anchor loads will depend on the vessel, of course. This is just an example.

Cheers, Mathias
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Old 23-02-2023, 07:56   #2
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Mathis,

Excellent! You show in images what I have been preaching for a long time, and is supported by all the anchor manufacturers I trust.

In the real world of anchoring in wind and waves a long and stretchy snubber is more important than the anchor you pick! So many people I see with a “snubber” that doesn’t even reach the water. Or a “snubber” made of Spectra or Dyneema because it is “stronger.” Totally useless! I avoid anchor downwind of such boats!
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Old 23-02-2023, 08:09   #3
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Anchored in 32-38 mph steady winds, with some gusts higher, I was in about 13 ft of water with sand bottom. Snorkeling the anchor I discovered that there was still catenary in the chain even at those wind speeds on a catamaran. It seems that it's heavy wind chop or waves that will make the chain pull up tight. If you are protected from long fetch creating rough water, the wind speed is not so much an issue.

But yeah, snubbers are important!
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Old 23-02-2023, 08:19   #4
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Quote:
Originally Posted by Cheechako View Post
Anchored in 32-38 mph steady winds, with some gusts higher, I was in about 13 ft of water with sand bottom. Snorkeling the anchor I discovered that there was still catenary in the chain even at those wind speeds on a catamaran. It seems that it's heavy wind chop or waves that will make the chain pull up tight. If you are protected from long fetch creating rough water, the wind speed is not so much an issue.

But yeah, snubbers are important!

Wave action definitely exaggerates the shallow vs deep water difference. The vertical movement from a 2 foot chop is far more significant in 10 feet of water than in 50 feet (vertical movement of 20% of depth vs 4% of depth).
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Old 23-02-2023, 08:36   #5
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

It seems to me that these animations show that while more of the chain is lifted off the bottom in the deep water example, thus showing that more of the chain's weight is doing work, it takes much less anchor load to do so than in the shallow water example. So presuming the chain doesn't break, the shallow water example can take more anchor load before the chain comes up off the bottom.
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Old 23-02-2023, 08:44   #6
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

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Originally Posted by Muaddib1116 View Post
It seems to me that these animations show that while more of the chain is lifted off the bottom in the deep water example, thus showing that more of the chain's weight is doing work, it takes much less anchor load to do so than in the shallow water example. So presuming the chain doesn't break, the shallow water example can take more anchor load before the chain comes up off the bottom.
Well not 'can take', but rathe have to take in order to balance things out. It is a bad thing if the anchor load goes that high. You want it as small as possible, so that the anchor has a chance to stay put.

Cheers, Mathias
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Old 23-02-2023, 09:29   #7
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Nice work Mathias. Just curious; how long is the snubber in your calculations?
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Old 23-02-2023, 09:32   #8
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Ok, so the animation is showing the load the anchor receives? given the same wind speed and boat? What is the scope and depth in each animation?


And given the same pull force on the rode, the deeper water sees less anchor load and more chain dampening?
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Old 23-02-2023, 09:40   #9
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

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Originally Posted by mikereed100 View Post
Nice work Mathias. Just curious; how long is the snubber in your calculations?
Thanks!

The funny thing about snubber is that all that counts is the actual stretch at a given wind strength, but not its actual length. What I assume for these calculations is a snubber that stretches by 1.6 metres at 8 BFT.

So, if you have a snubber that is 16 metres long, then this translates to 10% stretch. If your snubber is only 4 metres long, this would mean 40% stretch, which clearly is too much.

The snubber always needs to match the vessel, of course.

Cheers, Mathias
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Old 23-02-2023, 09:45   #10
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

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Originally Posted by Muaddib1116 View Post
Ok, so the animation is showing the load the anchor receives? given the same wind speed and boat? What is the scope and depth in each animation?
Yep. Same boat, same wind speed, same impact energy due to the gust. The depth ratio between the two is 2.5:1. (The actual depths I used in the calculations are 2.8 metres and 7 metres.)

I do not like the notion of scope. Scope is dependent on depth. Since in all these 4 cases there is enough chain so that the last link is still on the seabed, adding more 'scope' is obviously not achieving anything.

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And given the same pull force on the rode, the deeper water sees less anchor load and more chain dampening?
Yes, exactly!
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Old 23-02-2023, 10:26   #11
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

I only have 20ft of chain, so this probably doesn't have nearly the same impact, cool stuff though.
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Old 23-02-2023, 10:33   #12
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

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I only have 20ft of chain, so this probably doesn't have nearly the same impact, cool stuff though.
So you extend with a rope as needed?

A mix of chain and rope is very effective in terms of absorbing energy - provided you chose an elastic rope to begin with...

For a mix of chain and rope, it is not advantageous to seek deeper water. On the contrary. I might create an animated video for that as well. Need to do a bit of coding in the tool I use to create the animation with...

Cheers, Mathias
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Old 23-02-2023, 10:48   #13
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Good work ,Mathius, Over the years, I too have a firm opinion of anchoring v depth. So many variables - but considering a wind of 20 knots + and a sea bed of reasonable density , say sand/mud mix, then I would opt for depth and plenty of scope. Connected to my cat with a nylon bridle, approx. beam of boat ( 9 m ). So lets say a depth of 8m would be better than 2m. Same amount of chain, about 50m. The deep drop seems much more reliable and the boat doesn't wander in lulls.
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Old 23-02-2023, 10:56   #14
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Great stuff. I end up using 60’ of chain and 50’ of snubber when I’m in a typical shallow water anchorage. 4+’ freeboard, say 10’ water low tide 14’ high tide. 47’ heavy ketch rig sailboat. The snubber really works. I see the stretch.

Yes overkill a bit but only 2-1/2 boat lengths total.
If I need to shorten up in shallow water I shorten both chain and snubber.

Deeper water similar snubber, longer chain.
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Old 23-02-2023, 14:56   #15
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Re: Animation of anchor loads in shallow and deep water - the impact of gusts and swe

Anchoring is always interesting as so many factors come in to it. Therefore it is quite hard to generalize. I thought the main purpose of the chain is to keep the pressure on the anchor as horizontal as possible. Everything being equal I am quite happy in shallow water as that is achieved with less chain. Gusts and waves bring different factors to play and that is where the spring comes in. A nice 3 strand nylon warp hooked on to the chain and in extreme conditions taken back to the cockpit winches works well. That would be a total of about 15 metres - 11 on the deck and 4 or 5 in the water. Happy nights!
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