Animation of anchor loads in shallow and deep water - the impact of gusts and swell

[MathiasW]

Originally Posted by Uplander

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!

The chain has a number of functions, actually. Some are obvious, others not quite so:

- connect the boat with the anchor. (obvious
- make sure the anchor shank is pulled at horizontally, or at least as much as possible horizontally. (this is what you are referring to)
- Absorb shock loads in gusts and swell by raising more off the ground and temporarily storing energy.

If there were no gusts or swell, shallow water would indeed be best. But the chain cannot absorb energy very well in shallow water. The catenary needed for that disappears too quickly when the chain is running almost horizontal to begin with.

So, in this case, you can pay out as much chain as you like, and things will not improve. What does help, though, is to add a lot of elastic rope as you do. Then you are good also in shallow water.

My point is that many boats at anchor in shallow water do not use long enough snubbers for those to have any meaningful impact.

But you are in the camp of long snubbers, so that is good!

Cheers, Mathias

[Kd9truck]

Thank you for an excellent thread, I constructed a snubber this past fall to replace the snubber I had because I did not feel like there was enough “stretch/elasticity” in the system.

My new snubber has a 18mm technora sling with a chain lock shackle, this leg is static and holds the chain just under the waterline and ends with a threaded rated eye just before the windless on deck are a pair of 12 mm nylon threaded eyes that run from the bow cleat to bow cleat. In heavy conditions I have another length of the 12mm nylon with threaded eyes that run from midship cleat though the tech sling to the other mindship cleat. The longer heavy weather leg is built to take the load before the shorter “light duty” leg.
(All components are rated to a minimum of 7500 lb shock load)

having a over built static leg in my system puts all of the elasticity on deck in view for inspection. With the softer components on deck it’s more forgiving on the fiberglass.

A side benefit not initially recognized is that the whole system can be used as a snubber for a sea drogue or sea anchor when finding a place to wait weather out is not an option.

I have an all chain road, nylon rode does have stretch but in most cases not enough imho

My goal is to sleep well
Cheers

[valhalla360]

Originally Posted by MathiasW

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.

Unless I'm misunderstanding what you are saying...Nothing funny about it. The issue is a short snubber stretched by the same amount (nominal length) will be at a greater tension and potential for failure compared to a longer snubber.

Example:
- A 5ft snubber stretched to 7ft is a 40% elongation.
- A 25ft snubber stretched to 27ft is a 8% elongation.

The 5ft snubber is likely getting long term damage being stretched so much (assuming it doesn't fail immediately) and is operating at far greater tensions compared to the 25ft.

This is the same reason when you tie up at a dock, you don't want your dock lines to run 1ft from boat cleat to dock cleat. Much better to have 10-20ft from cleat to cleat so they can stretch further without reaching their ultimate breaking point.

[MathiasW]

So, I have now also done the animation for a mix of chain and (very elastic) rope. Half of the chain in the previous animations has been replaced by the rope.

https://trimaran-san.de/wp-content/u...1049-x-540.mp4

As long as the pulling angle at the anchor shaft is small, there is virtually no difference between shallow and deep water. Effectively, the rope is acting as a massive snubber.

Cheers, Mathias

[Don C L]

I don't see "shallow" and "deep," I see two different scopes.

[MathiasW]

Originally Posted by Don C L

I don't see "shallow" and "deep," I see two different scopes.

Well, in one case the keel is much closer to the seabed than in the other, isn't it. Here, deep means 7 metres, and shallow means 2.8 metres.

Scope is an incredibly unreliable way of telling how much rode has to be paid out. This is because the catenary shape of a chain is highly non-linear, unless it blows really hard and it is shallow. So, whilst in deeper water a scope of, say, 3:1 may be enough, this scope is likely not sufficient in very shallow water, where the chain is very ineffective in absorbing shock loads.

Cheers, Mathias

[HeywoodJ]

An interesting topic, but i think it assumes that the anchorages are identical except for depth. We, for instance, anchored for a tropical storm with winds measured at 70 knots in about 2m of water. The reason we anchored there was it was tight little cove completely surrounded by trees. Wave action at the height of the storm was nil (no fetch). Wind speed on deck never exceeded 25 knots, and at the top of our mast 35-40. Out in the 15m deep bay behind us the wind was measured at 70, and the chop was 0.5m. We survived just fine, and so did the boats anchored in the bay, but I think we had a much more comfortable time. I suspect, however, that there are other sailors that would feel hemmed-in where we anchored and would prefer more searoom in case something happened.

[MathiasW]

Originally Posted by HeywoodJ

An interesting topic, but i think it assumes that the anchorages are identical except for depth. We, for instance, anchored for a tropical storm with winds measured at 70 knots in about 2m of water. The reason we anchored there was it was tight little cove completely surrounded by trees. Wave action at the height of the storm was nil (no fetch). Wind speed on deck never exceeded 25 knots, and at the top of our mast 35-40. Out in the 15m deep bay behind us the wind was measured at 70, and the chop was 0.5m. We survived just fine, and so did the boats anchored in the bay, but I think we had a much more comfortable time. I suspect, however, that there are other sailors that would feel hemmed-in where we anchored and would prefer more searoom in case something happened.

Yes, you are right. This analysis assumes identical conditions for all but depth. What one gets with this is that one understand the effect of depth much better. But of course, this is just one variable to analyse when deciding where to anchor, and as you said, there are other reasons that can overrule the suggestions based on depth alone.

I have not yet sat in 70 kn of wind, and hopefully never will. I usually pick a spot at 8-12 metres depth, if available. This has the added bonus that I am not in the crowd but more on the edge, so not so many that can drift into me...

When you say 2 metres depth. Measured from the bow roller? Your draft must be pretty small for that to work out. In my case, the distance from bow roller to bottom of keel is already 3.5 metres...

Cheers, Mathias

[HeywoodJ]

2m water depth, we only draw 1. About another 1.2 from the water to the roller. Having seen how straight chain can get in shallow water I don’t doubt the results. One thing about shallow anchoring is we are basically anchoring with a rope rode. The snubber/bridle ends up very close to the anchor. In deeper water a better chance that the rope (presuming that is what people use) will hang in the water and clear of most things that cause chafe (other than the things on the boat end).

[MathiasW]

Originally Posted by HeywoodJ

2m water depth, we only draw 1. About another 1.2 from the water to the roller. Having seen how straight chain can get in shallow water I don’t doubt the results. One thing about shallow anchoring is we are basically anchoring with a rope rode. The snubber/bridle ends up very close to the anchor. In deeper water a better chance that the rope (presuming that is what people use) will hang in the water and clear of most things that cause chafe (other than the things on the boat end).

Ok, thanks for this clarification. So, I would call this an anchor depth of 3.2 metres, as I am always counting from the bow roller. That is the height difference that matters for the chain.

And when you are using mostly rope, then you are not affected by the poor performance of chain in shallow water, as this animation also shows. On the contrary, you will want to anchor in shallow water to keep the pulling angle at the anchor shank small.

So, it all makes sense...

Cheers, Mathias

[Cpt Mark]

I wish I could find it. May have been by and anchor manufacturer. They spoke at length about bridles and how long the need to be for multihulls.

How they based their calculations is the angle of he bridle. The Vee should be 90 degrees. That is the optimum length of bridle. On my 35 foot cat that worked out to 22 feet.

[Rimica]

Was anchored in 5m of water with 50m of chain and nylon snubber of 7m. Stayed out in 50kts of wind for 4 days. We were protected from the waves.

[Dockhead]

Originally Posted by HeywoodJ

An interesting topic, but i think it assumes that the anchorages are identical except for depth. We, for instance, anchored for a tropical storm with winds measured at 70 knots in about 2m of water. The reason we anchored there was it was tight little cove completely surrounded by trees. Wave action at the height of the storm was nil (no fetch). Wind speed on deck never exceeded 25 knots, and at the top of our mast 35-40. Out in the 15m deep bay behind us the wind was measured at 70, and the chop was 0.5m. We survived just fine, and so did the boats anchored in the bay, but I think we had a much more comfortable time. I suspect, however, that there are other sailors that would feel hemmed-in where we anchored and would prefer more searoom in case something happened.

Deep water is better for anchoring, even at the expense of scope (within reasonable limits). But shelter is even more important. Your example is a good one to illustrate that.

[MathiasW]

Originally Posted by Cpt Mark

I wish I could find it. May have been by and anchor manufacturer. They spoke at length about bridles and how long the need to be for multihulls.

How they based their calculations is the angle of he bridle. The Vee should be 90 degrees. That is the optimum length of bridle. On my 35 foot cat that worked out to 22 feet.

Hmm, I would not know what this rule could be based on.

Generally, the longer the snubber, the more elastic it is, and the better it therefore is. It will also be less stressed when it is longer.

[Cpt Mark]

Found the article

https://www.snubberhead.com/catamaran-anchor-bridles