Challenge: Anchoring on Slanted Bottom

[noelex 77]

Two anchors can be helpful to reduce the swing radius (as in the OP's example), but it is important to realise that you will still need a large, or very large scope on the anchor that is holding on the downhill slope. Even this may not be enough. I will explain why.

Note: Without an image of the terrain around the anchor, all we can work on in this example is the average drop in depth from the anchor to deeper water.

In the OP's example the depth drops 12’ in 45’, suggesting the downhill slope near the anchor may be about 15°. Thus if an huge amount of rode could be used and the wind was strong enough to straighten the chain near horizontal, the anchor shank would be pulled 15° to the substrate simply to get the shank horizontal, equivalent to the angle of pull on the shank with a 3.75 :1 scope on a flat substrate.

This suggests that if a strong wind is blowing the boat "downhill", a large anchor with a large length of rode is needed. In addition despite the large mathematical scope, you will need an anchor that performs well on a shortish scope ie 3.75 :1.

It is also worth noting that in situations like this two anchors does not add to redundancy. In fact it creates the problem that if either anchor fails the yacht will drag.

[s/v Jedi]

There is no slope when anchoring at the 22' depth as recommended. This is a 99% case as long as there is enough swing room for a 100' rode.

[Sos]

Looking at your last picture I would attached the safety rode to the first shackle thereby missing out the swivel. I would prefer a direct splice to the chain with multiplait.

[noelex 77]

Originally Posted by s/v Jedi

There is no slope when anchoring at the 22' depth as recommended.

How do you know this?

The only evidence the OP has presented (that I can see) is that the depth drops from 10’ to 22’ over a 45’ distance. There is no indication that the downslope magically levels out at the 22’ mark. Perhaps I missed this?

With the limited information it is possible the bottom would do this, but it is a dangerous assume this is the case with no evidence.

It does illustrate that it is very difficult to know what the slope is doing around the anchor. The slope makes a very significant difference to the holding power.

[s/v Jedi]

The OP seemed to find 22' too deep to anchor and 6' too shallow. We agree with 6' too shallow with the tides but not with the 22' and there has been no comments about not having room there or anything.
You guys also make this slant sound like a dropoff, while a 15deg angle is nothing like collapsing edges etc. that are mentioned. We anchor in sloping seabed all the time because when you anchor in the lee of an island, you have shallow water forward and deep back and anchor on a slope. Anchors just work on that, some better than others. The pull on the chain is always upwards from the angle of the bottom except when you drop it deep with the biat over shallow, which almost never happens. To press this point further: you could add scope to fully compensate for slant and create the exact same angle between seabed and chain.

[barnakiel]

Originally Posted by s/v Jedi

Okay Barna, 99% of all situations, okay?

100% OK. Listening, learning, happy.

Hugs,
b.

[noelex 77]

Originally Posted by s/v Jedi

To press this point further: you could add scope to fully compensate for slant and create the exact same angle between seabed and chain.

I agree sloping bottoms are common and typically with an offshore wind boats are anchored on a downslope. It is a factor that should be given more consideration.

I disagree that it possible to fully compensate for this downslope with extra scope.

If the downslope is 15° if the catenary is almost eliminated the anchor shank will be pulled up at 15° even with an extremely long scope.

In moderate wind chain weight and catenary can restore the pull of the anchor shank to parallel to the seabed, but as the catenary almost disappears in strong wind this effect is lost. The anchor shank will be pulled at 15° or greater.

I do agree that some anchor models retain more of their holding ability when the anchor shank is pulled up at an angle, but all models will have a significant loss at angles of around 15°.

This does not mean that it not possible to anchor safely on this sort of downslope. If you are confident that the wind will remain light or moderate the chain weight may be enough to keep the chain on the bottom close to the anchor and so keep the angle of pull parallel to the seabed. However, if there is a possibility of strong wind this catenary will be lost. To have confidence that your anchor will hold with the pull on the shank at say a 15° above the seabed it will have to be in good holding ground combined with an excellent anchor design, preferably oversized. Extra scope helps, but cannot compensate for the downslope.

[s/v Jedi]

Noelex, you don't understand what I'm trying to say: it doesn't matter which anchor or how shallow or what scope: you always pull up on the anchor when the rode goes taut because the seabed is lower than the windlass.
Now, when the seabed is slanted down 15 degrees, this angle between seabed and taut chain has increased by 15 degrees. But when you add scope, you decrease that angle. It is therefor certain that a downslope can be countered with more scope.

[noelex 77]

Originally Posted by s/v Jedi

Noelex, you don't understand what I'm trying to say: it doesn't matter which anchor or how shallow or what scope: you always pull up on the anchor when the rode goes taut because the seabed is lower than the windlass.

Yes, that is correct. This is the major reason for using a large scope in strong wind. It ensures the angle of pull on the anchor is as close to the seabed as possible.

So on flat ground assuming there is zero catenary, the chain is pulled up at 12° for a scope of 5:1. In preparation for bad weather if you let out 10:1 this decreases to 6°. The anchor's holding ability is greater with a shallow angle of pull closer to the seabed. So the holding power increases as more scope is let out.

Originally Posted by s/v Jedi

Now, when the seabed is slanted down 15 degrees, this angle between seabed and taut chain has increased by 15 degrees. But when you add scope, you decrease that angle. It is therefor certain that a downslope can be countered with more scope.

No that is wrong. The angle is decreased with more scope but the angle between the chain and the seabed can never be less than 15° (assuming the chain is taught with no catanery)

If we assume an infinite scope (obviously impossible but this is a best case scenario) the rode would be parallel to water surface (ignoring the curvature of the earth ). In diagram form it will look like this. You can see the chain is pulled up at 15° relative to the seabed. On a flat seabed the angle would be zero.



In practice, if the wind is light to moderate, the weight will ensure the chain will sit on the bottom and the pull will be parallel to the seabed, but in strong wind the catenary becomes minimal and the downslope will have a significant effect.

[s/v Jedi]

Of-course the angle can never be less than 15 degrees. It doesn't have to unless you want to match a 12 degree angle. But when you normally anchor in deeper water with a 3:1 scope then you can add more scope to compensate for seabed sloping down.

Most anchors are designed to dig in with an angle betwee the chain and the seabed that is well over the 0 degree angle you seem to like. With some anchors may be, but not my experience. Setting my anchor works better on short scope than on long scope, so the opposite.

[noelex 77]

Originally Posted by s/v Jedi

Of-course the angle can never be less than 15 degrees.

15° between the rode and the seabed is the same angle as occurs with 3.75:1 on a flat seabed. The holding power of the anchor is related to to this angle.

The holding power of the anchor will be significantly lower when the effective scope is 3.75:1. This is why, if possible, the scope is normally extended well beyond 3.75:1 when bad weather is forecast. Lowering the angle allows the anchor to bury deeper and increases the anchors holding ability.

15° is a long way from optimum and this assumes an infinite scope. Real world scopes will increase this angle further.

[Gone2long]

Originally Posted by wrwakefield

Hi G2L,

I'll offer some quick feedback. [And please keep that perspective; it is just feedback. There are many ways to accomplish your end goals, and all that work are viable... I also apologize in advance if I am missed any details that have already been mentioned...]

First, your set-up appears to be secure, but could be greatly simplified and streamlined if you wanted to.

It looks like you have a combination chain and double-braid rope rode. [I'm stating what I see so you can correct me if I'm misunderstanding...]

Your line appears to be double-braid so unless you are using a snubber [or preferable a bridle since you have a multi-hull?] it won't do much to absorb shock loads in inclement conditions [i.e., double-braid won't stretch unless it is dynamic climbing rope, which you wouldn't use for anchor rode, but is OK for a snubber/bridle...]

Therefore you have some choices as I see it [speaking from a monohull sailor's perspective... I would defer to the multi-hull aficionados for even more specific feedback...]

1) Keep the rode set-up you have and add a bridle of suitable length and material to relieve any shock loads you might encounter. [e.g., a 50 ft length of 3/8" to 1/2" (guessing again...) 3-strand nylon or appropriately sized dynamic climbing rope Prusik hitched to your anchor rode in the middle of the snubber length. [I'm assuming you would be connecting the snubber to the rope portion of your rode, but this will work whether you are attaching to the chain or rope portion...]

This same approach would work for a snubber as well...

2) If a snubber/bridle is not desirable- or if you want to streamline that rope-chain connection- you could remove the shackle connecting the chain to the swivel and set all that aside for recycling. Then splice an appropriately sized 3-strand nylon rode [5/8"? -just a semi-educated guess...] directly to the chain with a rope-chain splice. [Which is actually quite easy and satisfying to accomplish...]

Above are a couple of approaches to help improve and streamline your ground tackle set-up.

There are many more approaches, and sometimes certain situations require creative thinking...

In case this is helpful.

Cheers!

Bill

Thanks Bill. The links were especially helpful. But, wouldn't a direct connection of the rope to the chain cause twisting and potential chafing, if not other problems?

The latest photo should show that we have an admittedly ancient connecting swivel, and, connected to that is the shackle holding both the main anchor rope and the secondary "snubber" - the purpose of both ropes only being to absorb a bit of shock, and to create less racket than the chain does in the anchor roller, when we are hit by a bit of swell and winds up to 25knts. The "snubber" is also there to protect against a possible break in the main anchor rope.

In fact, I inherited the chain, swivel, shackle and spliced main rope configuration from the previous owner who has never made himself available to comment on any of this. Consequently, I am trying to learn, after the fact, how much of the set up that I inherited make sense, and how much doesn't.

I appreciate your comments and links, and tell me why you think it might be a bad idea to have the swivel and/or the shackle.

As noted previously, I think that the intent of the shackle attached to the swivel at the end of the chain was to make it easy to connect various lengths of rope, and perhaps more chain (which we also have aboard) to the chain line, as varying conditions might warrant.

The versatility that the shackle creates would seem to be worth any possible problem it might create, no? Furthermore, the shackle should be somewhat stronger than either rope connected to the chain, no?

I know; I am presenting lots of rather involved issues here, and they probably bring up a number of additional variables, but any comments you might have would be sincerely appreciated.

Thank you for your detailed responses.

G2L

[Xlantic]

Originally Posted by noelex 77

15° between the rode and the seabed is the same angle as occurs with 3.75:1 on a flat seabed. The holding power of the anchor is related to to this angle.

The holding power of the anchor will be significantly lower when the effective scope is 3.75:1. This is why, if possible, the scope is normally extended well beyond 3.75:1 when bad weather is forecast. Lowering the angle allows the anchor to bury deeper and increases the anchors holding ability.

15° is a long way from optimum and this assumes an infinite scope. Real world scopes will increase this angle further.

Looked at a different way, a taught rode at 5:1 scope gives an 11º angle against the horizontal, which added to a 15º seabed angles gives a total of 26º.

26º is the angle of a taught rode at a 2:1 scope in a flat seabed. And 2:1 is clearly not enough in a blow strong enough to make the rode taught!

Increasing the scope to 10:1 with a 15º seabed would still leave an "effective scope" of 2.7:1, again not enough in a blow strong enough that makes the rode go taught.

[s/v Jedi]

Originally Posted by Xlantic

Increasing the scope to 10:1 with a 15º seabed would still leave an "effective scope" of 2.7:1, again not enough in a blow strong enough that makes the rode go taught.

And that is where you are wrong. Please cite any reference that prove that a 2.7:1 scope is not enough when the rode goes taut.

We have been anchored in 50+ knot winds at those ratios with the only problem being to get the anchor back up again as it digs in deeper.

Also, when you anchor in deeper water, you need less scope anyway.

[Xlantic]

Here are a couple:

Peter Smith says:
"A 3:1 ratio of rode-length to water-depth should be considered minimal. Generally speaking about 5:1 is appropriate.

In bad conditions, the ratio can be increased up to around 8:1."
Scope vs catenary (Rocna Knowledge Base)

And the Morgan's Cloud site:
"Three to one scope is an absolute minimum, after setting, and five to one is a lot better.
If you have a decent anchor, seven to one scope is all you will ever need."
https://www.morganscloud.com/2014/01...t-2-technique/

In the same article above Peter Smith explains why greater water depth should not reduce the need for scope.