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Old 13-03-2016, 05:20   #1
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Challenge: Anchoring on Slanted Bottom

OK,

So everyone has probably heard the various formulas: 5 to 1 rode, more in bad conditions, chain as long or longer than your boat, etc. And, we have heard the various disagreements about some of the formulas.

My challenge is as follows: Lets say, you are going all chain. Its a 40 foot multihull with only 3' draft, and you are in 10 foot of water, with a 35 pound, old style Bruce anchor.

You have 45 foot of chain out and you can't let out more because 45 feet in one direction, the water is only about 6 foot deep, and you are seeing 3-4 foot tides and some rocks on the bottom here and there. In the other direction, the water drops off to about 22 feet, only 45 feet away. You have swung 180 in both directions and continue to swing with radically changing, winds which vary from about 10 -25 knots.

Is your 45 foot of chain enough to hold you when you swing into the deeper water and get hit with 25 knot gusts? If not, what is the solution?

Got some ideas, but hoping to hear from more experienced folks.

Thanks,

G2L
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Old 13-03-2016, 05:52   #2
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pirate Re: Challenge: Anchoring on Slanted Bottom

Why are you not anchoring in 22ft..??
That's only 7metres... and you'll lay more comfortably.. My formula which 'T's many off on here..
I am 'Old School' and never anchor in less than 4-5metres.. unless I want to dry out for some reason..
I use all chain and lay 3XLWL.. this was what I was advised back in the 60's before all this Techy 'Scope' stuff... and it has served me well.
Other boat have already lengthened rode by the time I'm nearing total chain lift.. the rest of the time in benign weather I swing in a limited circle with a lazy anchor.. the weight of chain does the work
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Old 13-03-2016, 05:53   #3
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Re: Challenge: Anchoring on Slanted Bottom

What matters is the relationship between the anchor line and the sea bed. Most commonly held notions about scope assume you are anchoring on a flat bottom.

If you are anchored on a steeply sloped sea bed the amount of line required changes depending on your relationship to the slope.

Anchoring uphill requires very little anchor line. Anchoring dowhilll may require more than you have on the boat. Suggest you move the boat.
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Old 13-03-2016, 06:12   #4
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Re: Challenge: Anchoring on Slanted Bottom

Yep, Boatman is right. Anchor in the 22 feet. That's quite shallow.

But for your hypothetical question maybe you could say the 22ft is 100 feet, what do yo do then?
MOVE!

Never anchor where it is untennable!

The next question of your hypothetical is how much more scope on a slope?
This is a seriously vexed question! I don't think there is an answer. Currently I am anchored next to a channel. My anchor is in 3m , my bow is 7 meters and 30m behind me is 14m deep! To make it worse we have had 3 days bad weather squalls 30+ knots, bottom grassy. Anchoring hell. Oh, and boats everywhere, quite crowded.

Because its so crowded one can't put out a stupid amount of chain, like 100m etc.

So I have out about 30 meters - actually, I just checked and it's about 40m -

So its 3x7m + double scope for storms




Kinda. Approx. Near enough.
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Old 13-03-2016, 06:37   #5
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Re: Challenge: Anchoring on Slanted Bottom

Quote:
Originally Posted by Delancey View Post
What matters is the relationship between the anchor line and the sea bed. Most commonly held notions about scope assume you are anchoring on a flat bottom.

If you are anchored on a steeply sloped sea bed the amount of line required changes depending on your relationship to the slope.

Anchoring uphill requires very little anchor line. Anchoring dowhilll may require more than you have on the boat. Suggest you move the boat.
Interesting. That is the kind of stuff I was wondering about. Other folks say, the slope doesn't matter. It is where you drop the anchor that you measure from. What you say seems to make more sense to me, but the problem is that flat bottoms are not to be found where I am at the moment.
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Old 13-03-2016, 06:44   #6
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Re: Challenge: Anchoring on Slanted Bottom

Quote:
Originally Posted by MarkJ View Post
Yep, Boatman is right. Anchor in the 22 feet. That's quite shallow.

But for your hypothetical question maybe you could say the 22ft is 100 feet, what do yo do then?
MOVE!

Never anchor where it is untennable!

The next question of your hypothetical is how much more scope on a slope?
This is a seriously vexed question! I don't think there is an answer. Currently I am anchored next to a channel. My anchor is in 3m , my bow is 7 meters and 30m behind me is 14m deep! To make it worse we have had 3 days bad weather squalls 30+ knots, bottom grassy. Anchoring hell. Oh, and boats everywhere, quite crowded.

Because its so crowded one can't put out a stupid amount of chain, like 100m etc.

So I have out about 30 meters - actually, I just checked and it's about 40m -

So its 3x7m + double scope for storms




Kinda. Approx. Near enough.
Hi,

Thanks for your reply, but I don't understand what you mean about your "bow" being 7 meters? Your bow is not 21 feet off the water, is it?

Please clarify,

G2L
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Old 13-03-2016, 07:06   #7
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pirate Re: Challenge: Anchoring on Slanted Bottom

Quote:
Originally Posted by Gone2long View Post
Hi,

Thanks for your reply, but I don't understand what you mean about your "bow" being 7 meters? Your bow is not 21 feet off the water, is it?

Please clarify,

G2L
He's saying he's in a sloping bottom situation.. he dropped the hook in a 3m depth.. by the time he'd laid out his chain/rode the depth beneath the keel had dropped to 7+m.. and to his stern over the next 30m the bottom sloped even further..
Where he is the wind is usually a constant ESE or a few degrees either way..
Basically the situation your asking about..
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Old 13-03-2016, 07:08   #8
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Re: Challenge: Anchoring on Slanted Bottom

The formula for scope is only dependent on the depth of water where the anchor is located. The depth under the boat is not used.

However, the scope calculation should include the freeboard.

So with 45 feet of chain the formula for scope at high tide is:
45/(10+4+5) = 2.3:1 (assuming 4 feet if tide above the 10 feet and 5 feet of freeboard).

If the boat swings into the 22 foot deep area, or the six foot deep area the scope remains the same, but the reasonably rapid change in depth suggests the anchor is on a downslope when the boat is in the deeper water.

Of course this is an assumption. It is slope immediately around the anchor that is important. The anchor could be on flat shelf, or in an area of steeper slope than average. The bottom is often not uniform.

While downslope does not effect the scope, it does effect the effective scope and the holding power of the anchor. The important factor is the angle of the chain relative to the seabed not the scope. We can mathematically calculate the effect of this downslope but in this case there is little need. The scope of 2.3:1 is inadequate even if we assuming that the anchor was dropped in patch of flat ground. A downslope makes the effective scope significantly worse.

See this thread:
Calculating Scope Can Be Very Misleading

The boat is not safe overnight.

As others have suggested it may be possible to safely anchor further from shore using more rode. Another alternative would be to stern tie to shore. Here the anchor will be kept on an upslope. This would significantly improving the effective scope.

Even though the forecast winds are only lightish (10-25 knots)I would still want more scope even if stern tying on an upslope, especially if I could only guesstimate the slope around the anchor and the anchor was on the small side. The variable wind direction would also likely result in some situations where the wind was beam on when stern tied.
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Old 13-03-2016, 07:09   #9
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On Anchoring Deeper, with a "Bridle"

Quote:
Originally Posted by boatman61 View Post
Why are you not anchoring in 22ft..??
That's only 7metres... and you'll lay more comfortably.. My formula which 'T's many off on here..
I am 'Old School' and never anchor in less than 4-5metres.. unless I want to dry out for some reason..
I use all chain and lay 3XLWL.. this was what I was advised back in the 60's before all this Techy 'Scope' stuff... and it has served me well.
Other boat have already lengthened rode by the time I'm nearing total chain lift.. the rest of the time in benign weather I swing in a limited circle with a lazy anchor.. the weight of chain does the work
Thanks boatman. Always good to hear from you. Not sure what the depth on the deep end of that 22 foot bottom might be, but if it is flat from there to the channel (which is a safe distance away), then your solution might be a good way to go. Right now, we are anchored close in, because our dinghy outboard won't start : o. But I'll save that for another thread : )

Would have to let out some rope to accompany my chain to anchor deeper, and I am a little worried about how sound the splice on the bitter end of my anchor line might be. A little too much of the stretch that those other guys mentioned think is valuable, and oops ... We definitely don't want that to happen. : ) By the way, as a reminder, I should note that "I am the windlass".

I will try to get some ACCURATE readings on the bottom starting at the 22 foot mark and look at moving deeper as an option. Also, if I am worried about the main anchor line failing, why not tie another line to the anchor line shackle where it meets the chain, and create a kind of "bridle". I would need two folks hauling on the double line to pull the chain up to where one person could reach it, but that should make safely pulling the anchor an easier job, while insuring that the anchor is not lost on a bad day.

Make any sense to you?

PS - Buying more chain is not an option at the moment.

Thanks again for all your comments,

G2L
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Old 13-03-2016, 07:09   #10
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Re: Challenge: Anchoring on Slanted Bottom

Quote:
Originally Posted by Gone2long View Post
Hi,

Thanks for your reply, but I don't understand what you mean about your "bow" being 7 meters? Your bow is not 21 feet off the water, is it?

Please clarify,

G2L
No.

The anchor is in 3 meters of water.
The bow of my boat where the depth transducer is, is in 7 meters of water.
Behind me the water depth is 14 meters.

So I am anchored a slope going from 3 meters deep to 14 meters deep in 100 meters...

So I have set my scope for the 7 meters depth at my boat, not the 3m depth at the anchor.

And I have enough out that if I dragged into the 14m I would still have 3:1. (But if I did start the drag along a grassy bottom the grass would build up on the anchor and make it less likely to hold. So I have been on my boat during the bad weather instead of being drunk ashore...)


Mark
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Old 13-03-2016, 07:16   #11
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Thanks for the Clarification

Quote:
Originally Posted by boatman61 View Post
He's saying he's in a sloping bottom situation.. he dropped the hook in a 3m depth.. by the time he'd laid out his chain/rode the depth beneath the keel had dropped to 7+m.. and to his stern over the next 30m the bottom sloped even further..
Where he is the wind is usually a constant ESE or a few degrees either way..
Basically the situation your asking about..
Hi again,

Yeah, I knew he was describing a similar situation, but I didn't get that 7 meter part.

Thanks, and please see my earlier, up thread, response to your suggestion on anchoring deeper.

G2L
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Old 13-03-2016, 07:24   #12
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Thanks for the Clarification

Quote:
Originally Posted by MarkJ View Post
No.

The anchor is in 3 meters of water.
The bow of my boat where the depth transducer is, is in 7 meters of water.
Behind me the water depth is 14 meters.

So I am anchored a slope going from 3 meters deep to 14 meters deep in 100 meters...

So I have set my scope for the 7 meters depth at my boat, not the 3m depth at the anchor.

And I have enough out that if I dragged into the 14m I would still have 3:1. (But if I did start the drag along a grassy bottom the grass would build up on the anchor and make it less likely to hold. So I have been on my boat during the bad weather instead of being drunk ashore...)


Mark
Thanks.


I know the feeling : )


G2L
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Old 13-03-2016, 07:35   #13
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On a Line to Shore v. Opposing Winds, Waves, Tides

Quote:
Originally Posted by noelex 77 View Post
The formula for scope is only dependent on the depth of water where the anchor is located. The depth under the boat is not used.

However, the scope calculation should include the freeboard.

So with 45 feet of chain the formula for scope at high tide is:
45/(10+4+5) = 2.3:1 (assuming 4 feet if tide above the 10 feet and 5 feet of freeboard).

If the boat swings into the 22 foot deep area, or the six foot deep area the scope remains the same, but the reasonably rapid change in depth suggests the anchor is on a downslope when the boat is in the deeper water.

Of course this is an assumption. It is slope immediately around the anchor that is important. The anchor could be on flat shelf, or in an area of steeper slope than average. The bottom is often not uniform.

While downslope does not effect the scope, it does effect the effective scope and the holding power of the anchor. The important factor is the angle of the chain relative to the seabed not the scope. We can mathematically calculate the effect of this downslope but in this case there is little need. The scope of 2.3:1 is inadequate even if we assuming that the anchor was dropped in patch of flat ground. A downslope makes this scope significantly worse.

See this thread:
Calculating Scope Can Be Very Misleading

The boat is not safe overnight.

As others have suggested it may be possible to safely anchor further from shore using more rode. Another alternative would be to stern tie to shore. Here the anchor will be kept on an upslope although I would still want more scope especially if I could only estimate the slope around the anchor and the anchor was reasonably small.
Thanks, especially for the final comment. That makes sense, and I have seen a lot of boats do that around here. To add some detail, I should note that we have held here for over a month in 25k winds, swinging all around, and one reason I have not put out a stern anchor or a line to shore is that I was worried about what might happen if the boat tries to swing into the wind when tethered to shore, especially in a 2' swell and 25knot wind pushing us around to wind, with a tide coming from the same direction. What is your take on that?

One of the reasons we may have held may be that there is a lot of coral and rocks around, and, when I dove the anchor, it seemed to be holding in sand, just a foot or so on the deep side piece of coral, so it is probably hooked under that coral as well.

Surrounded the anchor are other pieces of coral and rock, so when we swing deep, the anchor still holds. Granted, this is not the best of all possible worlds, so, we are definitely looking at going deeper, where we can let out more rode, and perhaps avoid the rocks, etc.

Thanks for your insights,

G2L
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Old 13-03-2016, 07:41   #14
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pirate Re: On Anchoring Deeper, with a "Bridle"

Quote:
Originally Posted by Gone2long View Post
By the way, as a reminder, I should note that "I am the windlass".
So am I if the boat is South off 35ft.. above that 7m of chain plus anchor converts good upper body exercise to Hernia..
I will try to get some ACCURATE readings on the bottom starting at the 22 foot mark and look at moving deeper as an option. Also, if I am worried about the main anchor line failing, why not tie another line to the anchor line shackle where it meets the chain, and create a kind of "bridle". I would need two folks hauling on the double line to pull the chain up to where one person could reach it, but that should make safely pulling the anchor an easier job, while insuring that the anchor is not lost on a bad day.

Make any sense to you?

PS - Buying more chain is not an option at the moment.

Thanks again for all your comments,

G2L
Regarding anchoring I ALWAYS circle the anchorage area to verify the charted depths concur with actual depths.. storm erosion, shifting banks and bars etc.. then I will pick my spot knowing what my surrounds are and possible get outa jail free routes.
Regarding your chain.. if your concerned about your splice at the shackle (bad idea 2 fail points in one) I would definitely run another line and make a bridle.. can be managed solo by hauling in first one half then tie off and take in the slack on the other till the shackles on board.. disconnect the bridle then deal with the chain.. IF you feed into an open locker.
One thing I would definitely do ASAP however is get rid of that shackle and do a proper tapering splice direct to the chain.
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Old 13-03-2016, 07:54   #15
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Re: Challenge: Anchoring on Slanted Bottom

If you don't have the right gear (only 45' of chain?? no rope rode? nothing?)

Anchor somewhere else.

Then, dock the boat, go ashore and get a job for a while.

Proceed to the local chandler and buy the following:

1) An anchor appropriate for a 40' cat (windage)
2) At least 100' of chain
3) Another 200' of 3/4" or so nylon rode (splice that to the bitter end of chain)
4) A bridle setup if desired

Basically, the question is similar to, "how do I drive a nail into solid oak with this stick of butter?"
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