Anchor in Shallow water

[StuM]

Originally Posted by MicHughV

I've anchored there a gazillion times and use the same setup pretty much everywhere I go. I use 75' of chain with maybe 10' or rode out,

So the other 65ft of chain is still on the boat?
"Rode" is not "rope". It's the name for the entire connection system between the anchor and the boat.

[Sojourner]

The usual scope rules don't apply in super shallow water. We're a deep mono (2.2m) but have anchored for long periods in 3m shallows. 30m minimum. That's 10:1. There is so little chain weight out any less than that will pull tight in any light breeze. We saw plenty dragging with less, even in clean sand. The other thing about anchoring so shallow is that in any slightly more than moderate storm/wind event I really dislike the waves coming close to breaking under us. We were hit with a simple 35kn thunderstorm anchored in 3m and the swell got real steep. Anything more would have been a 3am reanchoring in a packed anchorage, something no one enjoys. Just because you can anchor so close to shore doesn't mean you always should

[Jamme]

[Mike OReilly]

As I mentioned, I think the whole shallow anchoring discussion has missed the huge impact of chain friction. I often anchor in shallow waters. Most of the time we’re swinging from our chain. It takes some pretty heavy winds to lift it all. And at that point it acquires a catenary.

So I too am skeptical about this call for more rode in shallow waters. I can certainly understand how one needs LESS in deeper waters, but my experience tells me the standard ratios (5:1, 7:1) hold up well in shallow waters.

[Pete7]

Originally Posted by Shaneesprit

I see lots of Anchor questions, but never seen anyone discuss how much rode they use when Anchored in shallow waters?
I am thinking of less than 2 meters deep. Possible 1 or 2 meters of tide.
Obviously in the multihull section, as the fin keels will of fallen over by this point or is that what they say about us? ��

Us anchored in about 1.7m at low water just outside Dartmouth at Woodhuish . Think we had about 10m of chain out and .5m of depth under the keels We were laying to the chain, not the anchor.

Pete

[GRIT]

Shallow, to us, means the keel is very near the bottom at low tide. 3m is exactly our anchoring depth target, and has been since before we had our first manual windlass.

We feel like we're in deep water if we drop the hook in more than 6m of water. As has been mentioned earlier, any Bahamas anchoring is typically in 3m or less, and we find 3m a common depth near where we want to be, all over the north Atlantic. . As Mike said, it's convenient to shore too.

I understand some feel the need to keep the chain on the ground, in all wind conditions, but I don't subscribe to it.

There's room in the Anchorage for both schools of thought.

Cheers.
Paul.

[thomm225]

Anchoring in shallow water is great unless you drag then your boat will be laying on it's side on land or down in the mud.

Several times during the tide change plus with boat swinging I have seen 1' under the keel at like 2 am and have bumped off the bottom leaving the anchorage in the morning

I still use 75'-100' regardless of how shallow it is.

Where the wind gust is forward in photo is 2' deep (I draw 4') but forecast was for wind direction to stay on same course. Channel to stern is left to right

Home and dock on opposite side of creek.

[Lodesman]

Using a ratio for scope has never made sense to me. I much prefer 3 x boat length + depth of water to determine scope. That means in no wind/current situations, there is a standard amount of chain on the bottom regardless of depth, and therefore an equivalent capacity to absorb the force of wind/current.

[fxykty]

Originally Posted by GRIT

Hi Chris mac and fxykty. As regards "more scope" could you explain "why" for me?

I never really understood the more scope in shallow water thing. I pretty much always anchor with 5:1, deep water or shallow water. I figure the angle to the anchor is the same at 5:1, no matter the depth of the water. I'm happy with 5:1 up to 30 knots or so. If it's forecast to blow more, I'll likely put more out. One thing I should mention is that we're fond of what some would call oversized anchors. Presently, our primary (Ultra) has a mass of 60kg, and our secondary (Spade) 55kg. But regardless of the anchor size, I'm at a loss to understand why more scope in shallow water.

We anchored with less scope on our 39 foot Corbin (3:1), using a 30kg Genuine Bruce anchor. And the same before that with our SS 35, with a 15kg Genuine Bruce.

We cruised full time on each of these boats, from Canada to Trinidad, to the UK and the Atlantic coast of the EU. We always held great, and never drug, once set.

I suppose someone should define "shallow" too. Just so we're all on the same page.

I'm not being argumentative here, and I don't want to start a whole debate, but if someone could either explain to me why they use more scope in less water, or post the link to the thread someone mentioned earlier, I'd appreciate it.

Thank you.
Paul.

PS. To me, 5:1 means water depth plus anchor roller height, then multiplied by five. It offers a pull of, roughly, 11 degrees off horizontal.

Anchor rode calculator
https://www.cruisersforum.com/forums...d.php?t=228906

The load calculations show why more scope is needed in shallow water to make up for the lack of free-floating chain catenary. The damping effect of catenary is what keeps anchor loads from overwhelming the connection to the boat - lack of catenary in shallow water transmits more of the anchor load.

[Shrew]

IF anchoring with all chain rode, I go 5:1.

My calculation is based on actual depth (not depth of transponder)

PUS

offset from actual depth to high tide

PUS

Distance from water to bow roller.

For Example:

I'm reading 10 feet of actual depth, I'm in a 4 foot tide swing at mid-tide. There is 5 feet from water to bow roller.

((10 + 2) +5)) x 5 = 85 feet of chain to the the roller.

[GRIT]

Thanks for that link Fxykty. I'll read that today.

We use a bridle on our boat, which seems like it would handle just about anything the anchor could. The boat never seems to jerk on her tether.

I'm always happy to consider another point of view.

Thanks again.
Paul.

[Mike OReilly]

Originally Posted by fxykty

Anchor rode calculator
https://www.cruisersforum.com/forums...d.php?t=228906

The load calculations show why more scope is needed in shallow water to make up for the lack of free-floating chain catenary. The damping effect of catenary is what keeps anchor loads from overwhelming the connection to the boat - lack of catenary in shallow water transmits more of the anchor load.

Does this take into account the added friction from chain on the bottom? I missed it if it does. Anyone who shallow-water anchors knows that most of the time it is the friction of the chain that is holding the boat, yet I've never seen that included in any calculator.

I really think this is a key force in shallow-water anchoring. I can't say how equivalent it is to deep-water catenary, but it clearly plays a significant role in the forces at play.

Once friction is fully overcome, and the rode lifts, a shallow water rode has as much catenary as a deep water rode. Snubbers take the shock-load.

Seems to me the point about rode length is not so much that you need a lot more in shallow waters. Rather, you can get away with a lot less in deep waters.

[Franziska]

Originally Posted by Mike OReilly

As I mentioned, I think the whole shallow anchoring discussion has missed the huge impact of chain friction. I often anchor in shallow waters. Most of the time we’re swinging from our chain. It takes some pretty heavy winds to lift it all. And at that point it acquires a catenary.

So I too am skeptical about this call for more rode in shallow waters. I can certainly understand how one needs LESS in deeper waters, but my experience tells me the standard ratios (5:1, 7:1) hold up well in shallow waters.

Adding to this, the near horizontal pull on the anchor is also likely to increase its holding over an anchor which gets pulled from an angle above of 40degree something.

Still I do get the point that the missing catenary should be accounted for by a sensible extra amount of scope.

We tend to use 20-25m chain in most depths, if it's deeper we add plenty of rope or shackle in another 15m of chain.
So far that works very well.

Still an issue on à at us often that you anchor in shallow water, put out lots of chain and someone misjudged where your anchor is and anchors right on top of it.
An anchor marker helps and we see it frequently used here on the Iberian Atlantic coast.

Maybe a sort of board indicating how much chain you have out would work better.
Never seen this though.

If you use a long scope despite shallow water you sometimes almost get insulted for doing so...

[thinwater]

First, the OP is not talking about shallow (2 + 2 = 4 meters). In fact, I only anchor in water that deep ~ 5% of the time. Normal for me is 2 meters and shallow is 1 meter at high tide with a 1-foot tide.


Take the case of 3 feet. With 2 feet of freeboard on my F-24, that means 25 feet of rode at 5:1 (12 degree lead angle). The bridle is 20 feet, so that means 5 feet of rode. Any kind of projection and you will snag the trianglar bridle. Also, this means no chain weight at all. Really, with what amounts to all-rope and zero catenary in any wind about 7 knots, you need more like 10:1 scope to get good burying (6 degree lead angle). Yes, people claim lower angles, but they always have a lot more chain out than we are talking about. I have run the test and reviewed the actual test results done without chain (anchors up to 20 tons). At 5:1 with no catenary you give up almost half of your holding capacity.



And then there is the matter of just how little stretch and mass 20 feet of bridle has.



So IMO there is a practical minimum of about 50' unless I'm fishing from a kayak.


The scope vs. depth issued in moderate to deep water has been flogged to death and I' not going there. Without knowing the holding ground, anchor, rode type, fetch, and expected weather (I always expect a thunderstorm in summer!) there is no meaningful answer. But just like there is a minimum, with all chain there tends to be a max around 250 feet, where once you have that much weight out, more does not help unless the water is quite deep. This reasoning does not apply with rope unless the chain leader is quite long (over 50 feet, which it often is).

[GRIT]

Originally Posted by Lodesman

Using a ratio for scope has never made sense to me. I much prefer 3 x boat length + depth of water to determine scope. That means in no wind/current situations, there is a standard amount of chain on the bottom regardless of depth, and therefore an equivalent capacity to absorb the force of wind/current.

It seems like you'll be taking up a lot of room in a small anchorage. For us, using your technique, in 3m of water, we'd have 48m of rode out, that's 16:1 and a 100m turning circle, which is difficult to find in any popular anchorage.

Do you do this in places like St Augustine, Palm Beach, Georgetown Bahamas, Baiona, Kinsale, Cascais, or anywhere there are more than two boats?

I've no doubt, you're nearly always putting a horizontal pull on the anchor, but I haven't seen a manufacturer say they must have 0 degree horizontal pull. Those that mention scope, always use a ratio. You may notice ROCNA mention that above 10:1, there's little benefit.

Rocna says this: https://kb.rocna.com/kb/Scope_vs_catenary

Scope ratios
A 3:1 ratio of rode-length to water-depth should be considered minimal ("1" being the vertical distance from the seabed to the bow roller, not the waterline, at high tide). Generally speaking about 5:1 is appropriate. Typically you should set the anchor at the same scope it will be left at, in order to be sure the anchor can re-set itself again should it pull out. However, it may be necessary to set it at a higher scope and then pull in some of the rode.

In bad conditions, the ratio can be increased up to around 8:1. There is little benefit in going beyond this point, and boaters using scopes of 10:1 and higher are gaining almost nothing from their efforts and huge swinging circles. At 8:1, the maximum angle of pull on the anchor is already capped at just over 7°, and of course the scope must be doubled in order to halve the angle (i.e, even very large increases in the scope result in angle reductions of only several degrees). Considering the desire to lower the angle against the practical problem of longer rodes and increased swinging circles, 8:1 is roughly the 'sweet spot'.

Spade says: https://www.spadeanchorusa.com/how-to-anchor-a-boat.php

I won't paste their page, but it's basically the same as the Rocna site, other than they don't recommend 3:1, they start at 5:1 and go as high as 7:1 or more.

My personal experience mirrors what's said in these articles. 3:1 is a minimum, 5:1 is good for average conditions, and 8:1 is good for high winds. All this assuming an "all chain" rode.

Cheers, and happy anchoring.
Paul.