Anchor in Shallow water

[GRIT]

Originally Posted by Mike OReilly

This is not my experience; at least not on my relatively heavy monohull. Once again, in shallow anchorages most of the chain is sitting on the ground. This provides significant resistance to rapid acceleration. Combined with decent snubbers (mine are 40' long x2 since hung in a bridle), there is never any hard stops on the deck cleats.

When wind increases to the point of lifting the chain, shallow-anchoraged boats benefit from catenary, but in my experience this almost never happens, or doesn't happen fully. At this point you get a dual effect of chain friction and catenary. I've weathered many a Newfoundland gale and rarely have I seen the chain lift far, or for long.

Once again, my view is that the standard ratios are pretty darn good for shallow-water anchoring, assuming your gear is up to the task. What I think the evidence shows is that in deep water anchoring, one can use less than the standard ratios.

Our heavy monohull and light cat behave the same as what you're describing. Whether it's friction or catenary, unless it's blowing a gale, 5:1 works wonderfully for us. In a gale, we tend to let more out.

We have two 10m docklines as snubbers, with a Mantus hook. It doesn't roll off the chain when it drags on the ground.

Cheers.
Paul.

[GRIT]

Originally Posted by Jim Cate

^^^^
Paul, this seems right to me as well. The angle of the rode when the rode is lifted should not be related to the length of the anchored boat. I don't get the logic of such plans!

Jim

I thought that might be the case, with your experience.

I'll try to read that link today and comment. I'm leaving tomorrow morning for a run to the USA, and have a "to do" list before leaving; so if I miss commenting, it'll be a couple of days.

Cheers.
Paul.

[Lodesman]

Originally Posted by GRIT

And to be clear, I'm certainly not against having lots of rode out when it's needed, I'm just not a believer in 2 boat lengths plus the depth of water which someone on here was saying they used. It means a small boat will have much less chain out than a larger boat, which doesn't make much sense to me.

A 6m boat would only have 18m out in 6m of water, whereas a 15m boat would have 36m out. I don't see that smaller boat holding as well as the big one.

Whereas if they both had 5:1, they'd both have 30m out (slightly more due to anchor roller height). Assuming both boats have the "right" sized anchors and chain, I'd assume similar results. The anchor manufacturers also make that same assumption.

Larger boats generally have larger windage, and as it's related to the area, the force of wind increases exponentially with length.

Originally Posted by GRIT

At the extreme, anchored in 20m of water, the smaller boat would have out 32m of rode, and the larger would have out 50m. I think most folks would agree, neither of those lengths are enough. In a 5:1 ratio, they'd both have out roughly 100m of rode.


As I see it, basing rode length on boat length is just too arbitrary a way of deciding how much to put out.

Having a defined amount of chain on the bottom (@ no wind/current) means having a defined amount of potential energy in that unlifted chain. Irrespective of water depth, it will take the same amount of wind/current strength to completely lift that standard amount of chain. So you can be reasonably certain of having similar anchor performance (as "holding power") in all depths of water.

With the simple ratio, you have 10m of rode in 2m depth, but 20m in 4m depth. Really? You double your swinging circle within a small variation in water depth. Does that make sense to you? Do you not think you'd have too little chain out in the shallow water? Lots of other respondents state they arbitrarily increase that ratio in shallow water, so clearly that method is not the be all and end all.

It was 3X boat length + water depth, btw.

[rslifkin]

Originally Posted by Lodesman

Larger boats generally have larger windage, and as it's related to the area, the force of wind increases exponentially with length.



Having a defined amount of chain on the bottom (@ no wind/current) means having a defined amount of potential energy in that unlifted chain. Irrespective of water depth, it will take the same amount of wind/current strength to completely lift that standard amount of chain. So you can be reasonably certain of having similar anchor performance (as "holding power") in all depths of water.

With the simple ratio, you have 10m of rode in 2m depth, but 20m in 4m depth. Really? You double your swinging circle within a small variation in water depth. Does that make sense to you? Do you not think you'd have too little chain out in the shallow water? Lots of other respondents state they arbitrarily increase that ratio in shallow water, so clearly that method is not the be all and end all.

It was 3X boat length + water depth, btw.

Larger boats also have correspondingly larger anchors. And larger boats are actually easier to anchor, mostly because boats gain weight faster than windage as size increases. So a larger boat is less reactive to gusts and waves, making the shock loads proportionally easier to deal with. The larger anchors carried by larger boats also tend to work disproportionately better as size increases (by many accounts).

[Anders]

Originally Posted by Lodesman

Larger boats generally have larger windage, and as it's related to the area, the force of wind increases exponentially with length.

That's why larger boats have bigger anchors and heavier chain.

[Mike OReilly]

Originally Posted by rslifkin

Larger boats also have correspondingly larger anchors. And larger boats are actually easier to anchor, mostly because boats gain weight faster than windage as size increases. So a larger boat is less reactive to gusts and waves, making the shock loads proportionally easier to deal with. The larger anchors carried by larger boats also tend to work disproportionately better as size increases (by many accounts).

[GRIT]

Originally Posted by Lodesman

Larger boats generally have larger windage, and as it's related to the area, the force of wind increases exponentially with length.



Having a defined amount of chain on the bottom (@ no wind/current) means having a defined amount of potential energy in that unlifted chain. Irrespective of water depth, it will take the same amount of wind/current strength to completely lift that standard amount of chain. So you can be reasonably certain of having similar anchor performance (as "holding power") in all depths of water.

With the simple ratio, you have 10m of rode in 2m depth, but 20m in 4m depth. Really? You double your swinging circle within a small variation in water depth. Does that make sense to you? Do you not think you'd have too little chain out in the shallow water? Lots of other respondents state they arbitrarily increase that ratio in shallow water, so clearly that method is not the be all and end all.

It was 3X boat length + water depth, btw.

My apologies for putting 2x boat length, thanks for the correction.

I think we'll have to agree to disagree on the matter of scope.

Our boat has, probably, more windage than any other on this thread, and 5:1 has always worked well.

Yes, doubling the length of chain, for double the water depth does make sense to me. Keeping the same angle of pull on the anchor means I can expect it to work predictably.

I don't agree more scope is needed in shallow water, my experience is that 5:1 (with proper ground tackle, and snubbers) works quite well for anything up to 30 knots, at any depth of water.

The doubling of the swinging circle is of no consequence, as most other boats in the anchorage will be on similar scope; and it's still quite a bit shorter than 3x boat length plus water depth.

Catenary works, until it doesn't, and the last line of defense is to have the anchor do it's job, and manufacturers seem to agree that 11 degrees of pull is well within the design limits of the anchor. Using a short bit of chain, and the remainder being line, I'd still anchor using 5:1 scope, and be confident that it'll hold.

Using your method means the angle of pull on the anchor can change dramatically with water depth, making holding less predictable. In deeper waters, you have a larger angle of pull, once the catenary is taken away, lowering the holding strength of the anchor.

That's my opinion; worth twice what you paid for it.

Cheers.
Paul.

[Lodesman]

Originally Posted by GRIT

Catenary works, until it doesn't, and the last line of defense is to have the anchor do it's job, and manufacturers seem to agree that 11 degrees of pull is well within the design limits of the anchor. Using a short bit of chain, and the remainder being line, I'd still anchor using 5:1 scope, and be confident that it'll hold.

Using your method means the angle of pull on the anchor can change dramatically with water depth, making holding less predictable. In deeper waters, you have a larger angle of pull, once the catenary is taken away, lowering the holding strength of the anchor.

Catenary doesn't work at all if you don't have sufficient length and weight of chain to actually create it. My(*) method ensures zero degrees at the anchor up to storm force winds. Being within design limits does not mean that there isn't a loss in holding power; I think for most modern anchors an angle of pull of 7º (iirc) results in a 50% decrease in the holding power of the anchor.

Horses for courses.

* This isn't actually "my" method; it was mentioned by Boatman. I had previously used a variation of the Admiralty formula (square root of depth of water X factor = length of rode), but thought his method was much simpler and elegant.

[Lodesman]

Originally Posted by rslifkin

Larger boats also have correspondingly larger anchors. And larger boats are actually easier to anchor, mostly because boats gain weight faster than windage as size increases.

Actually they become correspondingly smaller as to their relation to the vessel itself. Looking at Rocna's fitment guide a 4K anchor is 0.1% the mass of the 2 t boat it can hold, but as we go up the scale to the 110K anchor, it goes down to 0.03% of the mass of the 165 t boat.

More mass also means more momentum.

[Lodesman]

Originally Posted by Anders

That's why larger boats have bigger anchors and heavier chain.

And if you look at anchorage charts, bigger ships are given bigger swinging circles. Do you think a VLCC anchors at 5:1?

[thinwater]

Originally Posted by thinwater

I think this chart requires some explanation

• The anchor were initially light set at zero lead angle and then shortened up. This is what happens when you have chain (does not lift until load increases).
• The real takeaway on Fortress is not that it can hold at short scope, but that it is very hard to recover if deeply set.
• Modern anchors are at about 80% at 6 degree lead angle (10:1 scope). However, most anchor ratings are at 10:1 scope and most test results you will read are at 7-10:1 scope for practical reasons.
• The reason the chart stops at 3:1 scope (18 degrees) is because the data becomes completely unpltable at that point; 1/2 of the anchors will failed at less than 10% and there is no reliability, particularly if thee is ANY yawing.

That said, this is the scope (lead angle) at the sea floor, not the overall scope. With any chain it will be better, depending on the wind strength and water depth. Personally, I like to think in terms of a 60-knot squall, because they are common in the summer and explaining why I didn't follow that rule after my boat is on the beach and someone is injured is going to be utterly useless. In shallow water you might as well assume the chain will be straight (tension a chain to 1000-2000 pounds and see how much sag you get, remembering that steel weights less in water). In deep water, with >75 meters out, there will still be some sag.

[Anders]

Originally Posted by Lodesman

And if you look at anchorage charts, bigger ships are given bigger swinging circles. Do you think a VLCC anchors a 5:1?

I have no idea. Do you?
Fortunately they don't anchor were I do and I don't really care if the have excessive scope like 10:1.

[GRIT]

Originally Posted by Lodesman

And if you look at anchorage charts, bigger ships are given bigger swinging circles. Do you think a VLCC anchors at 5:1?

According to this website, they recommend 6:1.

https://www.myseatime.com/blog/detai...choring-a-ship

Nothing mentioned about the length of ship dictating rode length.

Cheers.
Paul.

[sailingharry]

It seems I don't really use math in my anchoring. 125 ft almost all the time. If I anchor in 20 ft of water, plus height to the bow roller, that's 5 to 1. If I anchor in 35 feet of water, that's barely 3 to 1, but in winds less than 10 to 15, that's adequate (I almost never see the rode leaving the boat at that 3:1 ratio, so I'm pretty sure the chain is still on the bottom). I think I used 200, but I may have gone to 250 -- but there wasn't much swing room -- when I anchored in 75 ft. I can't recall the last time I anchored in less than 20 ft -- where I cruise, 20 ft of water is swing distance from rocks!

Don't read too much into this. I do the math every time. I consider the weather every time. I consider the anchorage and protection every time. But it's amazing how often the answer is 125 ft.

[sailingharry]

Originally Posted by Jim Cate

^^^^
Paul, this seems right to me as well. The angle of the rode when the rode is lifted should not be related to the length of the anchored boat. I don't get the logic of such plans!

Jim

I've often heard mooring lines should be specified based on the length of the boat. To me, that is equally irrational for the same reasons. But it's common.