Anchor in Shallow water

[sailingharry]

Originally Posted by Panope

I believe the "boat length" calculations attempt to account for different bow roller heights.

Steve

That's much easier to take into consideration by simply adding your freeboard to the water depth. So when I am in 20 ft of water, I use 25 as the water depth.

[GRIT]

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.

Thanks for that. It's surprising we had such good performance from our Genuine Bruce at 3:1. We anchored that way for years, with two separate boats, and two separate anchors. I'm not doubting your numbers, but if it worked well at 3:1, the Bruce must be a beast at greater than 5:1!

Cheers.
Paul.

[GRIT]

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.

I didn't download the app, but I have read the entire advert. That fellow did an immense amount of research, and it looks like an interesting app.

If his numbers are accurate, it would be an interesting tool to help understand what's happening. But I think most folks, after a year or two of cruising, will intuit most of what he said, even though they don't have hard numbers to back it. This app might help someone justify their setup, or perhaps see where it's lacking.

We don't have issues with our 5:1 in the shallowest of waters. Due in part to our snubbers, of course. We often anchor in waters less than a metre at low tide, and never have an issue with lack of catenary. There may be no catenary, or very little, but it's never been an issue.

An interesting read in any case. Thanks Fxykty, for the link.

Cheers.
Paul.

[s/v Jedi]

For shallow water it isn’t so much about scope as it is about having a minimum length of rode out. For our 3/8” chain rode, we set a minimum of 60’ from our bow that is 5’ above the water.

So when there’s 7’ of water at high tide, we set 60’ rode. When there’s more than 15’ of water (15’ + our bow 5’ = 20’ x 3 for scope is 60’ rode) then we set 90’ or 120’ as long as we can get a 3:1 scope. From 120’ we go up in 60’ steps.

[Lodesman]

Originally Posted by GRIT

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.

It took awhile for your link to open for me. An individual's blog is not necessarily a definitive authority. He apparently bases his opinion on the IACS guidance. Individual CS still apply their own rules and guidance as evidenced in this circular from Lloyds: https://maritime.lr.org/classnews2018-13

But even if using IACS as guidance, I think you can see in the accompanying snippets from their rules that as ships increase in size, not only does anchor and chain size increase, but the total length of chain carried also increases. So length of ship does dictate rode length or at least available rode length.

As to the 6:1 scope assertion. I can only find reference in the IACS docs to 6:1 being considered a minimum - and 10:1 considered 'normal.' Regardless, that's with the caveat that its recommendations are for sheltered anchorage. This seems to be covered in their anchor test section - whether that's equivalent to recommendations for usage is anyone's guess. I don't think IACS actually has guidance in that regard. It might be up to the individual CS, companies or even captains to determine scope required. In the latter case, I would assume many Brit captains would use the Admiralty formula: square root of depth of water in metres X 1.5 = number of shackles (shots for our American friends). I gather most companies would set a min length of rode, irrespective of depth. Maybe some commercial forumites can chime in?

Consider also that 6:1 is going to be in deeper waters generally - 20m or more. And also that being the 'minimum' it would be the equivalent of the 'lunch hook' situation - ie. while they wait in the roads for their assigned berth to open. Keep in mind, big ships keep a watch on at anchor, and frequently run the engine to reduce load on the chain.

[GRIT]

I'm going to withdraw from this conversation now.

I wish you fair winds and following seas.

Stay well.
Paul.

[Tetepare]

This is an amazing discussion, five pages.

I don't know what we're doing differently. I figure I can drop a bowling ball with 1:1 and still have to power the sucker out.

[thomm225]

So I have all my chain in regardless which is about 40’ then 25’ to 40’ of rope ride this from 5’ of depth up to 8’-10’ after that more …,,, usually holds pretty well.

[valhalla360]

34ft catamaran. 1.5ft draft (with boards and rudders retracted). Anchor roller approx 3ft off water surface.

30ft chain followed by rope rode (I know the horror of not having 300ft of chain).

For us shallow is going to be about 3ft deep at low tide.

We'll put out all the chain plus 15-20ft of rope to act as a snubber. We also do a bridle attached with a Pruisik knot.

At 6ft from roller to sea bottom, 50ft/6ft ~ 8.3 scope. If tide is big, we may let out more to keep scope at high tide reasonable.

One of the advantages of really shallow anchoring is you are often protected better from wind/waves (if you aren't why bother). You are also less likely to be fighting for space with other boats.

[thinwater]

Originally Posted by GRIT

Thanks for that. It's surprising we had such good performance from our Genuine Bruce at 3:1. We anchored that way for years, with two separate boats, and two separate anchors. I'm not doubting your numbers, but if it worked well at 3:1, the Bruce must be a beast at greater than 5:1!

Cheers.
Paul.

In fact, the Bruce is lower in holding capacity per unit weight than most new generation anchors (though MUCH better than the Lewmar Claw--they look similar but do not perform the same). The data was based on the Claw.


However, data from the original Bruce show that it is one of the better short scope anchors and that it does not fail suddenly at short scope as many others do. It just starts dragging slowly, sometimes not even noticed.


The short coming of the Bruce is hard bottoms and weeds, neither of which it penetrates well. Others do better in sand, but pretty much everything does well enough in good sand. In good mud, the original Bruce remains a solid anchor.

[Shaneesprit]

Originally Posted by s/v Jedi

For shallow water it isn’t so much about scope as it is about having a minimum length of rode out. For our 3/8” chain rode, we set a minimum of 60’ from our bow that is 5’ above the water.

So when there’s 7’ of water at high tide, we set 60’ rode. When there’s more than 15’ of water (15’ + our bow 5’ = 20’ x 3 for scope is 60’ rode) then we set 90’ or 120’ as long as we can get a 3:1 scope. From 120’ we go up in 60’ steps.

7ft of water is not shallow in a CAT, half that depth is shallow. - shallow is when you can climb off and walk to shore.
The real issue with scope in a busy shallow anchorage is that the surrounding boats cannot see your scope, I read here about people saying the swing is the same for all boats, but visually while calm it’s not. If you have 60ft of scope your boat can be facing in any direction. The other boats have no clue where your anchor lies. You could be 50ft west of the anchor with the rode behind you meaning your boat will move 100ft or more - (you being a general term for the reader not aimed at anyone)

[s/v Jedi]

Originally Posted by Shaneesprit

7ft of water is not shallow in a CAT, half that depth is shallow. - shallow is when you can climb off and walk to shore.
The real issue with scope in a busy shallow anchorage is that the surrounding boats cannot see your scope, I read here about people saying the swing is the same for all boats, but visually while calm it’s not. If you have 60ft of scope your boat can be facing in any direction. The other boats have no clue where your anchor lies. You could be 50ft west of the anchor with the rode behind you meaning your boat will move 100ft or more - (you being a general term for the reader not aimed at anyone)

The OP mentioned 2-4 meters depending on tide. 2 meters is about 7’ so I think my post was accurate and correct. If you anchor with a 10’ rode because you’re anchored in 2’ of water and think 5:1 scope is great, then I think you’ll get a nasty surprise when a 50kt squall hits!

[Shaneesprit]

Originally Posted by valhalla360

34ft catamaran. 1.5ft draft (with boards and rudders retracted). Anchor roller approx 3ft off water surface.

30ft chain followed by rope rode (I know the horror of not having 300ft of chain).

For us shallow is going to be about 3ft deep at low tide.

We'll put out all the chain plus 15-20ft of rope to act as a snubber. We also do a bridle attached with a Pruisik knot.

At 6ft from roller to sea bottom, 50ft/6ft ~ 8.3 scope. If tide is big, we may let out more to keep scope at high tide reasonable.

One of the advantages of really shallow anchoring is you are often protected better from wind/waves (if you aren't why bother). You are also less likely to be fighting for space with other boats.

I have less chain than you! Maybe I need another 5ft of chain 😂
Less likely to be fighting for space! Not in the UK, it’ can be a struggle to find space. Literally looking for someone leaving to take their space. .

[Shaneesprit]

Originally Posted by s/v Jedi

The OP mentioned 2-4 meters depending on tide. 2 meters is about 7’ so I think my post was accurate and correct. If you anchor with a 10’ rode because you’re anchored in 2’ of water and think 5:1 scope is great, then I think you’ll get a nasty surprise when a 50kt squall hits!

I am the OP with wind it’s easy.
I think I said in less than 2m which is why I said it’s a multihull question. But still listening to mono replies I am not a CAT diehard.
If there was the possibility of a squall then yes deeper water required.
I was interested in what everyone else does because in calm you can be anywhere facing in any direction inside your anchor circle.
I had scope hence why my boat moved across the anchor West to East I had about 10m of scope 7m of chain, possibly more but moved about 20m in the direction of another boat.
Boat at no point circled the anchor. Just a 20kt breeze arrived.
The comment/reply wasn’t directed at you personally.

[belizesailor]

I've done loads of shallow water anchoring. I usually go with 5:1 or more. If anchoring in 2m of water then 5:1 is not much rode so I expect that ultimately I end up with more (have never actually checked to see how much more).

Also, in 2m of water my usual bridle setup would be fouling the bottom so I shorten it. Mine is rigged to be adjustable so its easy.