How soft is "soft" mud when anchoring?

[troppo]

Without putting one's foot in it, how does one know if the bed where the anchor is about to be dropped is soft or hard? And how soft is "soft" mud?

Softness of mud bottom makes a major difference with holding power of anchors and I am very keen to know how others work it out.



Recently I read an article describing holding power of various anchors in extra soft bottom mud. I am not wanting to debate anchors but how to discern the bed make-up. Dipping an anchor toe into muddy waters - MySailing.com.au

Just having come back from a cruise where I was anchored in mud and compliments of a gale force breeze dragged about 50 metres before I released more rode and stopped, I have become more interested in how to recognise what bed is what.

Because where I anchor has high tidal flow, I just assumed the bed was rather hard mud and/or sand. Banks are hard mud covered with mangroves. I assumed soft mud would wash away. But then, if I tried to stand on the banks, I would sink so maybe the banks are not hard mud after all. Perhaps semi-hard or even semi-soft.

Are fish finders with the red line thing indicating the sea floor any use for checking bed consistency before anchoring?

So, how do people check quality of mud beds for anchoring?

[MarkJ]

Can be a bit difficult.
That damn local knowledge and a testing proceedure.

First thing I always do is a 'drive by' and check out the anchorage. If there is someone else there I will stop and ask how the holding is. Never been denied information

Then its a test plonk. Chuck in anchor in and see what happens. If it sets then I snubber it and then put it in astern at 2,000 rpm for a full minute. That sorts out if i am holding

But soft mud can be difficult. You plonk it in and it will drag every time.... So then you need to get creative... One trick is to let it settle in for 2 hours. Just drop it but dont pull back on it at all for a few hours, then just tap it in slowly, tap, tap tap. Slowly build up to a full reverse.

Difficulty where you are in Rockhampton is crocodiles have moved in, havent they? Muddy water, changing current from tidal flow. Just makes it more tricky.

The higest number of moves before I got set was 6, so dont worry, just have another go


Mark

[tkeithlu]

In a tidal salt marsh, the consistency of the mud varies with its location in the tidal stream and with the amount of sand blown into it from nearby dunes or islands. At the cut bank of a meander it may be pretty hard clay, but across the stream at the point bar (inside of the curve usually in the falling tide backwater) it may be fluff mud - too thick to swim in, too thin to walk on. We require students to own Chuck Taylor All Star High Tops, because they are the only shoe we've found that can't be sucked off a student's foot in soft mud. Of course, that means that the student is pretty well anchored, but we have not lost one yet. No. There is no sure way of saying how soft the mud is under you, unless you are ready to invest in some very expensive geological equipment or take cores. My observation is that cruising boats, seeking to anchor out of the strong current, tend to anchor on the soft stuff, since the soft stuff was deposited by the lack of current.

[Alan Mighty]

The big industrial users (submarine cable layers and the oil & gas industry) use a range of tools that are not available to cruisers. Those tools include:

* side-scan sonar using a mix of frequencies from 60 kHz to 500 kHz to image the bottom (e.g. the towed fish being used in the Indian Ocean to look for MH 370);


* sub-bottom profilers, such as pingers using frequencies 3.5 - 7 kHz that can penetrate a few metres or more; and boomers using frequencies 50 Hz - 5 kHz that can penetrate tens of metres.


Without access to those, we have to rely on:


* as tkeithlu wrote, looking at the chart and the landform and guessing where the recent fine and unconsolidated sediment is being dumped and where the bottom might be of older mud that has consolidated over time; and


* as MarkJ wrote, local knowledge and your own anchoring test - you are not testing the anchor, you are testing the failure or sheer strength of the soil. If the soil fails, you weigh anchor, move elsewhere, and try again.


I've found bottoms that fail to supply enough soil strength for anchoring. In SE Asia, overdevelopment and seasonal heavy rain has caused high soil erosion rates resulting in soft seabeds. Made worse near urban centres by trash. The result is very soft seabeds with plastic bags embedded, so that what should be a cohesive mud can never cohere and will always sheer along embedded plastic bags, disposed diapers, and other junk.


What can be done?


One is to recognise that most anchors, especially the 'new generation' anchors, are really optimised to work in sand (and are usually, as Jon Neeves noted in the article you cited, demonstrated and tested in clean sand).


Two is to recognise the nature of mud. It is cohesive - it sticks together, so if you work it in your hand it behaves like putty or modelling clay, not sand. It drains slowly - it takes years for recent fresh mud to drain and become cohesive, firm mud. Generally speaking, a mud soil is stronger with depth - so the deeper you can get an anchor to embed, the more firmly you are anchored (and note that heavy anchors are more likely to dive deeply into mud than light ones, sharp points are better than dull ones, and surface area counts).


Three are all the techniques that anchoring sailors have accumulated over years (none of this is new or original to me, but to restate the obvious):


* soak time. That's the 2 or many more hours that MarkJ wrote about before thinking about power setting. And, if expecting a squall, to anchor a day before because the mud soil may take 24 hours as your anchor and chain embed in the soil and the soil consolidates with each wind gust. Work slowly laying your anchor and chain, feeling the anchor into the soil, delaying power setting as long as possible (but still eventually do your power setting, because power setting (i) embeds the anchor deeper; (ii) consolidates the mud - so whereas you might only power set for 30 seconds in sand, in mud you likely will initially follow MarkJ's recommendation of multiple slow reverse attempts before, hours later, ramping up to your power test rpms very slowly and then hold the test for much longer (because you can consolidate the mud a little; doesn't matter how long you reverse against sand, you're not going to fuse the grains of sand together); and (iii) power setting is your only way of testing the strength of the soil).


* a longer scope than in sand. If we start by thinking about being anchored in sand, we can imagine the anchor chain starting with a few links embedded in the sand, many more links sitting and sliding on top of the sand, and then a catenary curve reaching up to the anchor roller. In mud, we want the anchor to embed deeper - if you're using a roll bar anchor, you want the roll bar to be under the surface of the soil. So allow more chain so it can embed in the mud. And even more chain that is sliding on or partly buried in the mud (the source of the idea that chain can add significantly to the total anchoring force). You need lots of sliding chain and embedded chain. If a 5:1 scope works in sand, then a 10:1 scope might be needed in a soft mud soil. (And good washing of the chain and anchor when retrieving).


* a longer drag length. If a your anchor embeds in sand in its own length, then expect to need to drag the anchor slowly at least 4 times that same length in mud. And not end with a jerk that brings the boat to a halt.


* a clean anchor. As Panope's video tests have confirmed, many new gen anchors including Rocna and Manson Supreme, accumulate soil and need to be cleaned to get a good set again. So soil failure requires retrieval and cleaning before trying again.

[scotty c-m]

By the way … the traditional method is to take a lead line and sound the anchorage. At the end of the lead is a concave cup. You fill it with a soft wax (a toilet bowl ring wax works well). When the wax on the lead hits the bottom, it absorbs a layer of the bottom. Bring it up and see what's on the wax. I've used this and it actually works.

Another trick in clear water is to make a water window (a box with a glass on the end) and look at the bottom.

In nice water you can dive the anchor and see it for yourself. (I never do that here in the SF area …. brrrrr… too cold!)

[Alan Mighty]

Good on you, Scotty, if you can tell very soft mud from soft mud from consolidated mud by looking at the grains stuck on the end of your lead.

The traditional navy approach was to send out a boat with a blunt wooden stake and a mallet. There was a table showing soil type against the number of hammer blows to drive the stake one foot into the bottom.


Very dense sand: more than 50 blows/foot


Sand: 25 - 50 blows/foot (i.e. from soft sand to hard sand)


Hard clay: more than 16 blows/foot


Consolidated mud/clay: 4 - 16 blows/foot


Soft mud: 2 blows/foot


Very soft mud/silt: you don't need the hammer


Those guidelines of hammer blows/foot penetration, the Soil Penetration Test, were still being used by the Royal Navy and USN in the early 1980s.

[JPA Cate]

troppo,

We work it out by the setting process, and to some extent, it doesn't matter. You drop your anchor, and slowly reverse, letting it pull back against the reversing lightly. You're probably using about 25 ft. of chain, and the rest rope rode, so when you've got out scope of 5"1, pull as strongly as you can. If you've anchored with the bows into the current, the current plus the engine will help you get a good tug. If you stay put, you probably will also stay put when/if the wind gets up. The further away from the mainland you get, the more sand will be in the bottom mix. Check the chart.

If you can't get the hook to set, it's probably thin mud, or hard sand, or a weed bottom. Not the preferred anchoring conditions, so you move, and try again.

Nice varnish on your toe rail, by the way.

Ann

[troppo]

Thank you for your various replies, folks. I have been enjoying reading them and find them interesting.

Louis

[barnakiel]

If you can approach the same site twice you can just sample the bottom.

I noticed many deeper spots without current have the softest mud. It gets harder closer inshore (on the slopes). But the very shore (to about 4 feet depth can be very soft again.

We often drop the hook in the deepest part, if this is far enough from the shore.

b.

[reed1v]

If its silty mud, do not anchor. Anything else, get out the danforth and lots of scope.

[noelex 77]

Originally Posted by Ann T. Cate

troppo,

We work it out by the setting process, and to some extent, it doesn't matter. You drop your anchor, and slowly reverse, letting it pull back against the reversing lightly. You're probably using about 25 ft. of chain, and the rest rope rode, so when you've got out scope of 5"1, pull as strongly as you can

Ann has given you a great answer.

Modern anchors these days are very versatile and with a good anchor it is very rare not to hold with the application of full reverse even in more challenging substrates, especially if it oversized. The anchors that failed in the article you referenced were a CQR and a smallish Excel. In my opinion both of these options are not great in soft mud.

If the anchor does not hold, it is worth moving and trying again although as Mark has said, allowing the anchor to rest or soak does seem to make a significant difference in soft substrate.

There are some rare substrates that are just unusable, although here rock is by far the most common problem.

It is very difficult to determine the detail of the type of substrate you are anchoring in. Fish finders give some clue. Some use the information on the maps or in pilot books or cruising guides, but I have found when diving and observing the bottom these sources do not always match reality (no crocodiles here ).

If the anchor fails to hold it will hopefully have set well enough to at least bring up a bottom sample. This is the best guide. It is especially helpful if the anchor has held and set deeply. This way you get some clue as to the make up of the deeper substrate (if you plan on revisiting the anchorage in the future.)

[noelex 77]

Originally Posted by Alan Mighty

One is to recognise that most anchors, especially the 'new generation' anchors, are really optimised to work in sand (and are usually, as Jon Neeves noted in the article you cited, demonstrated and tested in clean sand).

I don't agree with this. Modern anchors have more tip force so are better at penetrating hard and weedy substrates, but the other major development has been the introduction of much larger fluke areas and concave or flat blade shapes. Both of these innovations have significantly improved the resistance of the blade and the performance in softer substrates.

A major design goal is to make anchors far more versatile. Swapping anchor designs is a pain (literally ) especially on larger boats.

In soft substrates anchors like the CQR and Delta can set quite well, but can be reasonably easily pulled through the substrate. It is fascinating to watch underwater. Sometimes just the chain can be seen slowly dragging back at other times you can see the the disturbed sand above the anchor like a burrowing animal.

[nigel1]

I've anchored in the River Mersey a few times, plenty of places with soft silty mud.
As the ebb stream can be around 5 kts in places, I found it best to keep the prop ticking over in ahead to keep the weight of the anchor and allow it to set.
Just dropping the anchor, it can skip over the silt and never soak in.
Once soaked in it sets fine, and with the tidal stream, not any real need to use astern movement on the engine

[Cadence]

MarkJ's approach with the tap,tap,tap was interesting.

[wrwakefield]

Troppo,

The bottom types vary in our anchorages- like most places- but more often than not they are soft, sticky mud- often containing glacial silt.

Because our soft mud is also extremely sticky, it may be different from what you are describing as soft mud. [e.g., We often have to wash a soccer ball sized wad of it stuck in our anchor upon retrieval. That takes a few minutes slowly making way with the anchor just under the water...]

In most of our anchorages we just assume soft mud and anchor accordingly because soft mud technique also works in sand-mud and sand; hard or soft. It just takes a little more time and finesse as described by the techniques already mentioned by other veterans in this thread.

While this isn't a discussion about anchor selection, I want to mention our main bower only because it factors in to our approach for different bottom types. Our boat came with an 80lb adjustable SuperMAX anchor. It has a 3 position adjustable shank readily accommodating hard to very soft substrates. Opened to its widest angle of attack for a very soft [ooze] bottom, it sets quickly and deeply, and is a demon to retrieve. [There are more details on our Ground Tackle Inventory page if you are interested.]

I mention this because the fluke to shank angle does absolutely make a difference from our experiments. However, just as you are asking, one must determine the bottom type to optimize the angle setting. [Most of the time we just keep it in the middle position and never fuss with it...]

Back to your question about determining bottom type: because we typically anchor in deeper water [50-90 ft] we use the anchor to sample the bottom- but only if it doesn't set as we might wish [and we always set as though it will blow 60kts] Retrieving it will usually reveal what we are up against when dealing with different types of mud as it is a great sampling device too...

In hundreds of overnights since we acquired this boat a couple of years ago, it has only failed to set on first attempt 3 times, and only once did we have to relocate due to no set in a known predominantly rocky bottom.

I hope some of this is useful for you, and wish you safe anchoring always.

Cheers!

Bill