How should I anchor?

[JumbleDuck]

I like the wet cement analogy.

It's OK but potentially misleading, since not all substrates behave that way. Similarly, anyone who thinks that chain is the only good transmitter of sound needs to experiment with some string and two empty baked bean tins.

 

[Graham376]

If Thinwater comes along he may comment. He tried in line tandem anchoring.

First off its difficult to set 2 anchors in line connected together. Secondly if the furthest anchor holds then as the rode tightens, or straightens it results in a vertical component on the inner anchor (the one nearer the yacht) and simply lifts it out. You then have one anchor holding the yacht and the other simply acting as a kellet. Finally its difficult to retrieve two anchors joined together in line.

Tandem anchors worked OK for us. The old CQR lookalike used to drag for fun but shackling the Fortress with about 3 metres of chain to the tripping eye solved it. Main problem was the weight and handling on retrieval. CQR now resides at the bottom of a locker since we bought a Manson Supreme which hasn't dragged - so far.

 

[duncan99210]

If the wind does get up to stupid speeds, would there be any advantage to using the engine to drive the boat towards the anchor? - not enough to move forward, even if you can, just to reduce swinging and relieve the strain in gusts. After all, it's not as if anyone's going to be sleeping.

I'd also have my passage plan out at the ready, even if it is only "Our Father which art in heaven..."

I’ve not done that myself but some friends who got caught anchored off a lee shore once did exactly that for several hours before things abated enough for them to actually make ground into the wind and get away. There’s a picture somewhere of breakers with their boat almost completely obscured by spray.
I’ve been caught anchored with lines ashore been hit by an onshore squall which dragged the anchor, pushing us onto the rocks astern. The time it took to start the engine (doing the 10 second glow plug thing) was amongst the most worrying minute or so of my life... Lots of things going on at the same time; wife removing snubber and starting to retrieve anchor, me starting engine, realising we would have to depart in a hurry, cutting shorelines, dodging other boats in similar circumstances. Got away and out of the chaos of other boats and the had time to do things like turn nav lights on, get instruments going etc. We followed our agreed bug out plan of heading to a known safe anchorage about 3 hours away.
As mentioned above, we do always have a “what if” plan so that if it does all go horribly wrong we don’t spend time working out top what to do: we just do it.

 

[Neeves]

How to incorporate the Leaning Tower of Pisa into the comment on snubbers

No analogy is perfect, apologies. You don't need the wet cement analogy - you can simply stand on the shoreline, just in the water, and twiddle your feet, you slowly sink into the sand, the shear strength reduces. Most of us like to anchor in sand.......

And why be critical of my analogy of hitting the stretched wire - we are not talking about sound. If you stretch a string or wire between 2 tin cans - and tap both with a hammer, the wire is a better transmitter of the attack with the hammer. So please stick to the analogy and not invent red herrings.

Why do I think you are simply bored and being obtuse.

But another is the bungy jumper.

Jumpers are asked how heavy they are and the cord used varies as the weight of the different jumpers varies.

Now imagine the young lady who weighs in at a healthy 55kg. She has her cord attached to her ankles with a 'harness'. Alongside her we have a couple of lead ingots, weighing 55kg combined and they are attached to a steel cable. They are separated so that neither has the remotest chance of interfering with the other. the ground below the lead ingots and the young lady are cleared of spectators (which is of slight concern to the young lady (the lead ingots pass no comment). The young lady jumps but the lead ingots are pushed (they are reluctant knowing they are attached to a steel cable). Strangely, or not so strangely (according to Galileo who conducted a similar test off the Leaning Tower of Pisa) they fall at the same rate. The difference is that the young lady reaches the point where the bungy is straight at the same time as the lead ingots reach the point where the cable is straight, because both tethers are the same length. They both develop exactly the same kinetic energy, same mass, same acceleration. The young lady feels very little, she slowly decelerates, the ingots break one of the connections, maybe the swage (which was rated at a number of tons).

This how your snubber and chain works.

And in answer to JD - I would hesitate to think or guess on the tension at the end of the snubber - but you posed a Douglas Adams question (most unlike you) and the answer is 78kgs. You JD are the physicist - you will explain all this in a minimum of words and formulae whereas I will need analogies (of which you will be critical) and hundreds of words (of which others will be critical). (I wonder what it is liked to be loved )..

On length of snubber. 10m would be fine, 30m is better. 2m is a waste of time (it does take the load off the windlass). As your yacht moves, through chop, yawing or gusts it develops kinetic energy. Initially your chain absorbs that energy, as it straightens. But at 6m depth and 5:1 scope your chain starts to 'look straight at about 30 knots, it has used most of its ability to straighten (and absorb more energy). You lengthen your ,rode and increase the catenary effect. You can do the same with your snubber. The life of your snubber is contingent on the number and size of the stretch cycles (that's how climbing ropes are specified). Your yacht produces the same kinetic energy - better to spread it over a longer length.

To the OP, don't buy a Mantus, it has a similar hold to a Delta/CQR/Bruce of the same weight. Have a look at the Viking, it demounts and has roughly twice the hold of a Mantus about 1/3 bigger. You could also consider a Spade, which de-mounts, or a Kobra which folds (and is cheap) or the aluminium Anchor Right Excel.

Jonathan.

 

[JumbleDuck]

No analogy is perfect, apologies. You don't need the wet cement analogy - you can simply stand on the shoreline, just in the water, and twiddle your feet, you slowly sink into the sand, the shear strength reduces. Most of us like to anchor in sand.......

That doesn't work in all sand.

And why be critical of my analogy of hitting the stretched wire - we are not talking about sound.

Your "analogy" depends on the transmission of compression waves along the chain or rope. That's sound. Try giving a good heave to one end and see how much the force at the other depends on the material. The fundamental problem with your analogy is that all the mass concerned is in the cable, whereas only 1% or so of the mass in the boat-cable system is in the cable.

They both develop exactly the same kinetic energy, same mass, same acceleration. The young lady feels very little, she slowly decelerates, the ingots break one of the connections, maybe the swage (which was rated at a number of tons).

This how your snubber and chain works.

There are two fundamental problems with that analogy. First of all, it ignores the springing and damping contributed by the movement of the boat. More seriously, it assumes that the ingots move a significant distance before being brought up short, which is very different from the dynamic behaviour of a chain.

Here's a wee thought experiment for you. Two identical boats anchored side by side. One has 40m of chain while the other has 30m chain + 20m of snubber, of which - following your recommendation - 10m is over the bow and 10m is along the side deck. The wind is blowing steadily and with sufficient strength that there is no significant catenary. Let's say 20 kts, though none of the numerical values matter much.

OK so far? Now imagine a gust of another 20 kts. The boat anchored with chain stays almost exactly where she is and the additional wind loading is transferred almost at once to the anchor. The boat anchored with a snubber experiences the same additional wind loading. So far so similar.

What do you think the force in the snubber (and therefore at the anchor) looks like after that? Hint: unless there is a remarkable amount of damping in the system, the yacht will overshoot its final position and start accelerating forward again.

 

[Neeves]

I think JD you are showing boredom and need the University year to restart so that you can probe the knowledge of your students.

Sadly JD I deal in the real world and though I have measured wind speed and peak tensions, maybe 500 pairs of readings, load cell measuring tension and masthead wind gear measuring wind speed, at different scopes I have never encountered the imaginary situation you describe. However you obviously think it a valid question and as a senior academic at one of Britain's more prestigious Universities I might suggest you share your knowledge and give us the answer. After all this is not a tutorial when you tax the minds of your better students but a vehicle to share knowledge. Knowledge and education is of little value unless it is shared - which is one of the raison d'être of a University (and forum). As well as making the aforementioned 500 pairs of tension and windspeed readings I also measured wind direction variability, I simply check direction occasionally. Wind, in many locations is not constant, the wind veers even if the yacht might normally be stable at anchor. If the wind veers - then the yacht will veer - and sometimes sail at anchor. The biggest snatch loads that I have measured do not occur as a result of gusts but as a result of a change of wind direction relative to the yacht (allowing the yacht to sail) though the veer might be accompanied by a gust (compounding the problem). The average wind direction might be constant - but not the direction. Wind shear is hardly new - its a major item of research when locating wind farms (wind farms being located where wind shear is low).

In your posed question - if the wind was a steady and a guaranteed direction you would not need a snubber. If the wind simply increased in speed, constant direction - you would not need a snubber (just an anchor that is good enough). As you have pointed out the resistance of 'water' is sufficient to cushion the movement of a yacht, especially as it will be moving back wards with a gust

Now is you chance to share instead of setting up imaginary, and to my simple mind, unrealistic questions.

My experience is that though our yacht moves back and forward as the catenary straightens and the snubber stretches this idea that it 'accelerates' might misuse the normal understanding. Yes it is stationary and then moves but its a very gentle movement and, yes., it accelerates but unless you watch - its imperceptible.. In reality you simply don't feel it - the only indication being the catenary curve changes and you can see the snubber stretching - sit down with a glass of wine and it is simply undetectable. The reality is that the movement derived from the catenary is no different to the movement derived from the snubber the big difference is that beyond 30 knots the catenary no longer exists and the poor old snubber needs to do all the work by itself.

Maybe if you used a snubber you would be better able to understand its advantages. I do find it interesting the number of people who are experts on topics the practical use of which they have never tried (very common in anchor threads). Its amazing, but not extraordinary, the people who can condemn an anchor - they have never used.

I might add you obviously don't like the idea of using a snubber - maybe you can expand your thinking.

A couple of things for you to try. Dive on your anchor when the wind is 25 knots and all the rode is off the seabed. Similarly dive on your anchor when the wind is 35 knots and the rode is all off the seabed, preferably the same seabed same rode, same scope. Touch your anchor. You will find it twitches and twitches, continuously, with increased wind speed. Now tell me why your anchor is twitching if the 'engine' for the twitching is not the connection between yacht and anchor, the rode. If you want to extend this a bit further - now introduce a snubber, a decent length, and you will find the twitching is reduced.

Now I'd like to quantify the twitching but I confess I have been defeated. You need to do it yourself. If you can think of a cheap way of measuring the effect and the effect this twitching might have on the seabed - I'd love you to share. If you have a decent modern anchor and it is completely buried and some chain buries with it you will find buried chain reduces twitching. To measure the twitching of a buried anchor take a screw driver with you and prod about till you find the anchor and 'measure' or feel the twitching 'through' the screw driver.

As a theoretical physicist possibly you can hypothesise what impact a twitching anchor might have on the seabed in which it is embedded. As you obviously know so much about shear strength seabed variability maybe you can share the background as seabeds that have easily altered shear strengths are ones we all want to avoid.

Now there has been work on shear strength modification and its reduction of holding capacity but this has been related to earthquakes and though earthquakes might be the result of the twitching of anchors I have touched - I doubt it. I simplistically relate twitching to the movement of the yacht and the connection between yacht and anchor. Shear strength reduction is a major aspect of anchor research as so many items rely on anchor integrity.

Jonathan

 

[Adios]

What would be the recommended anchoring strategy in this situation?

Rather than dividing up the resources, eg having all the pull on one anchor and then lay half the other available chain and another anchor off at another angle not taking strain unless the boat swings that way and then expecting that weaker anchor to cope on its own, and have another anchor off in another direction in case it swings in that direction etc I think it makes more sense to trust in your main anchor's ability to reset as designed if the direction changes and make use of all the available weight in one system with all of it working all of the time.

I'd use the Manson and just its 50m of chain. Then I'd coil the other chain up and lower it by rope down the chain to get all its weight just off the seabed as a chum weight. Not sure of the exact physics but I imagine the benefit of that chains weight is as good if not better all in that lowest possible position than paid out as potentially problematic extra scope in a confined space.

And then one at a time after and bit less deep to avoid tangling with the coiled chain lower the other anchors as extra chum weights. Then all the anchoring gear is working all the time. If that fails I can't see how having less down there would have been better or how keeping one of the smaller anchors and a little chain as a spare on deck would save the day.

 

[tudorsailor]

My favourite way of anchoring is to drop the main anchor at the bow and run a line not ashore but on the bottom of the sea (chain around a rock and line to the boat). But this only when possible. I find it safe and I also don’t like the boat swinging around the anchor.

With a line ashore, what happens when the wind moves to be on the beam. Anchors resist inline pull well but not sideways. If the wind gets up isn't it more likely that your anchor will come unstuck if you have a line ashore than if you are swinging? If you anchor does loosen, it wont reset.

Of course I realise that a line ashore is preferable when there is not much room for swinging, but surely only when winds are light? I would point out that I too regularly sail in Greece. Taking a line ashore is not my favourite activity, but I do it ery occasionally!

Tudodrsailor

 

[RupertW]

With a line ashore, what happens when the wind moves to be on the beam. Anchors resist inline pull well but not sideways. If the wind gets up isn't it more likely that your anchor will come unstuck if you have a line ashore than if you are swinging? If you anchor does loosen, it wont reset.

Of course I realise that a line ashore is preferable when there is not much room for swinging, but surely only when winds are light? I would point out that I too regularly sail in Greece. Taking a line ashore is not my favourite activity, but I do it ery occasionally!

Tudodrsailor

Obviously when the wind is coming from the shore the line ashore is the perfect option with the tensioned anchor keeping the angle working, but even when it is one the beam you have one strong point ashore (usually two angled apart maybe 30 degrees) and the anchor pulling in the way it’s set with only a small angle and very little movement compared to veering about all over the place with snatch loads at different angles if just on the anchor. I have certainly been held solidly in 40 knot gusts on the beam tied that way.

 

[JumbleDuck]

As a theoretical physicist possibly you can hypothesise what impact a twitching anchor might have on the seabed in which it is embedded. As you obviously know so much about shear strength seabed variability maybe you can share the background as seabeds that have easily altered shear strengths are ones we all want to avoid.

I'm a mathematical engineer, not a theoretical physicist. A twitching anchor in a shear-thinning seabed will dig itself in more, unless the concept of "he struggled and rapidly freed himself from the quicksand" is a common one.

I might add you obviously don't like the idea of using a snubber - maybe you can expand your thinking.

I have nothing against them and I keep saying that I am sure there are boats for which they are useful. Heavy catamarans with lots of windage and plumb stems for example. I don't bother because my boat doesn't snatch. Ever. I think that's because she has a long keel and curved stem. She has high bows so she sails backwards and forwards something chronic in wind between 15 and 15 knots, ish, but even then she doesn't snatch at the ends, just dips her bows a graceful few inches as she absorbs the increasing force. She does, or did, make a bit of a noise as the chain moved from one side of the bow roller to the other, but that was solved with a chain hook and a couple of metres of rope which I use through a bow fairlead to hold the chain to one side.

I admire your messianic devotion to snubbers, as I admired your messianic devotions in turn to new generation anchors, to exotic grades of chain and to esoteric galvanising processes. Good luck.

 

[Neeves]

I'm a mathematical engineer, not a theoretical physicist. A twitching anchor in a shear-thinning seabed will dig itself in more, unless the concept of "he struggled and rapidly freed himself from the quicksand" is a common one.

I admire your messianic devotion to snubbers, as I admired your messianic devotions in turn to new generation anchors, to exotic grades of chain and to esoteric galvanising processes. Good luck.

It is interesting that the account of the Med storm to which I linked above was of a yacht suffering all sorts of calamities and terror. One of the most remarkable facts I gleaned was that the anchor being used was a CQR - about which not a complaint was made. In fact - it saved the day.

My devotion to snubbers, new gen anchors, exotic grades of chain and old fashioned galvanising (Sheradizing is hardly new) is only based on personal usage - which I have no hesitation in sharing, Strangely a number of people share my enthusiasm, particularly for new gen anchors - and one reason for my enthusiasm is that they are, largely, idiot proof and do not require decades of practice to use successfully. Neophytes like me can thus anchor like an old salt.

Jonathan

 

[JumbleDuck]

My devotion to snubbers, new gen anchors, exotic grades of chain and old fashioned galvanising (Sheradizing is hardly new) is only based on personal usage - which I have no hesitation in sharing

And more power to your elbow for doing so. The rest of us may have different experiences, of course. I think it interesting that the forum's three most enthusiastic proponents of snubbers all seem to have big catamarans.

 

[Neeves]

Obviously when the wind is coming from the shore the line ashore is the perfect option with the tensioned anchor keeping the angle working, but even when it is one the beam you have one strong point ashore (usually two angled apart maybe 30 degrees) and the anchor pulling in the way it’s set with only a small angle and very little movement compared to veering about all over the place with snatch loads at different angles if just on the anchor. I have certainly been held solidly in 40 knot gusts on the beam tied that way.

We often use a shore line (and 2 anchors in a 'V', or two shore lines in a 'V' and one anchor) to minimise yawing (and the inevitable snatches) as it quells the snatches and importantly allows more yachts to use tight anchorages. Its a sort of Med moor using nature to supply the strong points. We only draw 1m - its easier for us. We carry short lengths of chain to put round rocks and big strops, spliced at both ends for trees - for precisely this purpose. If you are not being blown from pillar to post by bullets and williwaws the tension on the rode is very much reduced as the yacht has no momentum (and you don't need a snubber . You do need long lines or be able to cobble together a long line.

In the linked article on the Med storm the authors did mention they could not get close enough to shore to effect this solution, the implication was - they had thought of it.

Jonathan

 

[Neeves]

And more power to your elbow for doing so. The rest of us may have different experiences, of course. I think it interesting that the forum's three most enthusiastic proponents of snubbers all seem to have big catamarans.

There is a danger that one (I am very guilty) forgets that experiences are not necessarily shared and that importantly people here have vastly different yachts from traditional yachts, long keeled, heavy displacement (long or short) to, maybe more common, lightweight plastic fantastics and what is ideal for some is heresy (sticking with the religious theme) for others.

I do detect, with some satisfaction, that the numbers of true believers in the religion of elasticity is growing and has been adopted even by some seasoned long distance monohull owners. I do like it when I see some quoting some of my stock phrases as their own . Sadly the religion of the exotic, lightweight chain is restricted to Oz and I am but a minor disciple in the other religion - the new gen anchor.

Jonathan

 

[Adios]

One of the most remarkable facts I gleaned was that the anchor being used was a CQR - about which not a complaint was made. In fact - it saved the day.

I think a lot of the bad opinions of CQRs are from people who have used a copy. It put me off until i a used a genuine one and found it significantly better. As it not even setting to at least it sets. They looked the same but must be different enough to matter. I still wouldn't carry one though, better options available.

 

[Adios]

I do detect, with some satisfaction, that the numbers of true believers in the religion of elasticity is growing and has been adopted

Seems like a nice addition to reducing the shock-loads and can be added on an already set anchor if conditions change. But the same goes for a chum weight which does about the same job and saves buying something else with a single use, as I already have a spare anchor to hang off it.

 

[RupertW]

Seems like a nice addition to reducing the shock-loads and can be added on an already set anchor if conditions change. But the same goes for a chum weight which does about the same job and saves buying something else with a single use, as I already have a spare anchor to hang off it.

No, the chum will be pretty hopeless in a breeze unless it weighs a vast amount - the chain will simply straighten pulling it up and you will have no tension.

 

[RupertW]

We often use a shore line (and 2 anchors in a 'V', or two shore lines in a 'V' and one anchor) to minimise yawing (and the inevitable snatches) as it quells the snatches and importantly allows more yachts to use tight anchorages. Its a sort of Med moor using nature to supply the strong points. We only draw 1m - its easier for us. We carry short lengths of chain to put round rocks and big strops, spliced at both ends for trees - for precisely this purpose. If you are not being blown from pillar to post by bullets and williwaws the tension on the rode is very much reduced as the yacht has no momentum (and you don't need a snubber . You do need long lines or be able to cobble together a long line.

In the linked article on the Med storm the authors did mention they could not get close enough to shore to effect this solution, the implication was - they had thought of it.

Jonathan

I bought 200m floating line which I deploy out of two separate bags - usually swimming to the shore with the upwind line first then back for the next one. I tend to use old warps as sacrificial attachments to rocks or trees with the lines looped around them. We don’t use them often enough to worry about chafe too much and polypropylene is very cheap to replace if it did happen to one of the two stern double lines one night.

 

[Adios]

No, the chum will be pretty hopeless in a breeze unless it weighs a vast amount - the chain will simply straighten pulling it up and you will have no tension.

In the OPs example we can add 35kgs of available anchors that wouldn't be doing anything else plus 40% of the chain's weight in additional chain (assuming same size chain), if 10mm that's probably another 46kg. Thats taking the weight of chain from 116kg to 196kg or equivalent of going from 50 meters of 2.3kg per meter 10mm chain to 50m of 3.9kg per meter 13mm chain. Calibrated Anchor Chain - Break Load and Weight Guide - Jimmy Green Marine

You really think that's insignificant? Nearly doubling the weight of the chain for free seems good to me. In fact the weight will be more effective (require more pull to lift it) the closer it is to the middle of the scope so it is probably in effect better than doubling the weight of the chain. It seems to me the best use of resources in that scenario.

The elastic on a snubber looses its benefit once it's stretched as well but reduces the shock getting there, same as a heavier chain or chum weight. But does it have any benefit in maintaining a low angle of pull on the anchor? I do wonder if the chum weight has the advantage of working all the time to keep the chain low while the snubber doesn't start working until the chain is straightening up and only reduces shock rather than helping keep the chain low.

For modest sized boats I'm thinking of, a very large boat with already heavy gear would need more weights to be significant and it gets less practical. For very small boats without a chain locker (on a muddy seabed not likely to cut a rope) its all I'd use, anchor, rope and chum. The token amount of chain some people use close to the anchor doesn't do much, a chum with all the weight closer to the middle of the scope is better in my experience and brings less mud into the boat.

 

[RupertW]

In the OPs example we can add 35kgs of available anchors that wouldn't be doing anything else plus 40% of the chain's weight in additional chain (assuming same size chain), if 10mm that's probably another 46kg. Thats taking the weight of chain from 116kg to 196kg or equivalent of going from 50 meters of 2.3kg per meter 10mm chain to 50m of 3.9kg per meter 13mm chain. Calibrated Anchor Chain - Break Load and Weight Guide - Jimmy Green Marine

You really think that's insignificant? Nearly doubling the weight of the chain for free seems good to me. In fact the weight will be more effective (require more pull to lift it) the closer it is to the middle of the scope so it is probably in effect better than doubling the weight of the chain. It seems to me the best use of resources in that scenario.

The elastic on a snubber looses its benefit once it's stretched as well but reduces the shock getting there, same as a heavier chain or chum weight. But does it have any benefit in maintaining a low angle of pull on the anchor? I do wonder if the chum weight has the advantage of working all the time to keep the chain low while the snubber doesn't start working until the chain is straightening up and only reduces shock rather than helping keep the chain low.

For modest sized boats I'm thinking of, a very large boat with already heavy gear would need more weights to be significant and it gets less practical. For very small boats without a chain locker (on a muddy seabed not likely to cut a rope) its all I'd use, anchor, rope and chum. The token amount of chain some people use close to the anchor doesn't do much, a chum with all the weight closer to the middle of the scope is better in my experience and brings less mud into the boat.

35kg won’t produce much curve above 20 knots wind so yes, pretty insignificant. A snubber can be stretched quite a few time before losing it’s elasticity but I always think they are worth replacing after a serious prolonged blow.