The great anchor debate - whats are take

[jimbaerselman]

Re: Two anchors

Attach the nearer anchor by its tripping eye, some 3 or 4 m up the rode. Don't use it as an anchor, use it as a weight. This technique has value only when you have very little swinging room and therefore can't lower the amount of chain you'd like to. It helps hold the anchor chain horizontal at the bottom end. It's just a really big 'chum'.

 

[roly_voya]

Re: Two anchors

As I understand it the idea of tandom anchors is that both will bed in but probably not fully set when laid. When they are fully loaded the will bed in and 'creep' foward untill they are sharing the load as the higest loaded one will always be the one to move most/first. For it to work the anchors need to be of a design where the do travel a little as they set (I'm talking feet not yards here) but must not travel far enough for the back one to come forward to where the first one started - I.e if you have a 5m link between them the total travel from drop to fully set must be less than 5m. It is important that any movement is part of the way the ancor sets and not dragging. It may be that this only works on some anchors and I havent yet had a good look the result exept that it has always held, but then one anchor on its own may have. I intend to conduct some tests nex spring by anchoring on a bank which dries.

The alternative of setting two anchors in a 'V' definatly cannot work. If you dont believe me try a simple test. Lay to two anchors with enough breeze to tighten the cables and get the boat shearing a bit. Put your boot on one cable and you will find that it alternatly loads and slackens as the boat moves. You are in fact laying to each anchor alternatly not ot both. It does have benifits (reduced shearing, backup if one breaks out etc) but it will not get an anchor to hold if the load is too high for it.

The obviouse answer is a single big anchor but I could neither stow it or handle it.
The other answer is a higher hold anchor of the same wt.

 

[craigsmith]

Re: Two anchors

[ QUOTE ]
When using tandem anchoring, they are a lot of chances that the most distal anchor will fall into the trench left by the first one..
and this is enough to explain why two tandem set anchors doesn't have twice the holding of one single anchor..

[/ QUOTE ]This is a bit of a false logic.

1) Frequently when an anchor falls into the trench of another, it sets better, not worse, as it sinks deeper through the softer ploughed material and into harder stuff

2) If both anchors drag so far that the tandem falls into the trench of the primary, you have a problem in any case.

 

[craigsmith]

Re: Two anchors

[ QUOTE ]
Craig
You say some of the maths in Alain Frayssee site is unsound. Would you dispute the idea that, in practice with sensible roads laid, during wind gust the anchor load rises by a factor of about 2x the increase in loading from lull to gust. This seems to be the determenting factor that causes anchors to break out and start a drag. If you do what sort of factor would you allow?

[/ QUOTE ]A 2x increase sounds reasonable, but it's entirely impossible to predict. It depends on the gusts, how long they last for, what rode is being used, etc. As I said, Fraysee manages to draw appropriate conclusions, they're just a bit simplistic.

[ QUOTE ]
Some interesting comments on tendom anchors and why they may not work, makes sense so food for thought. Those that said they found it didn't work what sort of gap wher you using between anchors? wonder if this makes a difference? Obviously if they where directly shackled together the back one would be in the hole dug by the front one. Alternitively if they where 20m apart this would seem unlikely.

The techneque I have used is to have about the water depth seperating anchors, allow boat to drift slowly back and lower first one, give it a check to get it to bite before the second one lands, then continue paying out to about 3x scope. Set and check holding then pay out to 7-10x scope. If there ain't room go elsware. Never had the chance to dive and view the effect but will definatly do that now.

[/ QUOTE ]Tandems are difficult to set, problematic, and poorly understood, hence the comments from the likes of Hylas. Their use has its place but only in extreme conditions and you should understand what you're doing.

The advice is usually to place a gap of a boat length between the two anchors. You must use chain, not rope, so as to maximize the rigidity of the system. One is compelled to reduce the distance between the two, since the less the gap, the better the scope for the primary. However, there should be a safe distance between them so as to minimize the chances of one fouling the other.

You should also dump both anchors on the seabed then set the primary, rather than setting the tandem first as you describe. This is because the set tandem can interfere with the setting of the primary. You may find that your primary is not setting at all, and just acting like an expensive kellet.

Other bits and pieces:

If you want the system to handle double the load, you must use double the scope. This is because the vertical force on the primary is doubled so must be halved again.

You should use two anchors of the same type. The same size is not necessary, but if there is a difference, the larger anchor should be the primary (aft most).

Realize that two small anchors are not the same as one large anchor. In fact two 10Kg anchors have more fluke area combined than one 20Kg anchor, so may hold more. However, the 20Kg is physically larger and may be able to get its tip down into harder material under the seabed. Which option is superior will depend on the seabed.

You should shackle the rode of the tandem to the rode of the primary, rather than directly to the front of the primary - unless you have a Rocna. It sounds partisan but it is true - we are unaware of any other anchor that has a specific tandem anchor attachment point that is in the correct location, designed and tested. The points that are commonly used on other anchors are for retrieval lines, not tandems, and are in the wrong location. Put enough force on a tandem rig using these points, and the primary will twist out (or you'll have other problems; the CQR's point for example is known to be quite weak).

5:1 scope, layer of soft mud with shell over hard clay.

There is a brief video demonstration here.

 

[jimbaerselman]

Re: Two anchors

[ QUOTE ]
The alternative of setting two anchors in a 'V' definatly cannot work. If you dont believe me try a simple test. Lay to two anchors with enough breeze to tighten the cables and get the boat shearing a bit. Put your boot on one cable and you will find that it alternatly loads and slackens as the boat moves. You are in fact laying to each anchor alternatly not ot both. It does have benifits (reduced shearing, backup if one breaks out etc) but it will not get an anchor to hold if the load is too high for it.


[/ QUOTE ]
A few points here.

1. 'V' anchoring either works; or the majority of Caribbean sailors (used to anchoring in 25kt winds and low currents) are using a worthless technique. I think it works. Why?

2. Your experiment shows that the load alternates between the anchors. I agree. You accept that sheering/sailing/yawing takes place but is reduced. I agree.

3. My contention: most anchor trips in strong winds are when older anchors (flat, CQR, whatever) are pulled with sideways snatch loads at the anchor, in combination with the higher load of the snatch, as the vessel sheers. (I don't have sufficient experience with modern anchors to comment on whether these may have similar reduction in holding).

4. Since 'V' anchoring reduces the degree of sheering, and therefore the load when the vessel tacks, this is a source of comfort.

5. With V anchoring, the anchor which comes tight to cause the sheer is aligned with its rode, so doesn't have any sideways snatch to reduce its holding power. A second source of comfort.

Do these points change your mind?

 

[geem]

Re: The great anchor debate - the critical missing factor

When in the uk prior to sailing across the pond in 04, I never used two anchors. We are lucky in the uk that holding is relatively good. Once you get to the Caribbean its a whole different storey. Tide is not a real issue but holding power is. We destroyed an aluminium spade anchor in the Canaries when it hooked under a rock in very light winds and the gentle bobing of our boat was enough to split the anchor all the way down the shaft! The manufacturer has since replaced this anchor free of charge with the steeel version. We picked up a very cheap 20kg Bruce copy in the Canaries that became our main anchor whilst in the Caribbean. We genarally set this anchor with 5 to 7 times the depth of water as a minimum but also added an additional aluminium anchor on a short length of chain and then warp. This was shackled to the main chain 10 feet down from our bows. This reduced the wrap should we swing through 360 degrees, which we didnt often. THis system worked extremely well for us in very poor holding on occations and reduced our swinging. We had the anchors set in a y pattern pointing into the prevailing wind. In gusty conditions it reduced the snatch considerably and proved more effective than a 30 kg chum. We were shown this method of anchoring by a friend from the Bahamas who sailed with 10 anchors and only a small outboard on a 16 ton 42 ft monohull. He was used to weathering huricanes( where he used his 10 anchors) so had huge anchoring experience. Interestingly, his anchors were mainly large fisherman ( the biggest being 50 kg) or Bruce. He could not afford to drag as his outboard was not capable of pushing him against strong winds.
We saw lots of British yachts on various anchorages in the Caribbean having anchoring/dragging problems. The anchor in use was invariably a CQR. The German boats we met generally used Bugle anchors. These always seemed to set well. We never saw one drag and all the guys we spoke to who had them spoke highly of them. I might well try one in the future. Anyone one want a free CQR? got one in my shed

 

[craigsmith]

Re: Two anchors

[ QUOTE ]
1. 'V' anchoring either works; or the majority of Caribbean sailors (used to anchoring in 25kt winds and low currents) are using a worthless technique. I think it works. Why?

[/ QUOTE ]It does work, and has its place, and so isn't worthless by any means, but in terms of pure holding power for handling extreme conditions, a tandem rig has advantages for the reasons given. I think the thread starter is interested in this context, rather than what Bahamian mooring is all about.

I.e.: I am in an isolated anchorage, fair holding, few other boats around, my boat needs to ride out a major storm, how shall I deploy the anchors.

[ QUOTE ]
2. Your experiment shows that the load alternates between the anchors. I agree. You accept that sheering/sailing/yawing takes place but is reduced. I agree.

3. My contention: most anchor trips in strong winds are when older anchors (flat, CQR, whatever) are pulled with sideways snatch loads at the anchor, in combination with the higher load of the snatch, as the vessel sheers. (I don't have sufficient experience with modern anchors to comment on whether these may have similar reduction in holding).

4. Since 'V' anchoring reduces the degree of sheering, and therefore the load when the vessel tacks, this is a source of comfort.

5. With V anchoring, the anchor which comes tight to cause the sheer is aligned with its rode, so doesn't have any sideways snatch to reduce its holding power. A second source of comfort.

[/ QUOTE ]This may indeed win the argument depending on the scenario, as above. I would argue that modern anchors are quite capable of handling snatching veering loads, even if the primary anchor is dragging around a bit between the boat and the tandem, but that doesn't invalidate your points.

There is however a basic fundamental logic involved with tandem anchoring. From the outset you are assuming that a single anchor on its own is inadequate, in that it will drag, even if only in bursts. Therefore you need a second. Placing it in a V, where the load may oscillate from one anchor to the other, will just "walk" the anchors backward, as each acts on its own, under these preconditions.

Some further advantages of tandems:

- The "walking" argument notwithstanding, you get superior performance out of both anchors as they are both on the same rode, and the pull on the tandem is always parallel to the seabed.

- The ability to deal with permanent veers, in that the primary should just drag around until it is lined up between the boat and the tandem again. With a V set-up, you might end up with a real mess, with the anchors either so close together as to have lost any advantages to do with dealing with lateral forces, or even fouling each other.

 

[jimbaerselman]

Re: Tandem anchors

[ QUOTE ]
However, in most other similar situations it is a fact that where two similar items are placed in series to 'double up' their effectiveness they don't do it.

[/ QUOTE ]
Agreed. Lets take this a bit further though.

1. If your rode is correctly matched to your bower anchor, it will cope with the maximum holding the anchor can offer, plus something to spare (double?).

2. If you increase the holding power of the stuff at the end of the rode, you're eating up the safety margin of the rode, so you should either increase the rode strength or reduce the pull from the boat . . .

3. If you reduce the pull from the boat, you don't need tandems . . .

OK, that's tongue in cheek, but the point is simple. The hook is only one part of the equation - deck gear, rode and winches etc must all be considered, together with the crew's ingenuity.

 

[craigsmith]

Re: The great anchor debate - the critical missing factor

[ QUOTE ]
We had the anchors set in a y pattern pointing into the prevailing wind. In gusty conditions it reduced the snatch considerably and proved more effective than a 30 kg chum. We were shown this method of anchoring by a friend from the Bahamas who sailed with 10 anchors and only a small outboard on a 16 ton 42 ft monohull. He was used to weathering huricanes( where he used his 10 anchors) so had huge anchoring experience. Interestingly, his anchors were mainly large fisherman ( the biggest being 50 kg) or Bruce. He could not afford to drag as his outboard was not capable of pushing him against strong winds.

[/ QUOTE ]With all due respect to your friend, I would suggest that if you show me someone who has so many different anchors, I will show you someone who has very little confidence in any of them, and is trying to compensate for this. While dumping 10 different anchors out might work, it isn't to say there aren't far easier and more efficient ways of achieving the same thing.

[ QUOTE ]
We saw lots of British yachts on various anchorages in the Caribbean having anchoring/dragging problems. The anchor in use was invariably a CQR. The German boats we met generally used Bugle anchors. These always seemed to set well. We never saw one drag and all the guys we spoke to who had them spoke highly of them. I might well try one in the future. Anyone one want a free CQR? got one in my shed

[/ QUOTE ]A common tale, and one that many UK sailors should take note of before defending their CQRs as religiously as they seem to. The recent SAIL / Yachting Monthly anchor testing (see our website) will no doubt interest you, as it is on hardish sand and the results will be of no surprise (the Buegel and Spade are tested also).

 

[craigsmith]

Re: Tandem anchors

[ QUOTE ]
OK, that's tongue in cheek, but the point is simple. The hook is only one part of the equation - deck gear, rode and winches etc must all be considered, together with the crew's ingenuity.

[/ QUOTE ]Certainly. It should go without saying. I touched on one factor above, in saying that the scope should be doubled along with expected performance. Otherwise don't expect doubled performance /forums/images/graemlins/tongue.gif . . . There are many others, including the strength of all equipment.

We have a note on our User's Guide under the "tandem anchoring" section that says something along the lines of "for extreme adventurers only"! /forums/images/graemlins/crazy.gif

 

[jimbaerselman]

Re: Two anchors

[ QUOTE ]
From the outset you are assuming that a single anchor on its own is inadequate, in that it will drag, even if only in bursts. Therefore you need a second. Placing it in a V, where the load may oscillate from one anchor to the other, will just "walk" the anchors backward, as each acts on its own, under these preconditions.


[/ QUOTE ]
Craig, I think the quote above has missed a couple of my points, perhaps because we start from different assumptions.

There are, of course, a selection of answers to dragging in these circumstances. Use a heavier (or bigger) anchor of the same design. Use a more efficient anchor of the same weight. Reduce the stresses.

I assume the design case for anchoring in strong winds, especially in short keel sailboats, is the dynamic forces exerted due to the boat sheering. I believe (perhaps wrongly) that the maximum force comes when the boat is brought up short and 'tacked' by the anchor. I'm ignoring wave effects here (which add a nasty short period cyclic surge on top of the sheer - which can be considerably reduced by a suitable length of nylon snubber).

If that assumption is right, reducing the sheer reduces the chance of an anchor being pulled out (by reducing peak forces). And you seem to have missed my point that with V anchoring, the snatch stress is aligned with the anchor doing the snatching, thus reducing the disturbing factors. Thus I'm stating that V anchoring, with any type of anchor, reduces the maximum forces applied at the anchor.

 

[hylass]

Re: Tandem anchors

<span style="color:blue"> Tandems are difficult to set, problematic, </span>
Already the first good reason for not using it..

<span style="color:blue"> and poorly understood, hence the comments from the likes of Hylas. </span>

Do you mean Craig that I’m talking about things that I don’t know??

- Jean Louis Goldsmith was the “technical Manager” of the “Glenans” the most French famous sailing school.. do you mean he doesn’t have enough knowledge?

There is one subject that you didn’t address: What happens to your tandem set anchors with a turn of wind or current??


<span style="color:blue"> Their use has its place but only in extreme conditions and you should understand what you're doing. </span>
Or use them when it is your last chance and only if you have a great experience of it


<span style="color:blue"> to minimize the chances of one fouling the other.</span> There is a risk??

<span style="color:blue"> You should also dump both anchors on the seabed then set the primary, </span> Very simple (in extreme conditions)

<span style="color:blue"> the set tandem can interfere with the setting of the primary. You may find that your primary is not setting at all, and just acting like an expensive kellet.</span>
Or with different words, it doesn’t work!



<span style="color:blue">If you want the system to handle double the load, you must use double the scope. </span> Not related to tandem set technique - valid also with a single anchor

<span style="color:blue"> Realize that two small anchors are not the same as one large anchor. In fact two 10Kg anchors have more fluke area combined than one 20Kg anchor, </span> but , tandem set, they have less holding

<span style="color:blue"> - we are unaware of any other anchor that has a specific tandem anchor attachment point that is in the correct location, designed and tested. The points that are commonly used on other anchors are for retrieval lines, not tandems, and are in the wrong location. Put enough force on a tandem rig using these points, and the primary will twist out (or you'll have other problems; the CQR's point for example is known to be quite weak). </span> Right..

One more point regarding the photos and the video.. Quite easy when you can perfectly put the anchors, flat on the beach.. but what in real situation?? ( <span style="color:red"> another good mean to send poeple looking at your WEB page! </span> )

 

[craigsmith]

Re: Two anchors

[ QUOTE ]
There are, of course, a selection of answers to dragging in these circumstances. Use a heavier (or bigger) anchor of the same design. Use a more efficient anchor of the same weight. Reduce the stresses.

[/ QUOTE ]Yes but I am trying to stay within the restraints of the thread starter's context.

[ QUOTE ]
I assume the design case for anchoring in strong winds, especially in short keel sailboats, is the dynamic forces exerted due to the boat sheering. I believe (perhaps wrongly) that the maximum force comes when the boat is brought up short and 'tacked' by the anchor.

[/ QUOTE ]Clearly that will be amongst the higher peaks of force placed on the anchor, and since it will be on something of an angle, dealing with it is critical. I agree. To my knowledge there isn't much in the way of data on this topic. I would like some time to install a loadmeter on several boats' rodes, and conduct something of a study on the results.

[ QUOTE ]
If that assumption is right, reducing the sheer reduces the chance of an anchor being pulled out (by reducing peak forces). And you seem to have missed my point that with V anchoring, the snatch stress is aligned with the anchor doing the snatching, thus reducing the disturbing factors. Thus I'm stating that V anchoring, with any type of anchor, reduces the maximum forces applied at the anchor.

[/ QUOTE ]Okay. But, presuming that force is applied to only one of the two anchors, the maximum force applied to that anchor must be half that of the maximum force applied to the same anchors in a tandem rig. All other factors notwithstanding. Or, if one assumes that the V set-up handles the angled pull a bit better, something a bit over half. I am not sure that dealing with the veering problem in this manner could ever come close to halving the figures, even on a light weight power boat on a bungee cord /forums/images/graemlins/tongue.gif - but I don't have any data to conclude one way or the other.

The real solution: instead of 2 x 10Kg in V or tandem, rather 4 x 5Kg, 2 tandems in a V /forums/images/graemlins/grin.gif

 

[jimbaerselman]

Re:Tandem anchors!

[ QUOTE ]
If you want the system to handle double the load, you must use double the scope. This is because the vertical force on the primary is doubled so must be halved again.


[/ QUOTE ]

I can't believe you said that! Do your diagrams again!

If I was to accept your contention that tandem anchors could (potentially) double holding power:

Your scope at most only needs to be increased by the distance between the two anchors, and if you're attaching anchors the way your diagrams show, no scope increase (to the nearer anchor) is called for.

Mind you, if you were to say that doubling the chain diameter to cope with the doubled forces was in order, I may go along with you

But frankly, tandem anchors are, for me, just an 'expensive form of kellet.' I'm beginning to suspect you haven't tried using this form of anchoring in demanding conditions . . .

JimB

 

[craigsmith]

Re:Tandem anchors!

[ QUOTE ]
I can't believe you said that! Do your diagrams again!

If I was to accept your contention that tandem anchors could (potentially) double holding power:

Your scope at most only needs to be increased by the distance between the two anchors, and if you're attaching anchors the way your diagrams show, no scope increase (to the nearer anchor) is called for.

[/ QUOTE ]If the assumption is that the load is doubled, the primary anchor's rode (the main rode) bears 100% of that load. If the anchors are equally sized - assume so for the sake of simplicity - the theory says that 50% of the load will be transferred to the tandem. Right?

No. 50% of the horizontal load is transferred to the tandem - 100% of the vertical (upward) force remains with the primary. Since this is "double", one must compensate with the rode. Either double the benefit of the catenary, or, assuming the chain grade is a static factor, compensate with geometry, i.e. scope.

I say "must", in the context of "doubling" performance. Otherwise the performance of the primary, when considered on its own, is actually lessened. (Usually this will be a practical compromise, and is one reason why the heavier anchor should be used as the primary if the two are unequal sizes.)

Let me elaborate. It is more obvious to see my point if one considers what happens when the tandem anchor is attached to the rode of the primary:

a is theta greater than b, and therefore the effective scope to the primary is lessened. The same thing happens with the rode attached to the front of the anchor, but the diagram becomes a little less clear.

 

[jimbaerselman]

Re: Two anchors

[ QUOTE ]
There are, of course, a selection of answers to dragging in these circumstances. Use a heavier (or bigger) anchor of the same design. Use a more efficient anchor of the same weight. Reduce the stresses

Your reply: Yes but I am trying to stay within the restraints of the thread starter's context.


[/ QUOTE ]

Or someone's mixing up threads and topics -
[ QUOTE ]
To my knowledge there isn't much in the way of data on this topic <peak loads while sheering>. I would like some time to install a loadmeter on several boats' rodes, and conduct something of a study on the results.


[/ QUOTE ]
Personally, I've got a lot of data. Once dug in (properly), with a single anchor, all my drags have been initiated when the wind got up, the boat started 'tacking', and on one of the tacks the bow continued to pay off. I've been there, done that in the Greek meltemi and in Caribbean nasties several times.

As a comparison, same bottoms, same single anchor, same boat, if tacking has been prevented with a shore line, I've sat secure without dragging.

I now watch as other vessels slide by at night, repeating the experiment, while I sit happily with my two anchors out.

Several replies have already added more data . . .

Do your experiment. I expect it will confirm what (to many of us) is obvious.

I won't comment on your muddled argument about tandem anchoring. It was obviously meant to be either a joke, or a pitch for someone's special anchor . . .

JimB
JimB

 

[jimbaerselman]

Re:Tandem anchors!

Your point about the geometry of load bearing tandem anchors is accepted. Sorry about my rant.

However, the thought of having to double my rode length writes this solution right out of my book, quite apart from the considerations of overloading a single rode and its equipment.

A problem I often have to deal with is a crowded anchorage with a lot of moorings around the place, when I'm forced to anchor in depths which are greater than comfortable and there's little scope for swinging. Hence my going on about using tandem anchoring solely as a 'chum'. It's a poor means of making a pull on a short scope more horizontal.

I can't see the utility of tandem anchoring as you describe it.

JimB

 

[geem]

Re: The great anchor debate - the critical missing factor

I dont believe you are correct regarding the lack of confidence. I suspect the reason why he used fisherman anchors predominantly was because good hurricane holes are generally in mangroves. The fisherman anchors would be set to hook into the mangrove roots. Also, the fisherman anchors could be folded and stored in the bilges.
So what would your technique be for weathering a hurricane? Have you any experience of this? Most of the people I spoke to who suvived hurricane Ivan in Grenada were using multiple anchor solutions during the hurricane and came through generally unscathed. I suspect most experienced cruisers. (I dont mean people who just sail in UK waters) would agree that multiple anchors are necessary for confidence and reliability.

 

[geem]

Re:Tandem anchors!

I agree. Anchors set in a V worked very well for me in 60 knot gusts in Cape Verdes 2 years ago. The snatch effect knocked us off our feet when the chain came up tight. We were in a 7 ton catamaran. Our friends were similarly set up in their 16 ton mono about 100 metres away. We both rode out a nasty storm like this. I would do the same again knowing it was an effective way of increasing the holding power. With the second anchor on rope not chain.

 

[Phoenix of Hamble]

Re:Tandem anchors!

Great thread this guys.... very informative and thought provoking... ta!