Anchors joined in tandem

[peasea]

I have seen occasional references to laying a pair of anchors in tandem with the first anchor joined by a short length of chain to the crown of the second anchor, supposedly giving better holding. Has anybody any experience of this technique?.
How effective is it and how difficult to lay and retrieve the anchors?.

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[Sybarite]

This is supposed to be a very effective means of anchoring and one that I would use before spreading two anchors.

However you must make sure that the chain between the two is longer than the depth of the water. If not you are lifting both anchors at the same time.

John

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[tcm]

Yes, this is correct. I anchored out in blimmin lot of wind, dropped anchor, gave it a tug at 20m, shackled kedge on and then laid out lots more. Efective in heavyish weather irrelvant otherwise imho.

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[Mirelle]

Agree. I've also done this, when a very fierce blow was predicted.

Note the advice, above, about the depth of water and the length of the chain between the two anchors. As you may inagine, the business of retrieving the first anchor, getting the shackle undone and getting the chain onto the gypsy is a disgusting, fingernail breaking, filthy, sweaty sort of a job. The only way to go about it is to get a rope stopper on the chain outboard of the nearer anchor, which is a LOT easier said than done.



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[Robin]

Why would you prefer it to 2 separate anchors? We have heavy anchor gear and have never felt the need yet for doubling up by either method but I must admit I would have chosen to spread anchors first.

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[Mirelle]

The Reason Why

There are studies which show that it is more effective - and certainly lying to two anchors at open hawse when the wind veers is an utter misery if you get a twist up!

Shackling one anchor to the crown of another - the recommended method - is horrible when it comes to retrieval. I am much more inclined, having a shackle that will go over my chain but not slide along it, to shackle the nearer one to the cable of the further one. But probably this is less effective.

It is an extreme tactic, not for regular use.

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[Sybarite]

I agree with Mirelle's points :-

In case of wind shift you don't have cables crossing.
I only have one bow roller.
Tests carried out in France showed that anchors in tandem had better holding power.
One must however make sure that the way the second anchor is attached to the first will not cause upward lift in anyway.

John


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[Roberto]

Extreme tactic

In ante gps days, we anchored in 50m depth (fog+shallow water all around) with full first anchor chain and cable + second anchor with its chain and cable, in tandem, roughly 150/200m of ground tackle now beat that!

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[drawp]

Standard offshore oil industry practice for floating rigs is to "piggyback" (connect in tandem) an extra anchor if a primary anchor drags. However the direction of pull on the anchor cable is constant and outer most anchor is retrieved by a pennant wire attached to the crown of the anchor. Hence advice in other posts re length of cable separating the two anchors is relevant.

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[oldsaltoz]

G'Day Andrew ,

Have you tried an anchor chain hook, easier than struggling with a rope and knots off the bow, and great for preventing anchor noise when sleeping on board if a short nylon strop is added.

Hope this helps. . . . . .



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[steffen]

Questions

This setup must be for safety, so one anchor backing up the other for i cant see how the load will be spread over both anchors equally; one always sets and thus prevents by nature the other from setting(?)
Also, what is the weakest link? Imho the chain or rope. If that breaks both anchors are gone.

Please enlighten me on the mechanical aspects of tandem anchors.

Happy sailing, Steffen

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by steffen on 17/06/2003 13:48 (server time).</FONT></P>

 

[Robin]

The problem I envisage is that when you anchor initially you would not (normally) do so expecting all hell to be let loose, so are in standard anchor mode. If later it looks like there could be a problem I'm not sure I would want to raise an anchor already set well especially if conditions were already worsening, wheareas laying a 2nd anchor on a sheer could be done. Twisting could be a problem unless you join the two chains at a swivel which is then lowered a few feet below the water.

As has been said this is an 'in extremis' method. We have confidence in our regular method and dig our anchor in gently then when set at full reverse, then add a nylon strop with a rubber snubber included on a chain hook to transfer the load from windlass to foredeck cleat. This prevents the huge snatch loads from sheering and swell/waves from jerking out the anchor.

I think in order of priority:

Have a good design properly sized anchor plus adequate all chain rode.
Set the anchor properly, letting the boat gently 'snub' on the chain to initially set it, then and only then gradually apply some power to dig it well in.
Have adequate length of chain out for the depth/conditions.
If possible anchor clear of likely 'draggers' upwind of you (especially in France!)







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[Sybarite]

I agree with everything you say and have never applied it in anger. However if I did not have the option of finding better shelter and knew that something nasty was forecast then I would seriously consider doing it this way.

One other adavantage is that if both anchors are biting they are doing so in line with the wind effort. Anchors sheered out are doing so at an angle and possibly not at their optimum holding potential. But here we are really talking about extremes.

John

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[Robin]

Steffen has a point too that one anchor might prevent the other digging in properly? I suspect with 2 sheered anchors the load would come on one only at a time unless laid like a mooring with chain between the 2 and a swivel/chain up to thre boat. If you laid 2 of different weights (even types), where would the weakest go as no 1 or no 2 in the rig?

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[hylas]

Re: May I dissagree?

again and old story..

yes, two anchors should have more holding than one alone.. but to achieve this you have to be sure that both anchors are well dug in..

After a near catastrophe with two anchors tandem set.. I did my own study.. in more than 62%, only one anchor was buried.. and you have no idea if the second one will dig in..

Mirelle says "There are studies which show that it is more effective"
I think I'm well informed on nearly all papers published since years about anchors and anchoring technics... and I never have seen any single study about this tandem set technic.. (please Mirelle, if you have one I will be very please to have a copy)

Up to my opinion, this technic should be avoided, this is a "Russian Roulette" anchoring technic.. difficult to put in place and even more difficult to retrieve.. mostly if you should do it in a hurry..

Moderns anchors have two to three time the holding of "classical" ones.. so keep it simple. a good anchor will always be more safer than two bad ones "tandem set"



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[tcm]

Re: May I dissagree?

Hm. My understanding is the primary anchor is the first anchor - the second anchor can be seen as an isolator for the first against heavy weather pitching and hence movement of the first.

Not sure how a "catastrophe" could develop and it seems a bit strong to suggest any one system is "russian roulette"!

With only one windlass sufficient to haul up or retrieve, two separate anchors ain't an option. And if one anchor moves cos it gets tossed around - why wouldn't two?

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[Mirelle]

Yes, but my chain is 7/16" and I cannot find a standard hook that fits - 1/2" slips off. I always mean to get one made but have not got round to it. I know I should!

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[Mirelle]

YM please investigate?

Good point.

There's really no way to be sure that the nearer anchor is set. Certainly if it is the chances are that the chain between the two is slack (this may not matter?)

I thought that I saw a study in a quite well known American book called "Oceanography and Seamanship", some years back, but I don't have a copy and I might be wrong

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[Mirelle]

May I compliment you!

Hylas, I read your postings on anchors and anchoring with interest and a great deal of respect - please keep it up!

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[Jacket]

Re: May I dissagree?

PBO or YM did an anchor test 5 or 6 years ago, using a chain jack to pull a range of anchors across a beach in shallow water. They tried two tandem anchor setups. In the first, the first anchor's chain was shackled to the crown of the second anchor. In this setup, the second anchor didn't dig in, so was doing little other than acting as an angel.

In the second setup, the chain of the first anchor was shackled to the stock of the second anchor, where the second anchors chain was attached. With this setup both anchors dug in, the first so deep that they had trouble digging it out.

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