laying to two anchors.

[fisherman]

Useful site here,
http://www.ropebook.com/information/vector-forces

vector forces when a load (e.g. boat) is applied to two strongpoints ( e.g. anchors). Should you spread the anchors 120deg apart the load on each is the same as if you lay to one. This was discussed on here at great length a while ago, and the conclusion was 'if one anchor is not enough you need a bigger anchor'. Interesting to see how much increased load can be applied.

 

[syneraida]

My experience is that the only good reason for setting two anchors unless one is infront of the other is to reduce your swing.

 

[BrianH]

The only time I laid out two anchors in a Vee, a CQR and a Danforth, was in 60 knots of bora wind in central Croatia. I did not measure the angle but it was about 45° off each bow. The holding was poor although shown as an anchorage on the chart, sand over rock as I found out later, and my yacht at that time, a Trapper 500, sheered from side to side putting the full strain on each anchor alternately and each one dragged as it did so.

I have never repeated using two anchors and if I did then it would probably be in tandem ... except I now have a Rocna and have used that alone in similar conditions (albeit in a better substrate) and held securely - in fact, it was difficult to retrieve.

 

[Neeves]

If winds are forecast to be consistently over 30 knots we lay 2 anchors in a 'V'. We do this quite frequently.

The idea you increase the hold is misleading, most of the time you 'lay' to one anchor or the other.

Lying to a 'V' reduces veering and veering can result in snatch loads. Most anchors when they drag do so as result of snatch loads so doing anything to reduce veering reduces the problem (as does using a snubber). Reducing veering makes for a more comfortable yacht. However wind is unstable and veers (or sheers) and having 2 anchors set does mean there is a much greater chance one anchor will be orientated in the wind direction and it will be that anchor to which you lie (as the wind veers you then lie to the other anchor). Having one anchor means that the anchor is constantly being loaded at an angle, as the wind veers, to the set direction and this will cause the anchor to continually reorientate. Anchors that might move are less predctable than ones that remains stationary.

We use a 8kg alloy Excel (which is the same area as the 15kg steel version) on a 38' cat which has the windage of a 45' AWB. Our second anchor would also be alloy, either 8kg A80 Spade or 8kg FX 23 Fortress (depends on seabed which anchor we might choose, mud FX; weed, Spade etc). Our rode is now 6mm HT chain and the second rode 6mm HT chain plus 12mm nylon.

Every yacht that ventures more than a days sail from home and especially those that venture much further should carry spare anchors that can be used as the primary (you might lose one or the seabed might not suit the one you have on the bow roller). It seems something of a waste to carry that extra weight and not use it.

Jonathan

 

[KellysEye]

As others have said we set two anchors in a V, we did it if the forecast was gale+. We alway flew a riding sail that cuts swing from over 90 degrees to 35 and thus snatch loads, dragging wasn't an issue.

 

[PaulJ44]

Two anchors in the same direction has always complicated life for me, with no useful return IMHO.
The only time I use 2 anchors is for minimum swing in an anchorage I intend staying on for a while: generally only if a multihull or light displacement boat arrives.
Drop first anchor, back down 2x required scope and drop second anchor then pull up half way.
Your in a good position for the regular ebb and flow. I bouy both anchors with my boat name on both bouys so new arrivals can see my setup. It gives minimum swing.
I've tried swinging from 2 spread anchors numerous times with no success. Primarily because the boat hunts in yaw axis from one to the other when in a blow and makes a hell of a racket. The boat is a Dennis Ganley 39 in steel. Your Results may vary of course.
As an aside:
I was a confirmed CQR guy for 35 years and now have ultimate faith in my Rocna (Okay, when it's really howling, that faith wobbles a little, but that's more my worrisome nature than a fault of the Rocna...

Remember that internet advice is always just one person's opinion and your mileage may vary

All the best....P

 

[jimbaerselman]

Laying two anchors ensures that each anchor only has a force applied to it which is approximately in line with the direction the anchor was set. This removes the probability that with a tide change, a wind change, or when "sailing" from side to side at anchor, a shift in pull direction will upset a single anchor, which has to re-set itself, with some risk of clogging or fouling.

In a tidal moor, or running moor, a first anchor is set up tide, a second is set down tide. The two rodes are lashed together, and the lashing point dropped below keel level. As tides turn, the up tide anchor usually takes the strain. In strong wind over tide, maybe the down tide anchor takes the strain. Mostly, tidal moors are in estuaries, with reasonable wind shelter, so strong sidewinds which will lead to significant "bowstring" effect are unlikely. If they were likely, you wouldn't leave the boat!

The forked moor, or Bahamian moor, is primarily designed to cope with the tendency of most sailboats "sail" from side in winds over 25kts or so. With a single anchor, the swing sector is likely alternately to pull the anchor with a 90 degree change in direction at each snatch. This has three effects:
(1) the bigger the swing, the higher the snatch load, and the closer you'll get to exceeding the anchor's holding limit.
(2) with each change in pull direction, the anchor works slightly down-wind.
(3) the direction change may un-set the anchor.

The forked moor overcomes these disadvantages. First, the sideways swing is much reduced by the splay. Second, this means snatch loads are much reduced. And thirdly, the snatch loads are taken "in line" by the current windward anchor, so there's much less re-alignment taking place.

So mooring with two rodes and anchors is not to do with "sharing" loads.

It's to do with (a) reducing inertia/snatch forces - much bigger than wind or tidal drag - when boats are checked whilst roaming around their scope. And (b) not forcing anchors to re-set direction (with the risk entailed).

Tandem anchors do not provide these benefits

 

[noelex]

As others have said, with two similar anchors in a "V" one anchor invariably takes most the load as the boat swings around, or in other words you 'lay' to one anchor or the other.

The boat will "walk" backwards as each anchor drags.

The net effect is that the holding power is much less than geometry calculations that assume the anchors are sharing the load.

The other problem is that you need two rodes. If chain is needed for abrasion resistance (as it is in some substrates) the total weight (let alone the hassle) is much greater than deploying a slightly larger primary anchor.

Even an extra 5m of 10mm chain, that might be used on the second anchor, is more total weight than adding 10kg to the primary anchor.

 

[rogerthebodger]

That in theory is correct but it assumes that there is no lateral force on the weight.

In real life when anchoring there will be some sideways load due the the swing of the vessel and the amount of sideways load depends on the amount ans rate of swing.

This also applies to when a dingy is towed using a bridle. If the centre point of the bridle is fixed the load on each side will vary depends on the swing of the dingy unless the bridle passes through block on the bridle and the block is attached to the towing boat from the block and is allowed to move along the bridle.

 

[Neeves]

People who actually use 2 anchors never mention that each anchor progressively drags. Possibly you have your own first hand experience that is different. As each anchor is not subject to very high loads, because there is no veering nor snatch loads, the anchors are secure - unless you are saying those that have posted here (and have actually used 2 anchors in a 'V') are kidding themselves.

As has been mentioned by many who have already posted the geometry is an illusion you do not enjoy double the holding capacity, or whatever the geometry might suggest - you rely on each anchor individually. I'm not sure what the hassle is - you lay everything out, you motor to your desired deployment point and drop the second anchor - seems and is quite easy. If you value the exercise you can row the second anchor out in a dinghy.

It would be accepted you could not use 2 anchors in a 'V' because your anchors are simply too big, at 60kg each, to manhandle - so I have to wonder where your first hand experience comes from?

It would be foolhardy for anyone straying far from home not to carry a second rode.

10kg added to the weight of, say, a 30kg anchor (so increasing to 40kg) will make no detectable different to the holding capacity of an anchor. To double hold you will need to increase weight by a factor of slightly less than 3 rimes.

Jonathan

 

[GHA]

Interesting thread. Given that the energy needs to dealt with is equal to 1/2 x mass x velocity squared, keeping the boat speed down seems to be a very good idea. Hands up, I've never tried twin anchors, single handed it just seems a lot of bother when a bigish anchor and riding sail might do just as well, ignoring swinging room in wind shifts though even then moving the boat and re anchoring would seem a lot easier than having to deal with 2 x rodes and anchors, especially if it really kicks off and you need to get out in a hurry at 3am.
Anyone tried a drogue off the bow to slow things down?
Of course, very much down to the boat and local conditions as well, it was gusting max high twenties here yesterday and the boat was slewing around a bit but nothing major, that's a heavy long keel boat though, light cat would probably behave very differently.
Gusting 30 plus forecast for a few days time, if I get round to it I might try to record wind/boat speed with different options, see how the real world behaves. Or maybe chicken out and go into a nice marina

The main argument for using 2 seems to be to cut down on the boat Sailing about, so has anyone actually recorded a gps track to measur the difference between lying to 2 anchors and something like a riding sail?

 

[noelex]

10kg added to the weight of, say, a 30kg anchor (so increasing to 40kg) will make no detectable different to the holding capacity of an anchor. To double hold you will need to increase weight by a factor of slightly less than 3 rimes.

I think all the evidence points to anchor holding being roughy proportional to anchor weight.

If we consider the rode weight deploying a second 30kg anchors when bad weather is forecast with just 20m of 10mm chain will add over 70kg! even with this small length of chain.

The cost/benefit or in this case the weight/benefit ratio of deploying two similar anchors on a "V" formation does not make sense when compared to the option of a slightly larger primary anchor.

 

[jimbaerselman]

Interesting thread. Given that the energy needs to dealt with is equal to 1/2 x mass x velocity squared, keeping the boat speed down seems to be a very good idea.

<big snip>

The main argument for using 2 seems to be to cut down on the boat Sailing about, so has anyone actually recorded a gps track to measur the difference between lying to 2 anchors and something like a riding sail?

Yes, it's all about energy. My earlier post was a precis of one of my web site pages: www.jimbsail.info/going-foreign/boat-prep/anchor-rodes which covers the whole issue of multiple rodes for various anchoring conditions.

It also suggests how to check what's going on, by comparing how long your rodes are slack, with how long they're doing work.

It's only visible to logged in site members at the moment. Hang on a bit while I switch access to the page!

 

[alahol2]

People who actually use 2 anchors never mention that each anchor progressively drags. Possibly you have your own first hand experience that is different. As each anchor is not subject to very high loads, because there is no veering nor snatch loads, the anchors are secure - unless you are saying those that have posted here (and have actually used 2 anchors in a 'V') are kidding themselves.

A slightly different scenario but, I think, wholly appropriate... We often tie alongside a fellow cruiser when at anchor. We lay our anchors in a 'V' formation. We have definitely suffered the 'walking backwards' syndrome when the breeze has got up more than expected, especially when we are in a reversing tide situation. Admittedly each anchor is smaller than the ideal size of anchor for our sized 'vessel' but the same principle would apply to a single vessel in higher wind strengths.
To those who have ever wondered why a rafted pair of yachts are motoring round in small circles in the morning, you can probably now work out why...

 

[jimbaerselman]

As others have said, with two similar anchors in a "V" one anchor invariably takes most the load as the boat swings around, or in other words you 'lay' to one anchor or the other.

Yes.

The boat will "walk" backwards as each anchor drags.

Why should the anchor drag? It seems you're assuming two under-sized anchors . . . neither suitable for use by itself.

 

[jimbaerselman]

A slightly different scenario but, I think, wholly appropriate... We often tie alongside a fellow cruiser when at anchor. We lay our anchors in a 'V' formation. We have definitely suffered the 'walking backwards' syndrome when the breeze has got up more than expected, especially when we are in a reversing tide situation. Admittedly each anchor is smaller than the ideal size of anchor for our sized 'vessel' but the same principle would apply to a single vessel in higher wind strengths.
To those who have ever wondered why a rafted pair of yachts are motoring round in small circles in the morning, you can probably now work out why...

I think you've mis-understood Neeves.

He said "people who actually use 2 anchors never mention that each anchor progressively drags". I think his intended implication was "that's because the anchors don't drag".

You've assumed he meant that those who use 2 anchors always hide the dreaded secret that each anchor progressively drags!

The reality is different. If you're swaying from side to side on a single rode, each time you twitch the anchor through 90 degrees, it can walk a little backwards. I've watched this in hard sand.

Of course, that may not be true in stiff clay with a well buried anchor.

 

[thinwater]

There are 2 scenarios that have not been mentioned. I've done a fair amount of quantitative testing on these.

1. The bottom is really soft. I don't care how big anchor you bring, my home Chesapeake Bay has bottoms so soft in some places that any anchor requires 10-15 meters to set. They may set in sand within a meter, but not in this soup. Even after setting for 10 meters, a 15 kilo anchor may only hold 2-3 KN. Several researchers have reported the same thing. If 2 anchors are set at a 60-90 degree angle, the weaker of the 2 sets/drags a little farther until they are both holding what they can. When the wind rises, they both move. Why not a single larger anchor? Because many of the creeks are so tight that a full swing + plus extended setting will put you in the shallows. So a bigger anchor will work, I'm just pointing out that in soft bottoms there is provably equalization.

2. In-line tandems can work in several specific bad bottom situations. First, rocky bottoms where the rocks roll or break, or the tide shifts a lot. Even the largest anchor will give if a friable rock breaks or rolls, and having 2 anchors helps to arrest the drag and allow re-hooking before the boat gathers too much stern way. Kind of like untangling barbed wire. The second is where all you have is friction (no possibility of burying). The advantage here is 2 fold. A, if you need to get out in a hurry, in-line tandems are much faster to recover. B, You already have you big anchor down, but need more friction. And you need at least 15:1 scope, since if the chain comes off the bottom at all, you are cooked (the anchors are not buried, so the nearest anchor becomes weightless).

I'm not suggesting smaller anchors. These tricks are for when that still isn't working right.

 

[OldBawley]

I use two anchors in “V” quit often.
Reason is that we anchor 365 / year at a lot of different places.

Main is a CQR, known to have its limitations. However, It has held the boat from the rocks the last 30 years, and buying a new high tech anchor is a big investment.

Big unnecessary investment, because we already carry four extra anchors. One Fortress fx16, a Fortress Fx 37, a Northill and a Greek fisherman's anchor.
Each of those is ready and can be easily deployed with our rigid dingy.
Rowing. Never use an outboard, that is asking for trouble going up 35 kn of wind and waves with that long anchor line behind you. As soon as I start worrying, one of the spare anchors goes in the water.
I agree, it is a lot of hassle, but then I love playing boat.

In most bottoms, the oversized CQR will hold us. Some places have bottoms that are hard to penetrate or are extreme soft. I know places where even the most sophisticated rocket science anchor will not hold – penetrate.

My favourite is the Northill. Will penetrate anything that is not solid rock, and once the stock digs into the bottom it is drag free ( For our modest 8 tons low windage )
Can only be trusted as long as the wind does not turn. One fluke sticks out of the bottom and the anchor will foul on itself.

I prefer a combination. Some experience helps.

 

[Neeves]

I think all the evidence points to anchor holding being roughy proportional to anchor weight.

If we consider the rode weight deploying a second 30kg anchors when bad weather is forecast with just 20m of 10mm chain will add over 70kg! even with this small length of chain.

The cost/benefit or in this case the weight/benefit ratio of deploying two similar anchors on a "V" formation does not make sense when compared to the option of a slightly larger primary anchor.

Anchor weight is proportional to holding but not on the basis of double weight double hold. No anchor, ever, does this. For our anchors its nearer double weight and increase hold by 1.5 times. There has been a lot of work conducted in this area. If doubling weight was the panacea you suggest then the Vulcan pocket would be filled with lead and the Fortress would not work, in comparison with a Danforth. In reality the Fortress, at half the weight, outperforms the Danforth every time and the holding capacity of an alloy Spade, or alloy Excel is the same as the same sized steel version. Its not weight but design or -comparing alloy Spade and Excel with their steel counterparts - same surface area, same hold (which underlines its not weight but surface area, and surface area and weight are obviously closely related, except one is a function of the cube and the other the square and as anchors get heavier surface area increases disproportionately less).

I think you are completely ignoring the repetitive comments from most posters who have tried anchoring in a 'V' that it reduces sailing at anchor. If you have never tried it - how wold you know? If you do not veer and sail at anchor well and good - but most yachts do especially AWB's and probably more so the newer ones that have high windage and are lightweight with very narrow keels. You can deploy one monster anchor you will still sail and veer, you can deploy 2 anchors and you will reduce this veering. Most people accept that sailing at anchor, veering, is the root cause to dragging.

I am still waiting for the evidence that deploying 2 anchors in a 'V' allows each anchor to drag progressively. This is absolutely rubbish and contradicted by virtually everyone who uses a 'V' formation.

There are other options and a riding sail is one, but very seldom seen. Ideal on a ketch. I have thought of a drogue, or steel bucket, but have feared getting the line wrapped round the saildrive (and I'd rather deploy a second anchor) Most 35 knot and + wind events last more than a few hours and the benefit in comfort in sitting relatively stable, not veering, far outweighs the minor inconvenience of deploying a second anchor - evidenced by the numbers who have actually tried the practice and used it.

GHA - something else to look at - simply check the bearing variation of the yacht under single anchor. I measured it once with a fairly constant NE'ly seabreeze of 35 knots at the masthead - the yacht veered through 75 degree. I have not measured it with 2 anchors - but it feels and is significantly less.

Jonathan

 

[alahol2]

I think you've mis-understood Neeves.

He said "people who actually use 2 anchors never mention that each anchor progressively drags". I think his intended implication was "that's because the anchors don't drag".

You've assumed he meant that those who use 2 anchors always hide the dreaded secret that each anchor progressively drags!

No, I didn't interpret Neeves' words in any way differently to yourself. I was offering evidence of a pair of anchors dragging successively when deployed in a 'V' formation and 'walking backwards'. Admittedly I'm talking about a pair of yachts at anchor, not using latest generation anchors. Each anchor is smaller than you would normally deploy from a single yacht with the same displacement & windage as the pair together. The fact of the smaller anchors makes the drag happen at a (relatively) low wind speed. Nevertheless I think the 'mechanism' would hold true for a single yacht on a pair of anchors at (relatively) higher wind speed.

I am still waiting for the evidence that deploying 2 anchors in a 'V' allows each anchor to drag progressively. This is absolutely rubbish and contradicted by virtually everyone who uses a 'V' formation.

I think you are overstating your case. If you take an extreme example... In high winds there will, as we are frequently told, be no chain on the bottom and the yacht will be ranging about. The ranging and snubbing may well be less because of the two anchors but it will still occur. At various times the whole force from the yacht will be on each single anchor, not shared by the two. If that force is high enough it will cause the anchor to drag, it's inevitable. Once one anchor has dragged a bit the force from the yacht will be on the other anchor for a greater proportion of the time and it will subsequently drag. etc. etc.