The great anchor debate - whats are take

[peteandthira]

Re:Tandem anchors!

Not replying specifically to MagnaCarta, sorry, but this seems a learned place to pose my question. I always have a bit of a dilemma when setting my main (Delta) anchor, and that is how much power to use in reverse?

I currently use 2000rpm applied gradually over about 10 mins, on a 36' AWB with a 29hp Volvo on a 3-blade folder. I am fearful of suddenly breaking out what was a perfectly good anchor set and having to do it all again. On the other hand, I always imagine that I should have put full power on and slept more comfortably.

I would love any comments from the assembled anchoriing committee!

Pops

 

[Phoenix of Hamble]

Re:Tandem anchors!

10 mins!

Wow.... I use about 20 seconds of reverse max!

 

[peteandthira]

Re:Tandem anchors!

I should add that most of those 10 mins are spent at lower than 2000 rpm in an attempt to pull the kinks out of the chain along the ground before I start to dig it in. My gradual increase in rpm to 2000 is included in that 10 mins.

Am I being a bit OTT, or isn't it enough?

 

[Phoenix of Hamble]

Re:Tandem anchors!

A bit OTT i'd say, but others opinions will probably have more value as I only anchor about 15 times a year...

 

[steveh]

Re:Tandem anchors!

The other advantage of a 'V' anchor is that you can put out an extra anchor to your original single anchor if the need arises, without having to start all over again. I've laid an extra Bruce out, with no chain, in the dingy, and have been amazed at how well it held and difficult it was to recover. This was in a muddy harbour in Morroco and had to get a local boat to pull it up!

 

[KellysEye]

Re:Tandem anchors!

We drop about 2 times depth then let the chain briefly pull up taught, then another two times and let it pull taught again but for a longer period. Then let out however much more we need for the conditions. If it's windy we don't use the engine at all - the boat sheers and is pulled back into wind as the anchor bites. If it's not windy we follow the same process using the engine.

 

[KellysEye]

Re:Tandem anchors!

[ QUOTE ]
advantage of a 'V' anchor is that you can put out an extra anchor to your original single anchor if the need arises, without having to start all over again...amazed at how well it held and difficult it was to recover.

[/ QUOTE ]

Entirely agree, that's been my experience too. In fact I'd go as far as to say I have never seen any cruising boat laying out tandem anchors. It strikes me it's an academic concept to discuss but the practical solution is a V setting (and I have seen 3 anchors laid out ahead).

And since much has been said about sheering and reducing loads can I just say again - make a riding sail, it really works.

 

[roly_voya]

Re: Two anchors

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?

I can see your point and if your major concern is shearing you may have a better solution. In my case I get little significant shearing, well under 10deg either way and it tends to be a slow crab sideways without changing the angle between the rode and the bow so very little snatch load consiquently I have never though about how to reduce it.

All boats being different they each need individual solution I would not expect a cat or an open 60 to use the same tech as I do

 

[roly_voya]

Re: Tandem anchors

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 . . .


When I started this post I was considering my current experience which is UK/N Europe based. My concern was that in poor holding due to a week seabed the single anchor would drag. By using the tandom anchors I am looking at returning the grip to what it would be in good holding or maybe getting a moderate increase in exceptional conditions. Even if tandom anchors do double the holding power that would as you say exceed the SWL of the road (10mm chain with 5T breack load & 2.5T SWL) if the holding is exellent. The goal is more likly a 50% increase in moderate holding but as much as possible in poor.

From what people are saying the experience in the Carribean and possible Med areas is that poor holding is caused more by the anchor not being able to penetrate the seabed and get a good grip than the anchor pulling through a soft seabed.

But widen it as much as you like 'cause I will probably get there one day

 

[roly_voya]

Re: Tandem anchors

In answer to questions of setting the anchor with astern power in a bad blow the load is going to far exeed anything you can get out of most sailing boat props although maby not a big motorboat

 

[roly_voya]

Re: Tandem anchors

Graig

If you want a voluteer to put a load cell on the end of the road while at anchor and measure the actual forces I would be delighted to take part in the research. Emirical data of this type does seem to be scarce

 

[wagenaar]

Re: Two anchors

quote:
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.

I find this an interesting observation, because it shows that the boating industry does not use what is common practice in other areas. In the building of airplanes a lot of this type of research has been done, where different components have been tested in flight by putting strain-gauges on them at the critical positions and the results used to test the components of new airplanes in laboratories.

 

[Robin]

Re: Two anchors

[ QUOTE ]
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).


[/ QUOTE ]

I have always felt that it is the snatch loads that cause anchors to break out, the loads are way in excess of the steady loads for the given conditions. Logic then says increase the maximum holding power (more scope, heavier chain, bigger/better anchor etc) such that this is in excess of the snatch loads that can arise. However there is another alternative and that is to reduce the power of snatch loads and ideas here for twin anchors seem to have some effect on snatch.

My firm belief is in the use of really effective snubbers. A length of nylon warp IMO is no more than a noise reducer, it doesn't have enough reactivity to be really effective. This is why we use a nylon snubber but with rubber mooring compensators in it, because a properly sized rubber compensator will dampen all the snatch. We actually carry two different snubber lines, each about 14/16mm dia nylon but one has our 'normal' rubber compensator and the other our 'storm' one which is much stiffer but still stretchier than the nylon alone. As the boat swings the rubber progressively stretches (limited eventually by the number of turns of the nylon weapped around it) and as the swing reaches it's peak the rubber contracts again, if the rubber reaches it's limits then it is time to fit the heavier one.

I may be preaching to the converted but I see lots of boats anchored with snubber lines but very few that have the rubber compensator as well. This is a method I have used for over 30 years over a variety of boats. In the early days we had home made compensators made from multiple lengths of shock cord whipped across a loop in the snubber line, worked very well but not as long lasting of course as the rubber sausages or Gondolastics.

It may well be that some modern anchors will hold heavier loads in various bottoms but unless the snatch is reduced IMO ultimately there are two possible results, either the anchor breaks out/drags anyway or something breaks, like the bow fitting, fairlead or cleat and it would certainly be very uncomfortable on board.

 

[DaveNTL]

Re:Tandem anchors!

[ QUOTE ]
10 mins!

Wow.... I use about 20 seconds of reverse max!

[/ QUOTE ]

And I don't even give it that. Not saying 'i'm right', but I just give a short and fairly gentle reverse, stand around for a minute and check that I'm holding and that's it.

Only dragged once, and I anchor a lot in every type of ground, the day after on a tide change anchored in grass.

I have a 45lb CQR, all chain.

 

[jimbaerselman]

Re: Two anchors

Roland,

Your last three posts were very interesting, and bear on many of the reasons why people disagree about 'best practice', 'best equipment' etc etc.

The principal reason is that different people start from different assumptions, which often are not stated. You've clarified your assumptions - many thanks.

First Assumption - Boat type. Your bio suggests you have a moderately heavy (for its size) long keeled boat. You are quite right - these do not sheer around the anchorage in a big wind. I referred to the behaviour of a short deep keel sail boat - the worst case. Light motor boats also yaw a lot, but, without a keel, they don't actually move much.

Second assumption - sailing areas. Tremendous differences in the need for and frequency of anchoring, and the threats to safe anchoring (bottom, tide, wind strength, wave exposure). I think your analysis of why views from the Med differ from UK views on anchor types is right on the nail.

Third assumption - Techniques. I think you'll find that the large majority of world girdlers use 6xdepth rode - at least - for anchoring, and routinely set their anchors using full astern to dig it in, re-dropping the anchor if it drags. In UK there seems to be a view that 3x or 4x rode to depth is adequate. In N Brittany estuaries (Treguier, Lezardrieux) an up-tide/down tide moor is essential to survive a tide turn - unless you duck the issue by using a marina or mooring.

Still techniques - Your view, that if you use tandem anchoring as a means of improving holding on a weak bottom seems sound. I'm still stuck with Craig's 'doubling the rode length' though - that wipes the idea out for my anchoring conditions. Perhaps not for you.

 

[jimbaerselman]

Re: Snubbers

Robin,

I completely agree with your need for something elastic to relieve the harsh jerk of a chain coming up taut. Years ago I did some crude sums and decided that I needed about 2 metres of 'spring'. With nylon that implied around 20m from chain attachment to bitts with 20mm, or 10m with 14mm, but a risk of permanent distortion in the thinner rope. I went for 20m.

It would be convenient to reduce that length. How much does your rubber-snubber give you?

 

[vyv_cox]

Motoring astern

10 minutes seems like a lot but won't do any harm, I'm sure. We use a Delta also, dependent upon conditions start off with about 1000 rpm astern, although if it's windy we probably would not bother with this step. Then go to 1500 while I am on the foredeck feeling for movement in the chain and crew is checking transits. If all OK I usually walk aft while revs increased to 1800 - 2000 and left for a minute or so. We both check different transits and if all OK shut the engine down. Mostly I will nowadays dive on the anchor to check that it is bedded OK but this is only really appropriate in the clearer and warmer waters of other areas than UK.

Our prop is an Autoprop so very efficient astern. I can see that with a folder you might need to go to 2000 rpm straight off.

I suspect that if the anchor breaks out at 2000 rpm it most definitely would in a decent blow, so I am never afraid to go to higher revs if we have any suspicions that there may be a problem.

 

[Robin]

Re: Snubbers

[ QUOTE ]
It would be convenient to reduce that length. How much does your rubber-snubber give you?

[/ QUOTE ]

We don't get anything like 2m of spring but the resistance is not linear so is maybe more effective. Our 'normal' one is about 12/15" long and will probably stretch by 60%, but it gets harder to stretch as it reaches its limits, plus there will also be some stretch as well in the nylon line. Our stretchable length of line is about 5m maybe, but excluding the bit on the cleat, and some of this is wrapped round the rubber. The normal one is usually more than adequate and works better in normal winds. We have only found the heavy duty one (15/18") needed when we had winds around F8 but with severe gusts off hills that came from all directions or on another occasion when the wind shifted 180 degs onshore and blew F6/7 but with then a 12ml or so fetch and waves rolling in so we pitched heavily. We put maybe 5 or 6 turns round the rubber.

It is quite fascinating to watch them in action and they definitely allow the pull to be applied gradually.

I'm reminded too of an article somewhere about moorings in shallow water where the chain broke because of snatch loads way above what would occur in deeper water. This was of interest at the time as my then club had such moorings and were concerned as they had one snap even with 5/8" riser chain. The solution then was to incorporate an old car tyre across a loop in the riser chain, a really heavy duty rubber snubber!

I can't find pics of ours but the big one is like the one here, the normal one is similar but not identical to the smaller one here.

Snubber pics/sizes

 

[jimbaerselman]

Re: Snubbers

Thanks for that. Yes, I've seen them. They'd give about 20cm of spring, the same amount that 2m more of nylon would give you.

On the subject of snubbers, or rather, elasticity of rodes, do have a look at Alain Fraysses site. I first found it about a year ago, and since he confirmed some of my (unfashionable) views, and also added a lot to my knowledge, I've never hesitated to recommend it to anyone debating anchoring techniques. But perhaps you've been there already?
http://alain.fraysse.free.fr/sail/rode/rode.htm

 

[Robin]

Re: Snubbers

I think it is more than that and I maybe underestimated the unstretched length. I know we use the one for one size rope dia bigger than it says it will accept. Also on reflection our 'storm' one is around 2ft long and over 1" in diameter. The amount of stretch is limited only by the number of turns wound round it and we use 5 or 6 turns I think again from memory. It has enough slack to double in length before the load goes on the line with 5 turns.

The amount of stretch isn't the whole thing IMO, because as I said it resists harder the more it is stretched and so applies a gradual breaking force to the swing. In our case there is nowhere near the same shockabsorber effect from the nylon alone.

We also carry 2 more of the 'normal' stretch ones to use for mooring in bad conditions which we use in a 2nd line with the main line(s) left slacker. We have had cause to be very thankful to have these, including one time in La Trinite visitor marina berth with wind against a SE8/9 blowing straight in and waves breaking over the finger. A boat in the next berth had a cleat pulled out of it's deck and flipped 2 berths away (no not a Bavaria...)