penultimate sail of the season....dragging anchor....

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Ghillie
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Re: penultimate sail of the season....dragging anchor....

Postby Ghillie » Fri Sep 25, 2009 10:49 pm

Having recently thought long and hard about the perceived superiority of the “new generation” anchors (NGAs) over the CQR plough, I have now convinced myself of the engineering arguments supporting this and thus together with the “epidemiological” evidence found in the fora now have two compelling reasons (and no excuses not to) to buy a new anchor. Xmas present solved!

These are my thoughts IMHO which are general and could well be flawed.

It seems that there are two main requirements of an anchor, its setting ability and its resistance to pull out. (let us not consider for now the turn of the tide). Consider that the direction of pull is more or less parallel (loads of scope, good chain etc) to a horizontal sea bed that is comprised of deep soil of some sort.

Turning to its horizontal resistance, (given that the anchor is set), this is a function primarily of the area of the blade, (presented to the direction of pull), the angle of the blade relative to the direction of pull, and the shear strength of the sea bed material. There are many other secondary factors eg densities of materials etc.

There is probably little difference in the “blade” areas between a CQR and NGAs. Their differing shapes however, are the key, as these would cause the NGAs to bury themselves on increases in load, whereas a CQR due to is configuration is unlikely to. This characteristic is the main difference. The CQR will become a plough (on which it is based) due to three reasons in my view:-

1. The angle of the blade to the direction of pull “looks” less than that of a NGA so there is less burying force and more force available for ploughing. (resolve the forces!)

2. Even if the CQR tried to bury itself, the area of its shank presented to the sea bed is several times that of the thinner shank of a NGA and this will resist burying.

3. The blade of a CQR is curved from tip to “ear” and as such, soil passing along its surface in this direction is turned and sheared upwards thus offering more resistance to burying and at the same time less resistance to horizontal pulling. (ie the area of the plough in the direction of burying is much much greater than that of an NGA - eg look at a Manson Supreme, and others’ cross sections)

In addition the CQR cross section (upside down flying seagull) tends to break apart the soil above it, the concave section of an NGA a bit less so)

I won’t discuss setting as I think that it is safe to say that it is accepted that the CQR does not set as well.

It should also be remembered that very soft muds/clays will only hold so much despite the anchor shape and will simply flow around the anchor as it pulls through.

Firmer clays may well provide an initial setting resistance, but once failure of the wedges of clay around the anchor occur, then the shear strength of the clay reduces to lower residual strengths resulting in “pop out” viz Dunvagan. A burying anchor should prevent this. This should not occur in sands....

Right, that’s off my chest......now, will an NGA fit my anchor bay? Anyone need a 35lb genuine CQR?

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Re: penultimate sail of the season....dragging anchor....

Postby Aja » Mon Sep 28, 2009 8:25 am

Shard wrote:It looked like the anchor had held well, but had pulled up a great chunk of seabed.


With respect to the above and to Ghillie's comments later on, It sounds that no matter what type of anchor you had used, it was the seabed that gave up first.

I think what I am saying is that you can have the latest generation anchor, the best technique in laying it, but it still comes down to the seabed and that's where I think complacency comes in. There is still every chance you may drag.

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Donald

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Re: penultimate sail of the season....dragging anchor....

Postby little boy blue » Sat Oct 03, 2009 12:17 am

Shuggy wrote:...... This latter yacht indicated that she wished to leave the anchorage and....
....How does one go aground at Puilladobhrainn? Any inside info? And why would you leave there in a force 7-8? The mind boggles... or perhaps the report is misleading?


as skipper of " this latter yacht " perhaps i can assist.
as i understand the position the yacht that ended on the rocks had 2 anchors out, at least one of which dragged. this was the second yacht to drag that night. the first yacht, which was furthest up the anchorage had left some hours before in daylight.
that left us as " second " in line. there was a westerly on our starboard bow. the yacht that ended on the rocks was behind us on the starboard quarter. there were another 6 or so yachts behind that.
the grounded yacht came up on our starboard side and level with the westerly and then suddenly veered off to starboard and onto the rocks. shortly after that our anchor started to drag. i reckon the main reason was the amount of swinging we were doing.
the westerly had alerted the coastguard to the grounded yacht and also later the we were in difficulty. i spoke to the skipper of the westerly next day at kerrera and it appears the the skipper of the grounded yacht had been distracted by his son and had taken his eye off the ball for the vital second or two in the confined space.
so far as leaving the anchorage is concerned i rapidly came to the view that " sea room " was preferable to confinement, no matter the wind strength.
having no windlass, hauling the anchor by hand was just at the limit of my strength in the conditions and it was only with great difficulty that i got it on board. if i had let the anchor down again and it had not held i would probably have been unable to pull it up for a second time. by this time the lifeboat had arrived and the its floodlights had virtually destroyed my night vision. i decided that it was untenable to dodge about in the anchorage for the rest of the night ( by this time it was about midnight ) and requested that the lifeboat lead us out.
for those of you interested in the great anchor debate ours is a manson supreme :roll: .
the westerly, whose crew sat in the cockpit all night with the engine running were hanging ( successfully ) on a cqr.

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Nick
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Re: penultimate sail of the season....dragging anchor....

Postby Nick » Sat Oct 03, 2009 12:33 am

.
Good call LBB - I would have wanted out of Puilladobhran as well in those circumstances.
- Nick 8)

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Shuggy
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Re: penultimate sail of the season....dragging anchor....

Postby Shuggy » Sat Oct 03, 2009 8:02 am

Understand - that sounded like a grim evening indeed. This thread has made me mentally re-assess PD as a 'fully safe' bolthole. Hope the night didn't dent your enthusiasm too much.
---
Shuggy

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little boy blue
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Re: penultimate sail of the season....dragging anchor....

Postby little boy blue » Sat Oct 03, 2009 11:38 am

the experience hasn`t dented my enthusiasm but has caused the crew to become reluctant to anchor so the increased mooring costs since then have certainly dented the cruising budget. the manson supreme was supposed to ensure that didn`t happen :x
looks like further expenditure on a windlass will now be required.

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Re: penultimate sail of the season....dragging anchor....

Postby puddock » Sat Oct 03, 2009 11:51 am

Hauling an anchor manually in strong winds is NOT an experience I wish to repeat, especially when in a close quarters situation. I must admit, I was surprised at the forces involved, despite having gone through the excercise "mentally" on several occassions.
Luckily I am fairly fit (for a 5'6" midget in his 40's) but I doubt I could have managed without Jim (Rowana) on the helm, making life much easier for me. I dread to think what may have happened had I been solo or less physically able.
Our own incident on Rosally certainly let me see the importance of having a RELIABLE windlass - I would not want to be in that situation again.

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Re: penultimate sail of the season....dragging anchor....

Postby Ghillie » Sun Oct 04, 2009 10:38 pm

LBB I am glad that you emerged unscathed from the trauma, and appreciate your posting an account of it. I can vouch for an electric windlass, but that is a very easy thing to say, and it came with the boat. Moving on..

Ghillie wrote:Having recently thought long and hard about the perceived superiority of the “new generation” anchors (NGAs) over the CQR plough, I have now convinced myself of the engineering arguments supporting this and thus together with the “epidemiological” evidence found in the fora now have two compelling reasons (and no excuses not to) to buy a new anchor.



little boy blue wrote:shortly after that our anchor started to drag.........for those of you interested in the great anchor debate ours is a manson supreme :roll: .
the westerly, whose crew sat in the cockpit all night with the engine running were hanging ( successfully ) on a cqr.


Well, with that evidence, bang goes my theory! I am undone! What to do now? (ie to CQR or not to CQR) I really thought that the way to a happy anchorage experience was to buy a New Generation Anchor and toss it overboard......with lots of chain attached.
(are spades better that manson supremes?). It is looking like AJA's sage advice is winning the day.

Perhaps a little mitigation may soften the blow my esteem has suffered.
LBB can I enquire of the particulars, ie what is the weight of your Manson Supreme, your boat, amount of chain out etc? Would you know whether the Westerlies CQR was oversized?

Indeed it would be illuminating to see a survey of experiences of anchor dragging with particulars......but I suspect that a proper survey would be difficult to organise.

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Re: penultimate sail of the season....dragging anchor....

Postby little boy blue » Sun Oct 04, 2009 11:09 pm

i got the 11kg manson - the recommended size for the boat. i`d have liked to go one bigger ( 16kg, i think ) but that would have been a step too far without a windlass.
the rode is 22 metres x 8mm chain + about 20 metres x 14mm octoplat.
we were possibly just about the position of the middle boat on the october calendar pic in about 3.5 - 4.0 metres with, initially about 20 metres of chain out. when the anchor started to drag i let out about 6 - 7 metres of octoplat. so about 29 metres in total.
i couldn`t really let out any more as a/ there was a yacht close behind by this time and b/ the amount of sheering we were doing. at one point we were beam on to the wind, the boat was heeled over quite a bit and i noticed 33 knots on the indicator. i think that the sheering about was the main cause of the anchor dragging that night.
i need to find a way of stopping that behaviour. observations suggest that the moody 31 appears to be somewhat more prone to that than other boats round about.
i may say that i had reset the anchor earlier in the day to take account of the anticipated wind shift and so far as i could tell it was well dug in.
can`t say whether the westerly`s anchor was oversized or not, i just noted its existence the following day at kerrera.

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Re: penultimate sail of the season....dragging anchor....

Postby Ghillie » Mon Oct 05, 2009 8:10 am

Thanks very much for that, I presume that letting out the extra 6-7m didn't stop the dragging.

Shearing about is characteristic of many boats now. I know people use riding sails successfully and remember posts from the skipper of a Sigma 33 who swears by it, and also extensively from Piota (who has ridden out some very high winds at anchor up north) on TOP. (I think Piota has modified his name at bit.....)

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Re: penultimate sail of the season....dragging anchor....

Postby sahona » Mon Oct 05, 2009 10:41 am

LBB, I think you'll shear more on elastic string than chain.
Any chance of introducing a fairly massive angel?
http://trooncruisingclub.org/ 20' - 30' Berths available, Clyde.
Cruising, racing, maintenance facilities. Go take a look, you know you want to.

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Re: penultimate sail of the season....dragging anchor....

Postby little boy blue » Mon Oct 05, 2009 6:59 pm

Ghillie - the extra rode did not stop the dragging.

Sahona - "LBB, I think you'll shear more on elastic string than chain.
Any chance of introducing a fairly massive angel?"

we were actually sheering just as much on the all chain rode, so not sure whether an angel would help much.
few days ago on a mooring buoy in craighouse in stongish winds, ie 20+ knots, we were sheering about and i was experimenting with part of a hank on storm jib up the backstay. that seemed to help but there was probably too much sail involved to properly assess the benefit of a riding sail.
i am looking for a bit of old hank on sail which could be cut to - say a 4 - 6 foot equilateral triangle to have a proper trial.

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Re: penultimate sail of the season....dragging anchor....

Postby Silkie » Mon Oct 05, 2009 7:12 pm

I've never had a problem in PD (touching wood as I type) but I've seen lots of people both dragging and having difficulty setting there and every time I've been close enough to see what's happening the problem has been insufficient scope.

Saw an Alba charter boat make at least six unsuccessful attempts to set a cqr there earlier in the season (it's usually their first night out when they come in) and after a bit of banter with the chap on the foredeck I asked how much scope he was using. "About 15 or 16m" was the reply and, trying to keep the tone light, I suggested that he "Horse it all out!" The skipper was called to the foredeck for a discussion (crew had presumably had his instructions and didn't feel he could change them on his own initiative) and more scope was employed resulting in the anchor setting at the next attempt.

Thinking about why they would be using so little scope and why he would quote such a precise range I eventually decided that they'd simply read the depth off the sounder and multipled by four. As a charter boat I'll bet that the sounder reads below the keel (probably with a generous margin for error) and that once they'd added the draught and the height of the bow they should probably have been multiplying six or seven by four.

Of course lbb wouldn't make any such amateurish errors or forget to add the tide either :) although I have to say that 20m doesn't sound like much scope in a rising wind regardless of what the sounder says. I'm sure I put all the details in the post with the squall video but I had about 40m out by the time the squall came in.
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Re: penultimate sail of the season....dragging anchor....

Postby Alcyone » Mon Oct 05, 2009 7:56 pm

We recently dragged for the first time, so I have been doing a lot of reading about it. I'm also in the process of putting down my own mooring, so it's doubly interesting.

From what I can gather, once the boat starts moving about and bouncing, the forces on the anchor and rode are multipled many times. I've read figures showing storm force winds on yachts pulling 600lb on an anchor. Once the boat starts bouncing in the swells, that seemed to rise tenfold.

As an aside, i dived to remove a riser chain from a mooring we had been using all summer, 15 miles up the river from Milford. I was very surprised to find the one ton concrete block we were moored to was upside down. It was not like that when I attached the riser. The forces can be huge......

edit. We stayed in PullTheDoorBrian one night and had a drink in the house with no trousers. Lovely.

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shearing about....dragging anchor....riding sails

Postby Ghillie » Mon Oct 05, 2009 8:26 pm

I too messed around with my storm jib fixed to the backstay, and in 15 to 20 knots of wind it did indeed make a big difference, reducing the shearing significantly, however, it is would be too big for stronger winds and I should have something bespoke made. I believe that a diamond shaped sail wrapped around a wire luff rope to form a V in plan section with an acute entry into the wind is the most stable.....


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