Anchor "toppling"

[SimonKNZ]

Here's an interesting one that caught me out recently. We were anchored near the south shore of a sheltered harbour which resembles a valley facing West (Bon accord harbour at Kawau Island) in about 6m (at HW) of water, good holding with 25m of chain and a generally reliable Delta anchor. Wind was around 15kts WSW and having been anchored for a bit over 24 hours, we were well dug in. Forecast during the second night was for a wind change to 15 kts E. I normally use an anchor alarm app without fail, but on this particular night, sod's law would have it that the phone was playing up and I turned it off (a mistake I know).
Anyway, wind shifts at about 0200 and being a light sleeper I woke up and spent the next 10-15 minutes keeping watch and making sure we were holding. Must have dozed off, and the next thing I knew, wind was 20-25kts E and we were drifting rapidly down wind. By "rapidly" I mean it was if someone had cut the chain above the anchor. I quickly started engine, got the crew up, retrieved anchor and re-anchored in a relatively sheltered spot. Slightly un-nerving but no harm done because I had chosen a suitable location originally. One interesting thing was that out of the 25-30 boats anchored in that area, about 6 had the same experience - some drifting 300-400 metres out of the harbour before noticing and getting things sorted out.
It seems to me that my anchor must have been "toppled" by the sudden reverse in pull, and then just skidded along the sea bed offering no hold. I'm interested if anyone else has experienced this, and whether something like a Rocna with its roll-bar would prevent this.

 

[sarabande]

You are in the right country to get the best anchor (hint) , as sending a UK Knox out there would be expensive.

It's hard to imagine half a dozen boats doing the same thing on their initial set of anchor in the 15k WSW, and then the chain piling on top of the anchor and neutralising it during the reset, so one must look for another possibility.

Does the "valley" of Bon Accord bay deepen as you go W towards Snells ? I was thinking of a W wind piling up the water in the BA bay plus some tide, then the water reversing to the W with a wind change and ebb. creating a sort of seiche and surge so that the anchor lifted bcs of a near vertical chain (which might give the symptoms of "rapidly" with the anchor not touching the seabed) .

Deltas are normally pretty good at resets, and I am thinking of possible causes for your anchor and the others all failing at the same time. Some hydrological event...

 

[Deleted member 36384]

There have been reports on rare occasions of Rocna not resetting after a turn. It has been suggested that roll bars can block with sediment causing the weight distribution to move towards the back of the anchor and stopping the tip from digging in. The rapid turn and fail to set has been described before for Rocna as well on harder sea beds. Discussion of both these factors can be found at www.morganscloud.com This is a subscription service.

The question you need to ask, is your anchor behaviour a common occurrence or a rare occurrence and is it otherwise very reliable. No anchor is perfect but many anchors are good enough by a long shot for reliable anchoring.

 

[sarabande]

Very much in agreement with you, A, but I am struggling to invent a common cause for it happening to 6 boats at the same time. A single point of failure on all the affected boats ?

 

[RJJ]

Very much in agreement with you, A, but I am struggling to invent a common cause for it happening to 6 boats at the same time. A single point of failure on all the affected boats ?

was the depth the same 6m or so?

Just wondering if you could have gone "off the shelf" and suddenly you're trying to reset using 25m of chain in 10-15m of water? I think (purely based on what I've read here) that you should expect a Delta to drag up to a boatlength or so in resetting itself, depending on the bottom.

Also, it would sound as though the bottom was suboptimal somehow. interesting.

 

[SimonKNZ]

The area is very benign really, no sudden depth changes and I've never seen any tidal surges or similar there. Bottom is muddy sand, normally after a night there my anchor needs a very good lift to break it free. Unfortunately I didn't get a look at the anchors of the other boats that drifted, they were modern yachts in the 40-45 ft range, so the common factor i can think of is quite high windage in a changing wind.
I don't think I'm going to rush off and buy a different anchor based on this but it was certainly a good lesson for me. An unfortunate coincidence that I've spent about 25 nights at anchor this summer and that's the only time I've dragged, and the only time I haven't had an alarm set.
Thanks for the thoughts so far

 

[SimonKNZ]

This shows the anchor location, I dragged just clear of the headland to WSW. There was another yacht slightly south of me who was very lucky not to go aground, I think he ended up with his anchor on the shallow water having drifted over it

 

[mainsail1]

Some places it just happens.
Newtown Creek is one in the UK.
Just a combination factors making holding unreliable.
No anchor is perfect.
Sounds as if you did all the right things.

 

[Mark-1]

Very much in agreement with you, A, but I am struggling to invent a common cause for it happening to 6 boats at the same time. A single point of failure on all the affected boats ?

This. 6 boats at the same time? Either Aliens or pure coincidence. (I'm betting on Aliens.)

 

[mainsail1]

The Aliens must visit Newtown Creek once a week in summer then.

 

[Deleted member 36384]

This. 6 boats at the same time? Either Aliens or pure coincidence. (I'm betting on Aliens.)

I agree. It is entirely reasonable that the greatest galactic mystery should be what is the best anchor type. Why would aliens be any less agreeable than humans over this intransigent issue?

 

[[2574]]

OP: it might be worthwhile you watching this Rocna anchor testing video:

 

[RupertW]

When you say you were well dug in after 24 hours do you mean you had 40-50 knot gusts to do that or that you did use full reverse when checking the seabed when you anchored?

I say that because I have set anchors more than once then realised they were slightly wedged in something or on a shallow substrate over rock or even held by weed roots that work well in 20 knot winds in one direction but would not hold in a strong wind or a reverse in direction.

 

[[2574]]

When you say you were well dug in after 24 hours do you mean you had 40-50 knot gusts to do that or that you did use full reverse when checking the seabed when you anchored?

I say that because I have set anchors more than once then realised they were slightly wedged in something or on a shallow substrate over rock or even held by weed roots that work well in 20 knot winds in one direction but would not hold in a strong wind or a reverse in direction.

Which is why Steve's (of SY Panope) veer tests are so important in identifying anchors that do not reset well. Anchor reset capability is just as important as holding power. Anchors that have previosuly been thought excellent due to high holding power attributes are now being identified as poor resetting anchors, this is siginificantly changing opinions of anchor type/design.

 

[Mark-1]

The Aliens must visit Newtown Creek once a week in summer then.

I've seen a helicopter land at Newtown Creek which is pretty much the same thing.

 

[noelex]

I suspect a big part of the problem was the change from an uphill slope to a downhill slope. This makes a very significant difference to the important effective scope. There is a good discussion of some of the factors here:

Calculating Scope Can Be Very Misleading - Cruisers & Sailing Forums

Once your bow roller height is taken into account your actual scope was about 3.5:1 (at high tide), plenty for an uphill pull in the conditions, but the effective scope could be significantly less with a downhill pull.

 

[RupertW]

Which is why Steve's (of SY Panope) veer tests are so important in identifying anchors that do not reset well. Anchor reset capability is just as important as holding power. Anchors that have previosuly been thought excellent due to high holding power attributes are now being identified as poor resetting anchors, this is siginificantly changing opinions of anchor type/design.

But at the same time a well dug in anchor will often slowly turn still buried rather than toppling and resetting with a wind change. I’ve seen that many times in the sand patterns of chain and anchor after a night with a reverse of wind.

 

[newtothis]

View attachment 110367
This shows the anchor location, I dragged just clear of the headland to WSW. There was another yacht slightly south of me who was very lucky not to go aground, I think he ended up with his anchor on the shallow water having drifted over it

I anchored a few metres closer inshore/south of there but glad I didn't end up with the same result. Fortunately what wind there was remained from the west.

 

[SimonKNZ]

When you say you were well dug in after 24 hours do you mean you had 40-50 knot gusts to do that or that you did use full reverse when checking the seabed when you anchored?

I say that because I have set anchors more than once then realised they were slightly wedged in something or on a shallow substrate over rock or even held by weed roots that work well in 20 knot winds in one direction but would not hold in a strong wind or a reverse in direction.

Yep, always use astern power to dig in and check holding when anchoring and we'd been at anchor for around 30 hours with mostly 15 kts and upwards. I have encountered the substrate effect previously at Great Barrier Island, but I don't think it is a feature of Bon Accord harbour. Weed roots are possible, but I anchor here quite often and I can't recall having weed on the anchor before.

 

[SimonKNZ]

Thanks all for the replies, some useful pointers there. I'll put this one down to experience, in retrospect I should have veered more chain before turning in for the night given the forecast and reset the phone then restarted the anchor alarm rather than turning it off due to the false alarms. Hindsight is a wonderful thing!