Help I've brainache - anchoring issue again

[asteven221]

Despite reading the books/forums and listening to advice I still can't get my head around an anchoring issue. I know the basics and I understand the theory except one thing. Everything I have ever read seems to assume that your boat lies to the anchor and tide/wind at 180 degrees - give or take a bit of vearing around, so the anchor is nicely dug in and the chain nicely parallel to the seabed with a nice curve up the boat. In fact many books show diagrams where the boat is sitting at the perfect angle to the anchor and then they put a dashed line/circle around the anchor on the suface to tell you that the boat can move around so make sure to allow space or you might to bump into your neighbours. That's fine (fairly obvious really) but they don't show you what happens below the surface, whilst the boat is auditioning for Strictly Come Dancing and following the dashed line on the surface.

So in my hypothetical scenario I set the anchor perfectly, 5 x depth at HW etc.... and the boat is pointing north. So far so good. Whilst lying in my kip, the wind/tide shifts 180 degrees to the south, so the boat is pointing south.

That is the route of my problem. The thing I can't get into my head is that at the very point the boat is pointing south, the anchor is doing nothing. It's physics - the boat will be doing it's best to rip it out the seabed. Logic tells me that the boat will now be not anchored - for want of a better expression! So am I right to say that unless the anchor resets itself (by luck? can't be anything else) we are enroute to disaster (remember I am asleep at this point)? Okay in reality I will have my anchor alarm gizmo's on, but say I didn't?

People tell me "no, no it's the weight of the chain that makes it all work out and it's really the chain that's doing the work. Hmmmmm.....well if that's the case the anchor would appear to be of relatively little importance, so why all the big debate about bruce/cqr/danforth/delta etc.....? It also makes the idea of picking the perfect anchoring spot and setting the anchor "to the book" seam of dubious benefit, if it's down to lady luck if the tide/wind turns the boat 180 degrees. No matter how I try and think about this, and take into consideration different chain weights/lenghts, anchors type/weight etc...... I always end up with the same conclusion - the anchor will have to trip at some point when the boat does a 180 degree turn. If it doesn't trip, then the forces of wind/tide are just not enough to make the anchor relevant, as the boat will simply be lying to the weight of the chain.

I am obviously missing something here! Hopefully someone will be along to explain how this all works so I can get a good nights sleep at anchor.

 

[NPMR]

Isn't that why we all wake up screaming in the middle of the night with the chain rumbling until it all settles down again?

I would agree that it has to 'reset' but as this is a gradual process as the flow reduces and then changes direction, it usually happens reasonably well. At least, in our experience. But not always, and if the chain tips the anchor upside down as it moves over the anchor .....

I think it pays to think of a check-up time (as the tide changes) to just have a look at what's going on.

It's why I can't leave the boat for long when it's at anchor and unattended. I like to stay within sight of it and have the outboard ready to fire up from the beach/party on the sand at a moments notice.

 

[jonjo]

Keep an eye on the weather, you should not be asleep riding on a single anchor when you expect a 180 degree wind shift and strong winds.

My guess is that in gentle conditions the anchor will reset itself well enough while you are asleep.

 

[bilbobaggins]

the anchor will have to trip at some point when the boat does a 180 degree turn. If it doesn't trip, then the forces of wind/tide are just not enough to make the anchor relevant, as the boat will simply be lying to the weight of the chain.

I am obviously missing something here!

Nope. You're missing nothing, except possibly some confirmation that you're quite right.

When the tide turns and runs the opposite way, or the wind does something similar, once the 'weight' of the boat comes on, the anchor will probably trip and you will no longer be anchored. What happens then can readily include an expensive wade ashore clutching your passport and Barclaycard. That's happened to more than a few.....

There will be someone along in a minute who'll swear on a stack of King James's that they've been single-anchoring for millennia, swung about all over the place in howling gales and ferocious tides, and never lost a minutes' sleep. Others will GUARANTEE IN WRITING that their patent anchor is designed to re-set itself in a nano' without human intervention, and there's no need to worry your precious little bonce..... Then there are those that place all their faith and their overdraft in a matchbox-sized box of electrickery gubbins whose marketing wonks on the other side of the planet also swear blind ( subject to the Laws of California or Korea ) this wee thing will wake you unerringly should your fragile barque stray more than a smidgeon from the circle/ellipse/geoid position that you choose before you turn in.

Me? I've always understood that it takes a lot of physical work and distrust, plus a fair few years' experience, to get a good anchor well dug in. Even then, I remain suspicious. That's what I'm there for. It stretches my credulity too far to accept that the anchor will do all of that, all on its own, unattended.

There's no way short of the Last Trump that I'm turning in to sleep without a very careful weighing up of the possibilities. And that includes anchor bearings, and a pre-planned route out of there, plotted and open on the chart table ready for urgent use in a black and zilch-visibility rain storm.

The only time I will sleep, at anchor, 'turned in' is when it's not my boat and HE insists on doing it HIS way. Even then, the only time I can recall was right out in the middle of a shallow, sandy sealoch near Crinan, with not a breath of breeze, no engine and 100 feet of chain out in 12 feet of water. We reasoned that the chain 'grumbling' would alert one of us to the beginnings of a breeze - but we had no other option.

That's why the 'ordinary practice of seamen' is to set and keep an anchor watch. But what do I know....?

 

[Daedelus]

. Then there are those that place all their faith and their overdraft in a matchbox-sized box of electrickery gubbins whose marketing wonks on the other side of the planet also swear blind ( subject to the Laws of California or Korea ) this wee thing will wake you unerringly should your fragile barque stray more than a smidgeon from the circle/ellipse/geoid position that you choose before you turn in.

Yes, and the ***** thing also goes berserk if it loses signal. I was fast asleep at Studland last weekend when had near heart attack - boat's moved - no just "Lost Signal".

Beautiful calm moonlit night at 0140 when I shot on deck so I suppose I'll have to forgive it.

 

[craigsmith]

People tell me "no, no it's the weight of the chain that makes it all work out and it's really the chain that's doing the work. Hmmmmm.....well if that's the case the anchor would appear to be of relatively little importance, so why all the big debate about bruce/cqr/danforth/delta etc.....? It also makes the idea of picking the perfect anchoring spot and setting the anchor "to the book" seam of dubious benefit, if it's down to lady luck if the tide/wind turns the boat 180 degrees. No matter how I try and think about this, and take into consideration different chain weights/lenghts, anchors type/weight etc...... I always end up with the same conclusion - the anchor will have to trip at some point when the boat does a 180 degree turn. If it doesn't trip, then the forces of wind/tide are just not enough to make the anchor relevant, as the boat will simply be lying to the weight of the chain.

I am obviously missing something here!

You're not missing anything; the boat will either work the anchor around in a turn, if the veer happens gradually, or if there is a sudden 180 degree shift then the anchor is quite likely to be pulled out backward and then have to re-set itself on its own. If it doesn't - well this is one of the main causes of dragging.

You have a point re "setting the anchor 'to the book'" - to some extent, the manual process of setting the anchor should be a test of the seabed and the anchor's ability to handle it on its own. In other words, if it requires fluffing about, and 5 attempts at dragging the thing for 50 meters before it bites, then it's not likely to handle an upset too well. A good anchor, given adequate rode and adequate sea-bed, will set instantly and reliably and require no special secret or technique. You need to worry about the situation etc, but anchors don't require 'operating'. Despite the rumors, it is not rocket science

 

[agurney]

You've set your anchor, with an all chain rode. Everything's neat and tidy, the chain will be vertical from the bow, though a slight catenary if there's a bit of a breeze. Wind/tide shifts 180 degrees. If there's little or no wind/current then you will pivot around the spot where the chain touches the sea bed. If there's a bit of wind then you'll gradually pull the chain the back so at some point your stern will be above the anchor

So, yes, you'll just be sitting to a lump of chain in this case.

If there's enough pull then then, again, yes, the anchor could be turned round and reset as you fear, but only if it's lying on the suface. A CQR in Puilladobhrain (thick sticky clay) for example is likely to be buried and stuck so hard that a gale won't break it out never mind turn it.

So, much depends on the type of bottom you are anchored on, as well as the type of anchor and how deeply it has been set.

 

[Lee_Shaw]

You're not missing anything; the boat will either work the anchor around in a turn, if the veer happens gradually, or if there is a sudden 180 degree shift then the anchor is quite likely to be pulled out backward and then have to re-set itself on its own. If it doesn't - well this is one of the main causes of dragging.

You have a point re "setting the anchor 'to the book'" - to some extent, the manual process of setting the anchor should be a test of the seabed and the anchor's ability to handle it on its own. In other words, if it requires fluffing about, and 5 attempts at dragging the thing for 50 meters before it bites, then it's not likely to handle an upset too well. A good anchor, given adequate rode and adequate sea-bed, will set instantly and reliably and require no special secret or technique. You need to worry about the situation etc, but anchors don't require 'operating'. Despite the rumors, it is not rocket science

I agree, my anchor just resets itself.

 

[MarkJohnson12345]

In Padstow, in a 2+ knot current, my boat was sitting in front of the anchor!

We set the anchor when its put down, after that as long as the holding is good, and you have heavy enough gear, and the wind and current are not too horrendous, just relax and enjoy your stay.

Only time I have dragged is in very light sandy bottom in a bay in the Scillies, and of course a c**p anchor who shall be nameless.................

And it eventually set anyway

 

[asteven221]

Thanks everyone for the advice. Glad to see that my idea of physics stacks up in real life. Hmmmmmm...... now I know why many of us use marinas / moorings! Peace of mind.

One other thing whilst airing my anchoring fears to the world! There are always reference's to choosing the right seabed for your anchor type. How do you know - for certain? Sure the pilots give a general guide, but in the last two times I have plucked up the courage to anchor, I could clearly see the seabed and watched my anchor land. I could even watch it start to set itself. As I could see the bottom I could also see that I in this anchorage of "hard sand" that there were significant areas of kelp. Possibly 60% perfect sand and 40% slippery kelp. My point is that if I hadn't seen the bottom then there was every chance I would drop the hook on top of the kelp. Or worse - in the sand - anchor dug in nicely - sound asleep - big wind shift - anchor trips - try's to reset itself but can't because of the kelp and hey presto - big problem! There seems a lot of unknowns and chance in this anchoring malarky! I suppose it's a case of managing the unknowns as best as possible using knowledge and experience. There will always be a margin of being lucky or unlucky. Am I about right in that summary?

 

[craigsmith]

There are significant differences in the relative performance of different anchor types, and many of the fears and problems commonly cited with regard to anchoring owe more to the flaws of old generation designs than any fundamental realities or impossibilities concerning specific bottom types. Some new generation anchors will install much greater confidence in you and your crew even in those more difficult sea-beds, including kelp, once experience is gained.

Experience again will help you with regard to familiarizing yourself with the anchoring process. A foot pressed firmly on the chain while the anchor chain is straightened and the anchor is set will give all the feedback necessary - to hazard a guess at the sea-bed type, and more importantly let you know when the anchor has set, how long it took to do so, and whether it took up sharply first time etc.

 

[jimbaerselman]

Why the oldies used to moor . . .

Blowed if I know how to get this quote stuff working . . .

Quote:
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Logic tells me that the boat will now be not anchored - for want of a better expression! So am I right to say that unless the anchor resets itself (by luck? can't be anything else) we are enroute to disaster
------------------------------- end of quote

You are exactly right. Not only that, but you've woken Bilbo up from his yawns so that he's made a highly accurate prediction of the responses you'd receive.

In the days when marinas were as rare as hen's teeth, anchors were also routinely difficult to set (yes, I'm that old). So the possibility of upset through turn of tide, or a shift of wind, had to be removed.

The method was to moor the boat. Moor it by dropping the main anchor, dropping astern by almost full scope, digging that anchor in with a firm reverse on the engine (or sturdily backing the sails). Then the kedge was dropped (maybe by sending a dinghy off astern) and the main rode was shortened (to whatever was needed for the depth and tidal range) while the kedge rode was paid out. Then the kedge was dug in by hauling it against the resistance of the main anchor (if it didn't dig, it was re-laid!). Finally, the rodes were lashed together, and paid out enough so that the join was below the keel (and prop if you had one).

Now, whichever way the wind/tide changed, you'll then be lying to a fully set and tested anchor, however ancient the anchor, however imperfect the bottom.

Modern anchors don't remove this need, however strongly worded the sales blurb. They may increase the probability that a reset will work, but they still may been dropped on an unsuitable bottom; say, where the original set was pure luck - snagging a lonely rock ridge.

 

[snooks]

I am obviously missing something here! Hopefully someone will be along to explain how this all works so I can get a good nights sleep at anchor.

The anchor should reset, and why not?

Think about how you anchor in the first place...Oh alright then, think about how I anchor for a moment. We go into wind/tide whichever is the dominant force. When the boat is stationary I lower the anchor, till it touches the bottom, by this time we will usually be going slowly with the dominant force, I keep paying out chain until I reach the length I want. Let it rest for a bit so the boat has taken up the strain naturally, then bung the engine astern to make sure it's held.

All the boat is doing differently by going 180º is having all the weight of the chain down low helping the anchor set in it's new direction, and not checking it has held firm.

My thinking is that if you've held once in a particular area, holding again a few metres away shouldn't cause too much trouble.

Well that's what I'm thinking before I go to sleep

 

[noelex]

A few observations about my Rocna anchor in the often hard sand and the weed in the Med (but few currents).

It has never failed to set first time.
The anchor has never taken more than 2 or 3 feet to set
Diving over the anchor during or after a significant wind shift shows the anchor stays buried and simply swivels around.
It will set even when traveling backwards at reasonable speed (make sure you are hanging on and the cleat is OK for the forces involved).

Given these observations it seems the chance of it not "resetting" with a change of direction has to be low.
an anchor alarm on the GPS is always a good insurance and set it to awake me with a significant change in wind direction.

 

[DaiB]

Resetting anchor

Quote" A few observations about my Rocna anchor in the often hard sand and the weed in the Med (but few currents).

It has never failed to set first time.
The anchor has never taken more than 2 or 3 feet to set
Diving over the anchor during or after a significant wind shift shows the anchor stays buried and simply swivels around.
It will set even when traveling backwards at reasonable speed (make sure you are hanging on and the cleat is OK for the forces involved).

Given these observations it seems the chance of it not "resetting" with a change of direction has to be low.
an anchor alarm on the GPS is always a good insurance and set it to awake me with a significant change in wind direction. "
unquote

Basically confirm the above with our Rocna but we have had some failures to set first time in weed. I quite often dive and see how the anchor has set and certainly on sand the Rocna goes in first time every time with no sign of taking more than a metre to set. Our CQR quite often took a few metres to dig in on the same bottom. Also noted that with a large direction change the Rocna seems to reset virtually on the same spot.