Do you dig your anchor in ?

[Storyline]

..... but RYA has this summer produced advice with help from BORG and the Wildlife Trusts for anchoring in areas of known sensitivity like Studland, which suggests you do not try to plough the anchor in as described above. More importantly the RYA advice is when raising it, to pull the boat up to the anchor under power to avoid dragging it through the seabed.

What does the panel think? I know well enough the need to ensure the anchor has to be set properly as I nearly always choose to anchor somewhere quiet and out of the way for the night, rather than go alongside.
....

So they advocate we drop our anchors and do not actually anchor

....
Back on topic, you only have to watch the Jamboree at East Head in Chi Harbour on a Bank Holiday Saturday when the tide turns at 2.00am! Cries of rage, and crunching of expensive Glass fibre echos for half a mile around as boats take up new positions to each other, and the odd one starts dragging through the crowd!

That is because they are not actually anchored in the first place.

As regards these sensitive areas it might be an idea for the conservationists to lay moorings, thereby just disturbing the sea bed once when they are laid.

 

[John Barry]

...Somewhere on the boat I have got a formula devised by the theoretical physicist we usually sail with when we were gale bound at anchor one day. Using this he worked out the force needed to lift the last link next to the anchor on a 6:1 scope. It was not enormous but I think it was more than a 3-4 knot tide could produce.

You don't need a theoretical physicist, you need Scuttlebut.

Force = W(Lr^2-H^2)/(2H) where W is the weight of the rode (kg/m), Lr is the length of the rode (m), H = the height to which the rode is lifted (depth of water) (m)

John

Edit: I realize that a force should be in Newtons, but the above gives kgf, which is what most people are more familiar with (the rode weight is actually kgf/m, and should be the weight in water of course).

 

[webcraft]

I guess on average we spend around 30 - 40 nights at anchor during the sailing season on the west coast of Scotland. Like many others, I suspect, we quite often watch new arrivals in the anchorage laying their anchors. It always amazes me that the overwhelming majority of people drop their anchor, gently motor backwards and then start looking at transits.

I have got to know what it takes for us to dig our anchor in and it goes something like this:

Wait for/encourage boat to line herself up to the direction we want to anchor.
Put engine into reverse. At 1800 revs the cable starts to come up. At 1900 revs the cable starts to get taut and the anchorplait snubber starts to stretch (I do not know how much catenary we have). At 2000 revs we are on the anchor so we start to check transits and my wife has her foot on the chain so she can feel what the anchor is doing. At this point we usually move a metre or two backwards and normally the anchor digs in. If in mud we might still move a little bit back then stop, if in sand we stop completely. If there is not much wind forecast I usually then just increase the revs up to 2200 just to check all is OK but if there is anything more than a F6 forecast I give it full welly for 30 seconds or so to be certain.

The only reason we do not always dig the anchor in at full revs is that the Rocna will sometimes pull out in muddy bottoms and it might take a few goes to get fully on which seems pointless on still nights.

As said above, this behaviour in other boats is very much the rarity.

What do you do ?

We do pretty much exactly the same as you.

- W

 

[stevebrassett]

To see what the holding is like. If it sets fine once then I reason that it will probably continue to do so.

That makes a lot of sense.

 

[GHA]

You don't need a theoretical physicist, you need Scuttlebut.

Force = W(Lr^2-H^2)/(2H) where W is the weight of the rode (kg/m), Lr is the length of the rode (m), H = the height to which the rode is lifted (depth of water) (m)

John

Would
F = w / (8*h) * ((2Lr)^2 - 4*h^2)

Work as well?

Straight out of Google

 

[noelex]

One thing I have never really understood is that people dig the anchor in nicely when they arrive, but what happens when the tide turns?

Good anchors remain buried when set properly and just swivel around under the surface. They develop a slight list but remain completely buried. I have seen this many times with wind reversal (not with current because you cannot dive then). It takes around 20k of wind to swivel the anchor around. The equivalent force with current is a lot of current so in most cases due to current alone the anchor will not move, only the chain.

 

[John Barry]

Would
F = w / (8*h) * ((2Lr)^2 - 4*h^2)

Work as well?

Straight out of Google

Well, it resolves to the same formula, but I think it's less clear.

John

 

[John Barry]

Good anchors remain buried when set properly and just swivel around under the surface. They develop a slight list but remain completely buried. I have seen this many times with wind reversal (not with current because you cannot dive then). It takes around 20k of wind to swivel the anchor around. The equivalent force with current is a lot of current so in most cases due to current alone the anchor will not move, only the chain.

1kn of current will have the same effect on a boat as 3kn of wind. This is an approximation, as the actual result depends on windage and underwater section, but it works quite well for most boats. Don't use it with more than 5kn of current (find somewhere else to anchor instead).

 

[GHA]

Well, it resolves to the same formula, but I think it's less clear.

John

Either of those are clear to you? Hats off

 

[prv]

suggests you do not try to plough the anchor in as described above.

So what do they propose instead? As my dad is fond of saying, "you can't push on a rope" - the only thing we can do to an anchor is to pull on the chain. Either we pull it after dropping it, or the wind and tide pull it later. Unless your anchor is so massive that it acts like a Norfolk mud-weight and holds the boat without digging in, it's going to get pulled into the ground sooner or later.

With a decent modern anchor, the "ploughing" idea is irrelevant anyway - I've dived on my Spade several times and there are never any drag marks leading up to it. Playing with it in the garden just after I bought it, it was quite clear that as soon as you pull on the chain, the tip dives into the ground. The damn thing is positively vicious - it even tried to penetrate my living room carpet when I started to pull it round for a better view after bolting the two halves together.

More importantly the RYA advice is when raising it, to pull the boat up to the anchor under power to avoid dragging it through the seabed.

Why do they imagine an anchor which was holding the boat fine will suddenly start to drag when you pull yourself towards it?

Pete

 

[rotrax]

Why do they imagine an anchor which was holding the boat fine will suddenly start to drag when you pull yourself towards it?

Pete

I wondered that myself.

 

[vyv_cox]

Why do they imagine an anchor which was holding the boat fine will suddenly start to drag when you pull yourself towards it?

I would imagine it might when the scope falls below about 2:1, although I cannot see this doing much damage to the bottom. Having seen the antics of people motoring towards their anchor when recovering it from stern-to berthing, often motoring across other peoples' anchor chains and dragging them for many metres, I think hauling the boat to the anchor is by far the least damaging of the two.

 

[noelex]

Unless your anchor is so massive that it acts like a Norfolk mud-weight and holds the boat without digging in, it's going to get pulled into the ground sooner or later.

In most cases an anchor will set itself as the wind builds up.

However to ideally set an anchor needs a slow steady increase in pull. With the a strong pull only when the anchor reasonably buried. If the wind picks up suddenly and the anchor has not been set this criterion is not met. Anchors can set with a sudden jerk, but it not as reliable as slow steady increase. Once an anchor is sliding at reasonable speed it rarely digs in and it offers little resistance to slow the boat down.

There are also occasions when the anchor does not set. Sometimes for a mysterious reason other times it has landed on some debris etc.

Once an anchor has dragged any reasonable distance it often picks up debris such as weed and it will never set.

Setting the anchor achieves two things:

1. If done correctly the slow steady increase in pull is the ideal way to bury the anchor under the substrate. Once it is buried it usually remains so even with a change in the direction of pull.

2. It provides a test that the anchor is holding. A steady slow increase and then full reverse for about 30 seconds on my boat is equivalent to about 30-35 knots of wind (assuming a lightish wind as well to add to the pull) If the anchor passes this test I know I am reasonably safe up to this wind strength. If it does not hold I try again, or move to a different area


There is a theory that anchors set better when given a rest before pulling on them. I don't understand why this is so (especially with little or no wind), but it does seem to have an effect. Some people therefore drop the anchor and then set the anchor a few minutes or hours later. With new generation anchors this is usually totally unnecessary, but it is useful as a last ditch attempt if you cannot get any anchor to set any other way. It may just be a placebo.

 

[ctva]

I guess on average we spend around 30 - 40 nights at anchor during the sailing season on the west coast of Scotland. Like many others, I suspect, we quite often watch new arrivals in the anchorage laying their anchors. It always amazes me that the overwhelming majority of people drop their anchor, gently motor backwards and then start looking at transits.

I have got to know what it takes for us to dig our anchor in and it goes something like this:

Wait for/encourage boat to line herself up to the direction we want to anchor.
Put engine into reverse. At 1800 revs the cable starts to come up. At 1900 revs the cable starts to get taut and the anchorplait snubber starts to stretch (I do not know how much catenary we have). At 2000 revs we are on the anchor so we start to check transits and my wife has her foot on the chain so she can feel what the anchor is doing. At this point we usually move a metre or two backwards and normally the anchor digs in. If in mud we might still move a little bit back then stop, if in sand we stop completely. If there is not much wind forecast I usually then just increase the revs up to 2200 just to check all is OK but if there is anything more than a F6 forecast I give it full welly for 30 seconds or so to be certain.

The only reason we do not always dig the anchor in at full revs is that the Rocna will sometimes pull out in muddy bottoms and it might take a few goes to get fully on which seems pointless on still nights.

As said above, this behaviour in other boats is very much the rarity.

What do you do ?

Ditto apart from feeling the chain as I just watch the transits.

 

[snowleopard]

The most important part of setting an anchor is the cup of tea. If I motor backwards straight away I'll charge back dragging the anchor after me. My routine is: 1. drop the anchor and enough scope 2. have a cup of tea while wind and tide ranges the cable and digs the tip in 3. if unknown bottom and/or wind expected, reverse while watching transits.

 

[NormanS]

Should this thread not be in the Motor Boat forum?
Does nobody else anchor under sail? We frequently do.

 

[prv]

Does nobody else anchor under sail? We frequently do.

I used to more often than not in Kindred Spirit. Anchoring and moorings, coming or going.

However, since moving to Ariam, which is a thoroughly modern boat with wheel, winches, fully-battened main and stackpack, I now see why fewer people do nowadays. The rig and the cockpit layout just don't lend themselves to close manoeuvring. Everything takes longer to do, you can't scandalise the main or stop it driving forwards, and you can't necessarily get it down in a hurry either. She just feels awkward and unhandy where I could get Kindred Spirit to do whatever I wanted to, with the tiller between my legs and every line within reach and light enough to pull with one hand.

I suppose I'll get used to her in time - and there's no denying that in open water (which is where we spend most of our time) the sailing performance is vastly better.

Pete

 

[jimbaerselman]

What do I do?

For a lunch stop in light weather and constant tidal stream when I'm staying aboard, I drop a weight on the bottom with just enough scope so it won't slide across the bottom. Makes it easy to pull up again after an hour or two. I'm a lazy feller . . . but I keep an eye on a transit.

If I'm leaving the boat, and there's no likely tide turn, nor wind direction change, I do as the OP suggests.

If I'm leaving the boat, or going to bed, and the wind is strong enough to cause the boat to sail from side to side, or if I'm expecting a wind shift of up to 90 degrees, but no tidal change, I make a forked moor with a second anchor, digging both in per OP. Often necessary in Caribbean trade winds.

If I'm leaving the boat, or going to bed, and I'm in a reversing tidal estuary, I make a running moor. Bower anchor dropped up tide, let out to full scope, dug in; then second anchor dropped (or sent down tide in the dinghy). Then the bower is pulled in the desired scope, letting out the second rode. Set the second anchor by pulling it in against the bower. Lash both rodes together, and lower them well below keel level. Then you either lie to one, or the other.

And, no, I'll never go to sleep or leave the boat if there's a probability that the anchor may be called upon to re-set in another direction.

I know, lots of other boats anchored around you may not take these precautions, and as a result may not predict your boat's swinging circle correctly. These are also the boats that sometimes find themselves unexpectedly dragging after a wind or stream change.

Of course, most of my favourite tidal turn estuaries (Lezardrieux, Treguier are a couple) now have marinas or mooring buoys in place, so anchoring with such caution is no longer a necessity for most cruising folk. Except vessels too big for the moorings or marina berths - such as Provident or Hoshi, big gaffers over 20m LOA (ignoring bowsprits). And I learnt on those . . . in the days before marinas. Old habits die hard.

 

[oceanfroggie]

One thing I have never really understood is that people dig the anchor in nicely when they arrive, but what happens when the tide turns? My anchor just resets in the mud, but then I didn't have to dig it in anyway.

Do people re-dig in their anchor? Does it reset itself (in which case why the need to dig it in in the first place)?

The main reason I dig in on arrival is to test what the holding is like on the bottom. There are some places with poor holding, so better to know from the start (eg shale rock, slate bottom, slab rock bottom with no sand, mud nor silt). I put my hand on the chain just beyond the roller and can feel the vibration if the anchor is scraping along a rocky bottom. This works no matter how much chain was put out. Obviously in anchorages we use regularly with proven good holding we tend not to worry as much about this.

 

[toad_oftoadhall]

Does nobody else anchor under sail? We frequently do.

TypicallyI just carry on with whatever I'm currently doing. If sailing, anchor under sail. I wouldn't bother to turn the engine on specifically to Anchor, but I wouldn't start sailing either. If I'm going deep into the Crowlin Isles (for the sake of argument) it's gonna be with the engine.

Obviously if you have a technique that requires an engine you don't have the choice.

Of course, some boats don't have engines.

I do wonder if some people are over-thinking this.