Anchoring in a blow?

We anchored very successfully on two three-year cruises in the USA/Caribbean - mainly sandy bottoms; we never dragged and had enough confidence in our system to sleep easily and to leave the boat for long periods unattended.

My system of choice for heavier weather and confined anchorages was two CQRs on all chain set at about 45 degrees apart but, when appropriate, we would use a Bahamian moor or a single anchor.

Each anchor system alone was capable of holding the boat. I certainly wouldn't use a two-anchor system in which each anchor could only handle half the load.

I used 10' long nylon snubbers to absorb snatching and keep things quiet in the forepeak.

We also carried a big Danforth which I like in mud.
 
Just done a rudimentary experiment with scales, string and a bag of flour. Obviously two anchors set very close together will share the strain 50/50, and as the angle increases so will the strain on each. My results suggest as follows: if your vessel is pulling 300 kg, when the angle (between anchors) gets to 90 deg the strain on each is 200kg, and at 120 deg, 300kg. All assuming the load is shared equally at all times.

(Duh, of course it does. 120 deg, three equal angles.)
 
Choice

The most important feature of anchoring securely is not the anchor, nor is it the cable, but it is knowing how to use them. At what point on the ship to make fast the cable.
If two anchors, how much scope on each? How much angle of open hawse?

Almost any anchor will hold given a long straight constant pull. The trouble is the wind is never constant.

Anchoring in the lowe Danube with no wind but a steady current measured at seven knots, Hosanna lay quietly. With her cross sectional area above the waterline that represented the force of a force eleven steady wind . Who ever heard of a steady force eleven.

When wind-rode in a storm, it is important to attach the cable to the right place in the boat. In bad winds, I used to disconnect the bobstay and bend the anchor cable on the outer end of that.
 
We have used two anchors in a 'V' on many occasions, very successfully. The big advantage is that the angle of yaw of around 140 degrees with one anchor reduced to about 90 degrees with two. On one occasion, at Galixidhi, with 40 knots or so of wind in gusts from various directions, boats around us on single anchors dragged, without exception, I assume as the large yaw angle continued to work the anchor from one side to the other. Only three of about a dozen did not drag - ourselves, another boat also with two anchors in a 'V' and a traditional style Dutch boat, around 35 feet, who we found later had 100 metres of 3/8 chain out.
 
7kts = F11? Surely not

binch said:
...a steady current measured at seven knots, Hosanna lay quietly. With her cross sectional area above the waterline that represented the force of a force eleven steady wind...
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We don't have the same boat probably, but F11 is so far off the equivalent wind force to a 7 knot tide for my boat that I think there must be something is wrong with your calculation.

I've used a university modelling program to calculate hull drag for my 42' boat, as well as measurements and my own mathematical model using the normal formulae involving Froude and Reynolds numbers. All are in very close agreement.

The force required to propel her at 7 knots is just over 1.8kN, or about 185kg, whereas the force exerted on her by a steady F11 is more like 1400kg.

185kg is about the equivalent of 27kts of steady wind.

I plotted this below:

effective_wind.png


Your boat will be different, but not that much I'd have thought.
 
jdc said:
The force required to propel her at 7 knots is just over 1.8kN, or about 185kg, whereas the force exerted on her by a steady F11 is more like 1400kg.

185kg is about the equivalent of 27kts of steady wind.
Click to expand...

This looks to be about in line with my rule of thumb which gives 1kn of current as the equivalent to 3Kn of wind (with an upper limit of 5Kn of current, after which I find another anchorage).

However, although 1400kgf looks about right to me for a F11 on a 12.8m monohull, the force exerted by the wind varies with the square of the windspeed, so taking F11 as being 63Kn, 27Kn would give 257kgf. I'd expect 185kgf from about 23Kn.
Are you working the force calculations from measured cross sectional area, or are you using an empirical formula? Have you used less yaw in the 27kn calculation?
 
The biggest loads on your ground tackle by far are nothing to do with currents, and only a little to do with the steady force exerted by the wind. They are inertia loads, to do with halting the motion of a boat as it veers from side to side, added to the snatch if a boat is pitching in a seaway. So the first rule of anchoring is to reduce snatch loads. Let's assume you've done the sensible thing, you have a nice long nylon snubber rope fitted (20m or more gives a 4m spring - softens the snatch no end).

Now look at other snatch reduction techniques. First, reduce yawing. A line ashore, good. Not possible? Yes, on some boats you can rig an aft sail. On others you may be able to set up a bridle so the boat "kites". Alternatively, use a forked moor.

It has been assumed by many above (and the writer of the original link) that the purpose of the forked moor is to share the anchor load. Vyv says it right.

1. The first purpose of the forked moor is to reduce yawing (in boats prone to it in strong winds) so the snatch loads are reduced.
2. The second purpose is to ensure that the anchor which takes the snatch load is aligned with the load (with a single anchor, each snatch alternates from side to side by up to 90 degrees, good for a slow backwards waltz even with the best anchors)

Yes, it fails if the wind direction changes by 90 degrees or more, so it's not a solution for all places. But in strong trade winds, or Mediterranean Meltemi, ie, whenever the wind direction is highly predictable, it's a winner.

The original link also seemed to miss the point of the Bahamian moor, or running moor. This is not designed for high wind situations - it's designed to survive reversing currents (typically tidal) in reasonably sheltered waters. It ensures there's a ready set anchor in each direction, so there's no risk of a trip when the tide reverses.

Of course, it is a pity that sod's law means the boat always swings one way, so there's a good twist or two to untangle when departing. But that's easy when one rode is a rope. And in emergency, ropes can be cut.
 
Caladh said:
I'm intrigued to learn how the OP's who opt to lay a second acnchor out if conditions persist or worsen, actually go about the task ? It's all very well having the second anchor "ready" but laying it ......
Click to expand...

In virtually every case I row out our Fortress. The article of mine in YM a couple of months back shows the equipment used - Fortress FX16, 7 metres of chain, 16 mm Anchorplait. I have cleats both inside and outside the transom of the dinghy, I usually use the inside one to hang the chain, with the anchor in the water. Row upwind, then to one side to make an angle of roughly 90 degrees to the bower chain. I usually need directions from the crew on the foredeck for this, as it is difficult to judge. When she tells me that most of the Anchorplait is out I kick the chain off the cleat and the anchor falls to the bottom and, in the vast majority of cases, sets.

I have laid it by motoring the boat out at about 90 degrees and then dropping the Fortress but it is difficult to estimate angle and distance. We have recovered it like this though.
 
vyv_cox said:
I have laid it by motoring the boat out at about 90 degrees and then dropping the Fortress but it is difficult to estimate angle and distance. We have recovered it like this though.
Click to expand...
Yes, that's my usual technique. I take a lateral transit dropping the bower, then dig it in. I then motor in a quarter circle, helm away from the anchor to keep the chain stretched, and when I hit the lateral transit, drop the kedge. Then drop back and dig both in with a good dose of astern, first on one, then the other.

I see you and Owen are running a seminar on Boat systems in February? That should be interesting!
 
There was a gale in Preveza anchorage with forecast winds from the SW so over 30 boats stayed at anchor. During the night the wind swung to East which was wide open. Two boats snapped their chain.

I spoke with one of the owners and he said that his chain snapped DESPITE the fact that he ran his engine in forward gear but he was gratefull he did that because he could motor out immediatly his chain snapped to keep his boat off the beach.

I maintain that the tactic of motoring in forward to take the strain off an anchor is what caused his chain to snap. In a lull the boat will move forward towards the anchor. When the full force of the wind comes back it will drive the boat back at speed and put a snatch load on the chain.
 
BlueTwo said:
There was a gale in Preveza anchorage with forecast winds from the SW so over 30 boats stayed at anchor. During the night the wind swung to East which was wide open. Two boats snapped their chain.

I spoke with one of the owners and he said that his chain snapped DESPITE the fact that he ran his engine in forward gear but he was gratefull he did that because he could motor out immediatly his chain snapped to keep his boat off the beach.
Click to expand...

Based on the work I did for the article in the current YM, I suggest that the cause was that the chain was faulty. If the chain was well made, as the vast majority were, the deck fittings would pull out, or the anchor drag, before the chain broke. Load sustained at fracture by the only bad one was around half of that taken by the good ones. Examination by magnifying glass showed that the weld had never been made - the crop marks could be seen in the wire at the fracture face.
 
John Barry said:
...
However, although 1400kgf looks about right to me for a F11 on a 12.8m monohull, the force exerted by the wind varies with the square of the windspeed, so taking F11 as being 63Kn, 27Kn would give 257kgf. I'd expect 185kgf from about 23Kn.
Are you working the force calculations from measured cross sectional area, or are you using an empirical formula? Have you used less yaw in the 27kn calculation?
Click to expand...

It's a good spot. Actually what I did was take towards the lower end of the wind speed for F11, which is 56 - 63 kts, and read from my graph the equivalent speed to 185kg taking the upper value I could get by squinting! between them they amount to most or all of the difference you spotted. I did it that way because I was trying to get the minimum F11 force and the maximum 7kt current force and still show that they were miles apart.

By the way, your calculation of equivalent current and wind speed works pretty well while viscous drag is the only force which counts. But of course once wave drag cuts in then they diverge exactly as you say.

drag_eqn1.PNG


These give the red line of total force shown here (the red stars are the university modelling program's output).

range_under_power_8.png


It bears out your "up to 5kts" rule of thumb extremely well.
 
vyv_cox said:
Based on the work I did for the article in the current YM, I suggest that the cause was that the chain was faulty. If the chain was well made, as the vast majority were, the deck fittings would pull out, or the anchor drag, before the chain broke. Load sustained at fracture by the only bad one was around half of that taken by the good ones. Examination by magnifying glass showed that the weld had never been made - the crop marks could be seen in the wire at the fracture face.
Click to expand...

No doubt that faulty chain was the main issue, as you said the fittings would pull out before good chain would break.

However, as the boat that snapped the chain was running his engine I reckon it was a factor that sourced the weak link.

I didn't talk to the other boat that allegedly snapped their chain because I didn't know who it was. These events do tend to get exaggerated so maybe there was only one boat that snapped the chain.

BTW there is a first hand story on the web describing how his chain was considerably shortened when it wrapped round a coral head. The chain jerked for an hour before snapping. Strong winds. That was also weak link but frankly I woudn't know what to do about it. Change the chain every two years???
 
BlueTwo said:
BTW there is a first hand story on the web describing how his chain was considerably shortened when it wrapped round a coral head. The chain jerked for an hour before snapping. Strong winds. That was also weak link but frankly I woudn't know what to do about it. Change the chain every two years???
Click to expand...

Impact loading is reckoned to be x10 the force applied in steady pull at the same load. In the case you describe I can only suppose that to be the cause.

Grade 30 and 40 chains are very ductile, so a severe loading could be expected to deform the links well before the chain would break. Measurement of 11 links is a standard inspection item. I will be posting the figures on the website when I get around to it.
 
vyv_cox said:
In virtually every case I row out our Fortress.
Click to expand...

I second everything vyv cox has said here.

We also have a Fortress as our second anchor with substantial rope rode (and 5m chain). The holding power of the Fortress is better than most anchors according to many tests, and it has never dragged for us.

It is so easy to handle that I can even place it on a small float (together with the 5m length of chain) and swim out with mask and snorkel to lay it exactly where I want it to be.

I can also retrieve it from the dinghy.

Cheers,
Per
 
That's fine and dandy if you put the second anchor out when it's still quiet, but if the wind has already risen, you won't be rowing out with it. Better get yourself organised to be able to lay out a second anchor, from aboard. that generally means using engine to motor ahead, and out to one side. If laying a second anchor, I always rig it with a tripping line, and buoy, in case it has to be slipped in a hurry. You can always pick it up later.

Incidentally, the last time I used 8mm chain was on a Folkboat. If you don't think that your chain is forming a useful catenary, your chain is too light.:)
 
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