westernman
Well-Known Member
So I have the anchor dug in, full power on reverse and it holds.
In how much wind will the anchor hold?
In how much wind will the anchor hold?
Anchoring and wind strength
- Thread starter westernman
- Start date
noelex
Well-Known Member
Assuming little wind when setting the anchor, I have read that sailboats can only apply force with the motor equivalent to about 20K of wind.
The chain does go bar tight and the snubber groans in protest on my yacht when I set with 2300 revs and a maxiprop. It certainly looks more than the equivalent of 20K knots of wind, but the static force of a motor in reverse and the dynamic forces seen when the boat sheers around at anchor are hard to compare.
With over 30k of wind the anchor will bury more than I can achieve under motor alone so I suspect the answer in my case is between 20 and 30 knots and in yachts without a feathering prop the answer is closer to the 20K often quoted.
The chain does go bar tight and the snubber groans in protest on my yacht when I set with 2300 revs and a maxiprop. It certainly looks more than the equivalent of 20K knots of wind, but the static force of a motor in reverse and the dynamic forces seen when the boat sheers around at anchor are hard to compare.
With over 30k of wind the anchor will bury more than I can achieve under motor alone so I suspect the answer in my case is between 20 and 30 knots and in yachts without a feathering prop the answer is closer to the 20K often quoted.
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webcraft
Well-Known Member
F6
If you are one of these, what it means is that you don't trust your anchor to hold if the wind gets up to F6 during the night. Does that mean you climb out of your cosy bunk when you hear the wind howling in the rigging and spend the night on anchor watch?
Your anchor should always be able to take a solid pull astern at or near full power on the engine IMO.
(Runs for cover)
- W
Probably about right but . . . lots of people don't use full astern to set/check the anchor though - in case it doesn't set.
If you are one of these, what it means is that you don't trust your anchor to hold if the wind gets up to F6 during the night. Does that mean you climb out of your cosy bunk when you hear the wind howling in the rigging and spend the night on anchor watch?
Your anchor should always be able to take a solid pull astern at or near full power on the engine IMO.
(Runs for cover)
- W
craigsmith
Well-Known Member
It depends entirely on the bollard pull of your boat in reverse. You then need to equate that with the windage level required to generate the same force. You can take a guess by considering what wind level is required to stop your boat from making headway at full throttle in reverse, although windage from ahead is less than that from aft.
You must also realize that it is the peak force levels that matter to the anchor, not the constant or average loading. The anchor will first drag when the rode snatches at the top of a wave, or when the boat is sailing around a lot. This is not always directly related to the wind strength.
It isn't always wise to talk about "full throttle" setting the anchor - in difficult bottom types this will not work, especially with a powerful engine, even with best performing anchors; you'll just straighten the rode, apply a lot of upward force on the as-yet-unset anchor, and lift it out - gradual setting is a better technique for tough locales.
shmoo
Well-Known Member
Anecdotal I know but we spent a night off Bembridge a few years back with fairly continuous 25 -35kts and gusts of 38kt (indicated at top of mast): 17kg Bruce, 25m 10mm chain and 50m 20mm anchor-plait, in about 5-8m water. Dragged about 10-15m / hour at worst, when tide flow was in same direction as wind, or about 60m over the whole night. Anchor came up very clean indeed!
As I recall the waves weren't that big, and there was very little yawing (and almost no yawning) although the boat does yaw at anchor under some circumstances.
As I recall the waves weren't that big, and there was very little yawing (and almost no yawning) although the boat does yaw at anchor under some circumstances.
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Salty John
Well-Known Member
Typically a sailboat with a good prop will produce bollard pull of around 7.5kg per horsepower. If you get 20 HP at the prop, you will apply a load of 150kg on your ground tackle.
The same boat in 25K of wind will apply a load of 100kg, in 35K it will apply 180kg and in 60K it will apply 550kg of load.
A typical 35 foot cruiser will probably be able to apply the equivalent of around 30K of wind by pulling at maximum revs with an efficient prop.
It takes a little while for an anchor to set to it's full potential, it needs time to wiggle itself into the seabed, so it's better to apply the power progressively.
The same boat in 25K of wind will apply a load of 100kg, in 35K it will apply 180kg and in 60K it will apply 550kg of load.
A typical 35 foot cruiser will probably be able to apply the equivalent of around 30K of wind by pulling at maximum revs with an efficient prop.
It takes a little while for an anchor to set to it's full potential, it needs time to wiggle itself into the seabed, so it's better to apply the power progressively.
Chris_Robb
Well-Known Member
Knowing the bottom in Priory bay quite well, its very good holding once the pick is in. I am very surprised that you dragged at all. This means that your anchor is not providing enough digging force, but going like a plough through the ground. Perhaps you should change it. I had heard that Bruce anchors have never been the same since the oringinal Bruce ceased manufacture, the copies being somewhat less efficient.
westernman
Well-Known Member
From experience, my boat will sail down wind under bare poles in an F8 somewhat slower than I can motor at full throttle in calm conditions.
Assuming that going the other way all things are equal (they are not quite), then full throttle reverse should generate a constant pull equal to more than an F8 right?
Also when we test the anchor in reverse at full throttle, that is in addition to any existing wind etc. So presumably it is already blowing a F4 when we do this, we can sleep soundly if it gets up to an F9 during the night????
Assuming that going the other way all things are equal (they are not quite), then full throttle reverse should generate a constant pull equal to more than an F8 right?
Also when we test the anchor in reverse at full throttle, that is in addition to any existing wind etc. So presumably it is already blowing a F4 when we do this, we can sleep soundly if it gets up to an F9 during the night????
westernman
Well-Known Member
I was talking about applying full throttle once the anchor is set of course.
The only case where the anchor will catch when whizzing backwards under full throttle is when you hook a submarine cable ;-)
lw395
Well-Known Member
Obviously hard to answer because the ratio of thrust to windage varies from boat to boat.
However, I would think a greater variable would be the amount of chop on the water. If there is a fetch of a mile or so across the harbour, or any swell, it is a very different proposition to being on flat water.
The peak loads exerted by even quite small waves can be significant. Once the bow starts moving up and down even a few inches, it can be like hammering on the anchor, if the rode is light enough to go anywhere near straight.
These forces will add to the steady forces of the wind. The peak of the combined forces may be much more than the average force.
I think the 'ideal conditions' answer might be a lot of wind, whereas the 'reliable minimum performance' will be a lot less. Unless you are anchored in a small atholl or something!
Neil_Y
Well-Known Member
Depends on the size of the swell or the fetch into the anchorage, if the swell wraps around the island or land you are sheltering behind.
lw395
Well-Known Member
If you were fairly close in, the wind at the top of the mast would be significantly more than at deck level, so the whole windage is not exposed to the whole wind.
Sounds like a pretty respectable performance from your anchor though, I don't think I'd want to test it much further. If the tide is flowing the same way as the wind, it doesn't have to swing far before you've run out of shelter?
shmoo
Well-Known Member
Before the OP question can be answered perhaps we need to agree on what our expectations are. There are two quite different responses to my account above:
"Perhaps you should change it." [Chris_Robb]
and
"Sounds like a pretty respectable performance from your anchor" [lw395]
I must say I didn't expect the anchor to hold with no dragging at all under those conditions, but perhaps my expectations are too low.
"Perhaps you should change it." [Chris_Robb]
and
"Sounds like a pretty respectable performance from your anchor" [lw395]
I must say I didn't expect the anchor to hold with no dragging at all under those conditions, but perhaps my expectations are too low.
Salty John
Well-Known Member
The OP's original question was about the correlation between the holding power of his anchor when tested under power and when tested by the wind.
A mid-sized boat, say 35', can simulate conditions in wind up to around 30 Knots or so with a typical size engine and prop combo. If you sail a tug you could simulate anchoring in hurricane conditions!
I think the thread is dragging!
A mid-sized boat, say 35', can simulate conditions in wind up to around 30 Knots or so with a typical size engine and prop combo. If you sail a tug you could simulate anchoring in hurricane conditions!
I think the thread is dragging!
Chris_Robb
Well-Known Member
This is a difficult one to quantify. My main experiance with anchors are CQR and Manson.
The CQR would - once I had managed to set it - gently and predictably "plough" its way accross the bottom. The experiance with the Manson, is significantly different. We rode out a gale in Brittany this summer and we had not appeared to have moved an inch after 48 hours. Recovering the anchor was really hard work, having to be nudged out with the engine, and I suspect the rising tide. I thought that we had fouled something on the bottom. It appears that rather than plough across the bottom, it just digs itself in deeper.
The CQR would - once I had managed to set it - gently and predictably "plough" its way accross the bottom. The experiance with the Manson, is significantly different. We rode out a gale in Brittany this summer and we had not appeared to have moved an inch after 48 hours. Recovering the anchor was really hard work, having to be nudged out with the engine, and I suspect the rising tide. I thought that we had fouled something on the bottom. It appears that rather than plough across the bottom, it just digs itself in deeper.
dur
Well-Known Member
Also makes me wonder about the loads on the chain / shackles and cleats or sampson post when the boat starts to pitch. As said, the constant wind load on its own must be pretty small beer compared with the inertia behind 5 tons of boat rearing its head.
westernman
Well-Known Member
So up to what wind strength do you guys sleep soundly when at anchor?
At what point do you feel the need for a permanent anchor watch?
At what point do you feel the need for a permanent anchor watch?
jimbaerselman
Well-Known Member
Which implies you're not a motor boat, nor even an over-engined motor sailor!
Salty John
Well-Known Member
In some anchorages a Force 10, in others a Force 4. Is it a hurricane hole or an open anchorage? Can it turn into a deathtrap if the wind shifts, or picks up?
A lot depends on the circumstances: How crowded the anchorage is, what are the implications of dragging (onto rocks, or out to sea), whether the anchors are set well or you are a bit concerned about the holding capability in this particular anchorage. Do the currents reverse during the night?
Then there is the concern about other boats; if some boater has come in and plonked himself upwind of you on a lunch hook and bit of string you are going to be more inclined to keep an anchor watch than if you're on your own.
I reckon you have to weigh up each anchoring situation and manage it accordingly.
