noelex
Well-Known Member
Anchor thread
- Thread starter geem
- Start date
geem
Well-Known Member
We power set our anchor and often bury it. Maybe that's why it doesn't drag. According to the engine power graph at 1800rpm I am using 60hp?
NormanS
Well-Known Member
I note that you try to anchor where it's not subject to 30 knot winds. That's fine for you.
For those of us who anchor where it quite regularly reaches into the 60s (and on one scary occasion 74 knots), an anchor which might be bigger/heavier than the manufacturer's recommendations gives some extra reassurance. As I have said before, when it's blowing a hoolie, I have never wished that I had a smaller/lighter anchor.
[194497]
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I definitely take the point about not spending more money than necessary, and safe handling is an issue too.
But intuitively at least I would have thought that a heavier anchor would be more likely to set, all things being equal, given that it would put more weight on the tip and thus be more likely to penetrate into the sea bottom.
I think I'll maintain my policy of generous but not ridiculous anchor sizing, long rodes with nylon rope as well as chain, and at least two full sets of ground tackle on board.
It's interesting stuff though, and great to read other people's experiences, which are much broader than my own - my sailing has mostly been East Coast and Bristol Channel, with a lot more time spent thinking about tides, depths and rode length than about holding power and setting.
Koeketiene
Well-Known Member
I too subscribe to AAC.
While the advice on there can at times be invaluable, John's word is not always gospel (even though he can sometimes act as if it is).
There have been occasions where his advice conflicts with my own - first-hand - experience.
geem
Well-Known Member
What anchor are you referring to? If it's an old generation of anchor it likely has half the ultimate hold of a new anchor. So yes, maybe going up a size on an old anchor may have some merit if you are likely to test that ultimate hold. If its a new anchor you are already there without oversizing.
[194497]
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I was thinking more about initial setting - with more weight on the tip, a bigger anchor seems more likely to penetrate and start digging itself in.
But really just natural caution, I think...
noelex
Well-Known Member
For those who advocate following the sizing tables, make sure you read the fine print. For example, the Fortress table is only appropriate up to 30knots of wind and they insist on the deployment of two anchors if “a change in wind or current” is expected.
Fortress state:
“for storm conditions we recommend using an anchor one or two sizes larger”.
Fortress Selection Guide - Fortress Marine Anchors - PDF Catalogs | Documentation | Boating Brochures
Fortress state:
“for storm conditions we recommend using an anchor one or two sizes larger”.
Fortress Selection Guide - Fortress Marine Anchors - PDF Catalogs | Documentation | Boating Brochures
RunAgroundHard
Well-Known Member
NormanS
Well-Known Member
I note that you try to anchor where it's not subject to 30 knot winds. That's fine for you.
For those of us who anchor where it quite regularly reaches into the 60s (and on one scary occasion 74 knots), an anchor which might be bigger/heavier than the manufacturer's recommendations gives some extra reassurance.
It would appear that I had already submitted this. Old age comes to us all, if we're lucky. ?
boomerangben
Well-Known Member
I think the mechanics of anchor holding is extremely complicated and my earlier post was written wondering if a shallow buried big anchor is better than a deeply buried smaller anchor. Clearly anecdotal evidence here suggests it is.
In my elemental understanding of soil mechanics, the deeper the burial, the higher the shear force the soil can withstand before failure. In other words, the deeper you bury the tighter the hold the soil has on the anchor. I think it’s fair to say for a given anchor, the deeper it is buried, the better it holds. What then matters is the relationship between the rode force and the depth of burial and the depth of burial verses the holding capacity. Are the relationships simple and linear or more complicated? Perhaps (and I don’t know this) an oversized anchor will bury so far where it’s holding capacity is say 2000kg but a smaller anchor can bury deeper with the same ride load and thereby make use of stronger soil and give a capacity of say 2500kg. Or big anchor in weak soil not as good as small anchor in strong soil? I don’t know enough about soil mechanics and would be interested if someone on here had that knowledge.
From a practical point of view, as Neeves points out, it’s probably academic anyway since we never get close to theoretical holding capacities anyway - the 3D dynamic reality of riding to anchor makes the 2d static pull presumably used in most anchor tests rather simplistic.
I like NormanS’ thoughts about reassurance, which is the fundamental need we have of our anchors - will our anchor let us sleep well. It is only time and experience of our own boats that provides that and I wonder if the OP is any the wiser for this discussion.
Ps I have an over sized anchor because it’s the smallest they make.
In my elemental understanding of soil mechanics, the deeper the burial, the higher the shear force the soil can withstand before failure. In other words, the deeper you bury the tighter the hold the soil has on the anchor. I think it’s fair to say for a given anchor, the deeper it is buried, the better it holds. What then matters is the relationship between the rode force and the depth of burial and the depth of burial verses the holding capacity. Are the relationships simple and linear or more complicated? Perhaps (and I don’t know this) an oversized anchor will bury so far where it’s holding capacity is say 2000kg but a smaller anchor can bury deeper with the same ride load and thereby make use of stronger soil and give a capacity of say 2500kg. Or big anchor in weak soil not as good as small anchor in strong soil? I don’t know enough about soil mechanics and would be interested if someone on here had that knowledge.
From a practical point of view, as Neeves points out, it’s probably academic anyway since we never get close to theoretical holding capacities anyway - the 3D dynamic reality of riding to anchor makes the 2d static pull presumably used in most anchor tests rather simplistic.
I like NormanS’ thoughts about reassurance, which is the fundamental need we have of our anchors - will our anchor let us sleep well. It is only time and experience of our own boats that provides that and I wonder if the OP is any the wiser for this discussion.
Ps I have an over sized anchor because it’s the smallest they make.
geem
Well-Known Member
Not all anchors are the same. A Bruce doesn't really have much of a tip. Some anchors have a sharper tip than others. In my mind it's about design not weight. If you can bury a large fluked anchor successfully then you will achieve ultimately more holding power than the same small fluked anchor. Not all large fluked anchors need to be heavy to achieve this holding power. Look at the Fortress anchor. Will a heavy anchor penetrate this tough seagrass that we experience here in the Caribbean? Not necessarily unless the tip is as sharp as the smaller version of the anchor. Then it will be able to apply more pressure on the seabed. Does the anchor actually put most of its weight on the tip when you are setting the anchor? Not necessarily. It depends on the design. I think there is so many variables it is hard to say that a heavier anchor will set better than a smaller anchor. If it will set deeply it will provide more ultimate hold but first you have to set it in what may be a difficult seabed. The seabed here is so unlike what we used to experience in the UK when anchoring that I now have a different view of anchoring than I did when I only sailed around UK waters
noelex
Well-Known Member
After setting, the anchor will achieve only a small fraction of the maximum diving depth that it will bury before its ultimate holding ability is reached. At least you hope that is the case.
The setting force using a yacht engine is typically only equivalent to around 25-30 knots of wind. If your anchor is so small that it has already achieved its maximum depth below the substrate at this point then the anchor will break out if the yacht is exposed to a higher force, so beyond about 25-30 knots you will drag.
Even with a small anchor you would expect (or at least hope) that there is capacity for the anchor to dive deeper after setting. This is exactly what happens when you observe anchors in real life. Large anchors will dive deeper than smaller anchors (assuming the same design and material), but it takes more wind before they reach their ultimate diving limit.
If the anchor was buried so that its holding power was 2000kg (to use your numbers) and it was exposed to 2500kg it would bury deeper so that it would hold this new force. That is providing the anchor has not reached its limit of maximum diving depth and holding capacity for that particular substrate, scope etc.
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vyv_cox
Well-Known Member
In the 2006 anchor test by YM, Sail mag and West Marine they pulled 35 lb anchors to over 5000 lbs, about 2.25 tons. The new gen anchors held at this pull 3 out of 4 times, the old gen failed miserably. Delta about half way between best and worst. It would be a very strong wind indeed, even using the very conservative ABYC figures, that would exceed these loads. I am not at all sure that my deck fittings would survive.
MathiasW
Well-Known Member
Have mulled this over a bit, as I too like scaling arguments.
What you are forgetting is the vertical dimension. The soil can take more shear pressure when it is deeper in the ground. If I assume this to be a linear relationship with depth, then I get another geometrical factor in your scaling argument above and voila, the holding power scales in exactly the same way as the weight. In reality it isn't quite, but it seems to me that this solves the mystery as far as scaling arguments are concerned...
Gryphon2
Well-Known Member
Tranona
Well-Known Member
That is not what he said - he was simply comparing the load placed on the anchor by using the motor compared with the wind speed that would achieve the same load.
Worth putting some real data into the mix. The 30kg for 10hp is confirmed by the YM test on propellers in 2009 yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807 As you will see the test boat had a 20hp engine and the best propellers achieved around 250kgs in forward, but less than 180kgs in reverse. In terms of setting anchors using the motor the rule of thumb is optimistic. However as noted it is roughly equivalent to the pull from 25-30 knots of wind - which is also the range of windspeed when catenary disappears and the pull is direct from the boat to the anchor.
The next force acting on the anchor is the boat's pull as a result of wind speed and this in turn is a function of weight and windage. This is discussed at length here practical-sailor.com/sails-rigging-deckgear/anchor-testing-and-rode-loads Note that the boat used was well over twice the weight and windage of the boat used in the propeller tests (a Beneteau 323) but the maximum loads they measured were less than 400kgs in 25-30 knots of wind. They then extrapolated from the extensive known data to 60 knots of wind which would generate a load of around 1000kgs.
This links well to the data on holding power of anchors in the test referred to in post#114, summary table here https://pdf.nauticexpo.com/pdf/sea-tech-fun-spade/yachting-monthly-anchor-test-nov09/2213. The anchors tested were in the 15kgs range which is typical of the size used in the boat in the propeller test (although the boat would show on most sizing charts at the top end of 10kgs and bottom of 15kgs, so effectively "next size up"). As Vyv says the best anchors could ultimately hold to well over 2 tons pull in a perfect holding substrate. To reflect real life though they also tested on a wide variety of substrates to derive an "average" holding power and these are shown in the table. The best general purpose anchors were in the range of 1-1.3 tonnes - that is more than the calculated load in wind speeds of 60 knots for a 45' 12 tonnes boat (rather than a 33' 4.5 tonnes!)
You may want to revisit the statement you made in post#77 in the light of this data. as an anchor DOES know what boat it is attached to as it is the size, weight and hp of the boat that determines the load that is applied to it and therefore the holding power the anchor can achieve. As the data shows the loads are substantially less than the ultimate holding power of the recommended size. So why fit a larger anchor when the boat is not capable of accessing the potential additional holding power?
However I can see why you might consider going up a size if you had one of the older poor performing anchors which had 50% or less of the holding power of the NG types. That is what anchor development has been about for the last 30 years, designing shapes and geometry to achieve higher performance for the same weight of materials and physical size. Seems odd therefore that some choose to change anchors for a type that has anything up to double the holding power (and other benefits) but still think there is value in going up a size.
geem
Well-Known Member
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Nicely put.
NormanS
Well-Known Member
You can write as much verbage as you like, but I can assure you that no anchor known to mankind knows anything about the boat to which it is attached. ?
MathiasW
Well-Known Member
The advantage of a larger anchor is not to have more maximal holding power than I ever need.
As said before, if the maximal holding power is more than what I need, I can use some of that to shorten my chain and thus anchor where it is crowded. And yes, severe yawing may wiggle out the anchor if the chain is pulling at an angle, so I have to take some precautions to make sure yawing is less than 30 degrees, which the folks in Jonathan's link claim means there is no harm done to the anchor.
Also, you may be anchoring in a very muddy place where the maximal holding power is greatly reduced. I had to do this for 9 months during Covid, when we were not allowed to move, and I learned to love my somewhat oversized Spade anchor then. Yes, above 26 kn of wind it would drag a little, regardless of how much chain and snubber I would put out, but not by much and as the wind came from different directions, it was a kind of random walk... Every two weeks or so I would weigh the anchor and go back to where we were originally. In all that time it never broke free, it always stayed buried.
And finally, when the substrate is a layer of sand over rock, then there is a natural limit as to how deep a smaller anchor can be buried. The best anchor here is one that just fits into this layer of sand to make optimal use of it.
Thus, to me, it all depends on the circumstances whether a larger anchor makes sense or not. I am currently also in the Caribbean and I have never regretted my choice of anchor.
Cheers, Mathias
As said before, if the maximal holding power is more than what I need, I can use some of that to shorten my chain and thus anchor where it is crowded. And yes, severe yawing may wiggle out the anchor if the chain is pulling at an angle, so I have to take some precautions to make sure yawing is less than 30 degrees, which the folks in Jonathan's link claim means there is no harm done to the anchor.
Also, you may be anchoring in a very muddy place where the maximal holding power is greatly reduced. I had to do this for 9 months during Covid, when we were not allowed to move, and I learned to love my somewhat oversized Spade anchor then. Yes, above 26 kn of wind it would drag a little, regardless of how much chain and snubber I would put out, but not by much and as the wind came from different directions, it was a kind of random walk...
Every two weeks or so I would weigh the anchor and go back to where we were originally. In all that time it never broke free, it always stayed buried.And finally, when the substrate is a layer of sand over rock, then there is a natural limit as to how deep a smaller anchor can be buried. The best anchor here is one that just fits into this layer of sand to make optimal use of it.
Thus, to me, it all depends on the circumstances whether a larger anchor makes sense or not. I am currently also in the Caribbean and I have never regretted my choice of anchor.
Cheers, Mathias
