Big anchors

[thinwater]

Bad bottoms, like grass, are hard to define. If the roots are really thick, nothing will penetrate and it may just be a bad place to anchor.

There are "grassy" areas in the Chesapeake where a good anchor will bury ok (eel grass). And then there are some heavy weed areas where you can barely stick a screw driver into the mat (grass in firm clay). So comments about how a certain anchor does in weed should always be ... suspect.

 

[vyv_cox]

My thoughts on this well-aired issue are expressed in this article Oversize anchors – necessary?

Interestingly, the anchor originally specified for my Sadler 34 was a 25 lb CQR copy. This would be considered light in the extreme today but for ten years or so it was considered adequate. I now carry a 15 kg Rocna, previously a 16 kg Delta, both of which outperform the 25blb CQR by a considerable margin.

 

[webcraft]

My thoughts on this well-aired issue are expressed in this article Oversize anchors – necessary?

Interestingly, the anchor originally specified for my Sadler 34 was a 25 lb CQR copy. This would be considered light in the extreme today but for ten years or so it was considered adequate. I now carry a 15 kg Rocna, previously a 16 kg Delta, both of which outperform the 25blb CQR by a considerable margin.

Great article Vyv

- W

 

[Neeves]

Maybe the tide is changing. Canute simply chose the wrong venue.

Just a pity for those who followed the flock.

J

 

[Tranona]

"

Sizing Example​

Let’s look at an example: a Valiant 40-foot (12-meter) 30,000-lb (13.6-metric ton) monohull.

Note that I have been realistic about displacement in that the Valiant specification calls for around 24,000 lb, but I have upped that to a more typical cruising weight.

For this boat I would recommend:

• SPADE: S160, 77 lb (35 kg)
• SARCA Excel: #7, 79 lb (36 kg)"

Taken from morgans cloud website.

I can only echo Neeves comments - zero data to support this. Where there is reliable data it points in the opposite direction. Hold of anchors is a direct function of load applied as demonstrated in all the comparative tests. 15kgs and above anchors can develop ultimate hold way in excess of 2 tonnes which in turn is way in excess of the kinds of loads that a boat of the size for which the anchor is specified can ever generate. The two determinants of load are either thrust from the engine or the action of wind and waves on the boat.

To take a simple example - a 30hp engine in a 10m boat with a good 3 blade propeller will generate around 300kgs of pull in reverse. 30 knots of wind (force 7) will generate much the same with that size boat. Yours will of course be higher because of the bigger engine and greater weight and windage of the boat but note how small this load is compared with test loads required to generate maximum holding power.

People get fixated with maximum holding power and extreme conditions when in reality the vast majority of anchoring takes place in conditions that get nowhere near this. The test of a good anchor is how well in works with your boat in your conditions. That is does it set easily and stay set in relation to the load the boat and conditions can apply. In other words is the anchoring system (chain, connectors and anchor) matched to the boat and conditions.

The two main causes of failure to set and hold are the nature of the seabed and the impact of changes in the direction of load. As your video shows and others explain weed is one of the main problems of getting the anchor set - and in many cases the larger the anchor, the more difficult it is to get it to penetrate. On the breaking out and resetting tests have shown that a smaller well buried anchor is less likely to break out following a change in direction than a larger anchor with much of it not buried - even though the actual hold may be similar.

On a slightly different tack, you say your boat is 38' long and 14 tonnes - is that right as that sounds awful heavy. Suggests a Disp/L ratio approaching 500 way above even well known super heavy displacement boats like a Vancouver 36 or Saga 40 which are around 400. Remember weight/displacement is only one component of load - windage is far more important when the load comes from the wind.

 

[noelex]

Also, a video of how the anchor behaves in Turtle grass. In my world, the anchor isn't set if you can move it.... and if you can see the flukes.

Thanks for the video. It is always helpful to see how anchors perform in the real world. The steel Spade is an good anchor, but in that substrate the Spade is obviously struggling and will not have high holding power.

The answer is not a smaller anchor. This will just have a lower ultimate holding power in the same substrate.

In poor substrates It is possible to exceed the ultimate holding power of modern anchors and drag, as you experience shows. The best defence is to fit the best design and the largest anchor you can comfortably manage. This is why experienced cruisers such as Morgans Cloud use oversized anchors where possible (I think in his case, two sizes over Spades recommended sizing).

 

[Tranona]

The answer is not a smaller anchor. This will just have a lower ultimate holding power in the same substrate.

Do you have any concrete data to support that> Are you really saying "ultimate" holding power when I think you mean actual holding power in that substrate. The ultimate holding power is so high for even a small anchor that a boat of the appropriate size will never be able to exceed it. If the substrate is poor that the hold is less than the load applied such that the anchor drags a larger anchor will only be able to generate the same level of hold.

We have had this debate before and I have seen nothing to convince me that simply increasing the potential ultimate holding power is any advantage when the failure is because of the holding power of the substrate NOT the ultimate holding power of the anchor.

 

[[167227]]

I can only echo Neeves comments - zero data to support this. Where there is reliable data it points in the opposite direction. Hold of anchors is a direct function of load applied as demonstrated in all the comparative tests. 15kgs and above anchors can develop ultimate hold way in excess of 2 tonnes which in turn is way in excess of the kinds of loads that a boat of the size for which the anchor is specified can ever generate. The two determinants of load are either thrust from the engine or the action of wind and waves on the boat.

To take a simple example - a 30hp engine in a 10m boat with a good 3 blade propeller will generate around 300kgs of pull in reverse. 30 knots of wind (force 7) will generate much the same with that size boat. Yours will of course be higher because of the bigger engine and greater weight and windage of the boat but note how small this load is compared with test loads required to generate maximum holding power.

People get fixated with maximum holding power and extreme conditions when in reality the vast majority of anchoring takes place in conditions that get nowhere near this. The test of a good anchor is how well in works with your boat in your conditions. That is does it set easily and stay set in relation to the load the boat and conditions can apply. In other words is the anchoring system (chain, connectors and anchor) matched to the boat and conditions.

The two main causes of failure to set and hold are the nature of the seabed and the impact of changes in the direction of load. As your video shows and others explain weed is one of the main problems of getting the anchor set - and in many cases the larger the anchor, the more difficult it is to get it to penetrate. On the breaking out and resetting tests have shown that a smaller well buried anchor is less likely to break out following a change in direction than a larger anchor with much of it not buried - even though the actual hold may be similar.

On a slightly different tack, you say your boat is 38' long and 14 tonnes - is that right as that sounds awful heavy. Suggests a Disp/L ratio approaching 500 way above even well known super heavy displacement boats like a Vancouver 36 or Saga 40 which are around 400. Remember weight/displacement is only one component of load - windage is far more important when the load comes from the wind.

Yep, 13,800 kg with almost full tanks. It's steel, and carries a lot of fluids. And all my crap, as we are full time cruisers.

 

[noelex]

Do you have any concrete data to support that> Are you really saying "ultimate" holding power when I think you mean actual holding power in that substrate. The ultimate holding power is so high for even a small anchor that a boat of the appropriate size will never be able to exceed it. If the substrate is poor that the hold is less than the load applied such that the anchor drags a larger anchor will only be able to generate the same level of hold.

We have had this debate before and I have seen nothing to convince me that simply increasing the potential ultimate holding power is any advantage when the failure is because of the holding power of the substrate NOT the ultimate holding power of the anchor.

I was referring to the ultimate holding power of the anchor in the substrate where it is dropped. This will vary with the substrate and other factors such as the scope.

The anchor will drag when this ultimate holding power is exceeded.

There is an abundance of data to support the common sense view that an anchor’s holding ability increases as the size of the anchor increases (assuming the anchors are the same design and construction material). I am not sure why anyone would dispute this.

You can look at work done by Professor Knox as just one example. His tests included various sizes of steel Spades (amongst many other designs), so the results are particularly relevant to Rhysmate‘s case of dragging a Spade.

For example, in his tests a 6kg steel Spade had a UHC (ultimate holding capacity) of 120 kgf and the 15kg steel Spade had a UHC of 420 kgf (in the substrate where the tests were conducted). See the table below:

https://static1.squarespace.com/sta...02103d72392/1489319853474/JHK+PBO+article.pdf



He even produced a graph showing the linear relationship between the ultimate holding power and the anchor weight (assuming the same design and construction material).


https://static1.squarespace.com/sta...b/1509856649414/Practical-Boat-Owner-2002.pdf

 

[srm]

OK hands, up:
I have an oversized Rocna anchor:
Reason, previous experience of anchoring for a few hours in 60 plus knots and relatively sheltered location with oversized Bruce patent anchors.

My current boat came with a big pretend CQR that I left behind as a garden ornament.
On the Rocna website my boat came on the cusp of 15kg and 20kg and at that time recommended going for the larger. I bough a 25Kg.

Experiences:
I have slowly dragged a few times, but in each case suspect that the problem was a thin sediment layer over rock.

In normal choice of seabed, such as west Scotland mud, the anchor sample on recovery usually showed that it was fully buried. Sand is more difficult to judge as most washes off, but have seen large quantities streaming away as the anchor is recovered so suspect there was good burial of the fluke.

Just my observations over the five years it was in almost daily use for five or six months a year. We never experienced more than 50kn gusts though.

 

[[167227]]

OK hands, up:
I have an oversized Rocna anchor:
Reason, previous experience of anchoring for a few hours in 60 plus knots and relatively sheltered location with oversized Bruce patent anchors.

My current boat came with a big pretend CQR that I left behind as a garden ornament.
On the Rocna website my boat came on the cusp of 15kg and 20kg and at that time recommended going for the larger. I bough a 25Kg.

Experiences:
I have slowly dragged a few times, but in each case suspect that the problem was a thin sediment layer over rock.

In normal choice of seabed, such as west Scotland mud, the anchor sample on recovery usually showed that it was fully buried. Sand is more difficult to judge as most washes off, but have seen large quantities streaming away as the anchor is recovered so suspect there was good burial of the fluke.

Just my observations over the five years it was in almost daily use for five or six months a year. We never experienced more than 50kn gusts though.

I find with the oversized SPADE in proper sand bottoms or mud, its very hard to lift out with the powered windlass and comes up usually completely covered. I have to motor over it sometimes. My biggest issue is more difficult bottoms like grass. Its a tough one, because i have never met a full time cruiser that says their anchor is too big. But i also see that i struggle to set this big bastrd sometimes. Sadly my eardrum is pretty badly ruptured and i cant go take a photo of the anchor right now. I dove on it yesterday and regret it this morning... ear is not good.

My assumption of the manufaturers data was that it doesn't take into account anchoring in 50 plus knits which we do and have done. Its very hard to ignore anecdotal data too though! Anyway, I'm stuck with what I have and when my ear is fixed I'll go back to taking photos of each set... data

 

[thinwater]

Do you have any concrete data to support that> Are you really saying "ultimate" holding power when I think you mean actual holding power in that substrate. The ultimate holding power is so high for even a small anchor that a boat of the appropriate size will never be able to exceed it. If the substrate is poor that the hold is less than the load applied such that the anchor drags a larger anchor will only be able to generate the same level of hold.

We have had this debate before and I have seen nothing to convince me that simply increasing the potential ultimate holding power is any advantage when the failure is because of the holding power of the substrate NOT the ultimate holding power of the anchor.

An uninitiated (not used to anchor threads) reader could take away some odd ideas from a thread like this.

• A smaller anchor has the same holding capacity as a larger anchor.
• A large anchor is always more secure.

In fact, the first take away should be that a larger anchor of the same design always has more holding capacity than a smaller one. A small boat may not be able to set the anchor well enough to make it stable under yawing, but the capcity is there if the straigh line force becomes great enough. That same small boat probably can set the up-one-size anchor in very soft mud, and that is probably what it needs, since the holding capatiy of any anchor in very soft mud is 3-1 times less than in good sand.

The second take away is that an excessively oversize anchor may not fully set in firm bottoms. With just the toe engaged, it will be vulnerable to yawing (the shank is not buried, which damps anchor movment). It may wiggle around, and as a result either trip or foul (the risk of fouling goes up the farther an anchor moves). Some anchors reset so well you can hardly tell, others not so well. Pivoting fluke anchors jam and even new generation anchors can become packed with sticky mud, which upsets the balance so badly they do not reset. You really want the fluke fully engauged, even if the shank does not bury.

Is it better to have an anchor deeply buried so that it resists yawing, or free to rotate with persistant shifts (certinly not every yaw--such an anchor will move too far overtime and is likely to foul)? With a Fortress anchor, the answer is that deeper is better. It may be a bugger to recover, but the anchor won't reset, so you better make sure it is so stable it does not need to. With anchors that rotate very well (Excel, Mantus, Spade, and others) I'm not sure the answer is as obvious. It might be better to rotate as the storm does, than hold in one direction until forced to do a 180 flip. I've seen many tests that tickle the edge of the question, but I'm not sure it's clear which is better.

I do know what my conclusion is, for my cruising area. I'm pretty sure the exact answer varies regionally. For a Fortress anchor, it should be the recommended size if it is to be the bower anchor and will see wind shifts. It needs to dig deep in a storm, and expect recovery to be difficult. If you intend to use it as part of a V or triangle mooring for major storms, then larger is better because it will not bury so deep and will be much easier to recover. A kedge can be bower-size, because it will still be light (easy to row out--use a rope rode) and will be easy to recover under lighter loads.

As for new generation anchors, it should bury and hold in the softest bottom you are likely to encounter with some frequency. You should be able to bury at least the fluke in firm sand. Thus, the right size depends to some extent on your cruising area, because the holding capacity of the anchor depends on the bottom.

Typically (several boats, several bower anchors used on each), I cannot power set my anchor any more than just the fluke in good sand. It is deep enough to resist yawing and will pull deeper in a thunderstorm and has holding capacity t0 spare. In very soft mud the same anchor will be just adequate, will need to be set in stages, and will require hours to reach full holding capacity (the mud must consolidate). Thus, it is neither too small nor aggressively oversized. On that rare, rare occasion where I feel the need to more anchor, I have another anchor and will set them in an asymmetric V (typically a very soft bottom, hail storms coming, and I don't feel like moving--rowing out and linking the kedge takes only a few minutes).

 

[Tranona]

I was referring to the ultimate holding power of the anchor in the substrate where it is dropped. This will vary with the substrate and other factors such as the scope.

The anchor will drag when this ultimate holding power is exceeded.

There is an abundance of data to support the common sense view that an anchor’s holding ability increases as the size of the anchor increases (assuming the anchors are the same design and construction material). I am not sure why anyone would dispute this.

You can look at work done by Professor Knox as just one example. His tests included various sizes of steel Spades (amongst many other designs), so the results are particularly relevant to Rhysmate‘s case of dragging a Spade.

For example, in his tests a 6kg steel Spade had a UHC (ultimate holding capacity) of 120 kgf and the 15kg steel Spade had a UHC of 420 kgf (in the substrate where the tests were conducted). See the table below:

https://static1.squarespace.com/sta...02103d72392/1489319853474/JHK+PBO+article.pdf

View attachment 159296

He even produced a graph showing the linear relationship between the ultimate holding power and the anchor weight (assuming the same design and construction material).
View attachment 159295

https://static1.squarespace.com/sta...b/1509856649414/Practical-Boat-Owner-2002.pdf

I am well aware of that, but I am not sure it says what you think it says. It would indeed be disappointing if a larger (in this case weight used as a proxy for size) anchor of a given design did not have higher holding power. What is missing is the load applied to the anchor by the boat which in the scenario of dragging is constant.

Take my oft quoted example of my boat with its 30hp engine which can achieve something like 280kg bollard pull in reverse. That will achieve the same hold in a given substrate with a 10 kg or a 15kg anchor. If the substrate cannot take that load both will drag. Unless the pull of the boat increases neither anchor will generate higher holding power. Holding power is a function of fluke area and design NOT weight. As Neeves reports (and Fortress show) Aluminium versions of anchors achieve the same holding power size for size as the heavier steel versions.

The measure of efficiency in use is the conversion of load applied to level of hold.

 

[noelex]

Unless the pull of the boat increases neither anchor will generate higher holding power.

If the wind picks up the force on the anchor will increase. If this exceeds the ultimate holding power (in the particular substrate) the anchor will drag. This force is lower for the the 10kg anchor than the 15kg model (assuming they are the same design and construction material).

 

[vyv_cox]

Yep, 13,800 kg with almost full tanks. It's steel, and carries a lot of fluids. And all my crap, as we are full time cruisers.

I doubt that the weight/displacement makes any difference other than maybe the effect of momentum when yawing. A floating boat has no effective weight. When Prof John Knox was developing his rode loading expression he found that cross sectional area was the critical value but since this pretty much runs in conjunction with length he used that as being easier to measure.

 

[Tranona]

An uninitiated (not used to anchor threads) reader could take away some odd ideas from a thread like this.

• A smaller anchor has the same holding capacity as a larger anchor.
• A large anchor is always more secure.

In fact, the first take away should be that a larger anchor of the same design always has more holding capacity than a smaller one. A small boat may not be able to set the anchor well enough to make it stable under yawing, but the capcity is there if the straigh line force becomes great enough. That same small boat probably can set the up-one-size anchor in very soft mud, and that is probably what it needs, since the holding capatiy of any anchor in very soft mud is 3-1 times less than in good sand.

The second take away is that an excessively oversize anchor may not fully set in firm bottoms. With just the toe engaged, it will be vulnerable to yawing (the shank is not buried, which damps anchor movment). It may wiggle around, and as a result either trip or foul (the risk of fouling goes up the farther an anchor moves). Some anchors reset so well you can hardly tell, others not so well. Pivoting fluke anchors jam and even new generation anchors can become packed with sticky mud, which upsets the balance so badly they do not reset. You really want the fluke fully engauged, even if the shank does not bury.

Is it better to have an anchor deeply buried so that it resists yawing, or free to rotate with persistant shifts (certinly not every yaw--such an anchor will move too far overtime and is likely to foul)? With a Fortress anchor, the answer is that deeper is better. It may be a bugger to recover, but the anchor won't reset, so you better make sure it is so stable it does not need to. With anchors that rotate very well (Excel, Mantus, Spade, and others) I'm not sure the answer is as obvious. It might be better to rotate as the storm does, than hold in one direction until forced to do a 180 flip. I've seen many tests that tickle the edge of the question, but I'm not sure it's clear which is better.

I do know what my conclusion is, for my cruising area. I'm pretty sure the exact answer varies regionally. For a Fortress anchor, it should be the recommended size if it is to be the bower anchor and will see wind shifts. It needs to dig deep in a storm, and expect recovery to be difficult. If you intend to use it as part of a V or triangle mooring for major storms, then larger is better because it will not bury so deep and will be much easier to recover. A kedge can be bower-size, because it will still be light (easy to row out--use a rope rode) and will be easy to recover under lighter loads.

As for new generation anchors, it should bury and hold in the softest bottom you are likely to encounter with some frequency. You should be able to bury at least the fluke in firm sand. Thus, the right size depends to some extent on your cruising area, because the holding capacity of the anchor depends on the bottom.

Typically (several boats, several bower anchors used on each), I cannot power set my anchor any more than just the fluke in good sand. It is deep enough to resist yawing and will pull deeper in a thunderstorm and has holding capacity t0 spare. In very soft mud the same anchor will be just adequate, will need to be set in stages, and will require hours to reach full holding capacity (the mud must consolidate). Thus, it is neither too small nor aggressively oversized. On that rare, rare occasion where I feel the need to more anchor, I have another anchor and will set them in an asymmetric V (typically a very soft bottom, hail storms coming, and I don't feel like moving--rowing out and linking the kedge takes only a few minutes).

Agree with a lot of this - although I cannot imagine many people would use a Fortress as a general purpose bower anchor when there are so many better alternatives.

With regard to power setting, if you have a decent engine you should be able to apply loads equivalent to 25-30 knots of wind. This is a generalisation but is reasonable for cruising boats between 30-50' with appropriate sized engines. As Vyv's article from YM shows this is sufficient to bury the recommended size of a Rocna, but would not bury fully the next size up. This is nowhere near the maximum potential holding power of the anchor and even going up to 50 knots of wind would still have reserve holding power - assuming the substrate can take it. The limitation in extreme conditions is not the anchor (assumi9ng the recommended size), but the holding power of the substrate.

As to yawing, again it is instructive to look at Vyv's photos (and similar from Neeves) which show how well a buried NG anchor deals with changes in direction of pull. Equally it has been shown many times that poorly buried anchors (such as happens with oversized anchors) are more prone to tripping and poorer at resetting.

 

[Tranona]

If the wind picks up the force on the anchor will increase. If this exceeds the ultimate holding power (in the particular substrate) the anchor will drag. This force is lower for the the 10kg anchor than the 15kg model (assuming they are the same design and construction material).

But they will drag equally because the constraint is the hold of the substrate NOT the potential of the anchor. That is the point I am making. For any given load the holding power will be the same.

 

[Tranona]

Yep, 13,800 kg with almost full tanks. It's steel, and carries a lot of fluids. And all my crap, as we are full time cruisers.

As explained earlier, length and displacement are not the biggest determinants of load on anchors. Wind resistance is. Last sentence of post#15 gives a practical example derived from some empirical tests carried out in Australia. Length of boat is used as simple proxy as generally for monohulls with single masts windage does correlate fairly well with boat length. However on an offshore cruising boat this can diverge rapidly once you add sprayhoods, biminis, stern arches solar panels, davits, dinghies and so on. Far more than your extra displacement for size - never mind the consequences for stability!

 

[noelex]

But they will drag equally because the constraint is the hold of the substrate NOT the potential of the anchor. That is the point I am making. For any given load the holding power will be the same.

The substrate is vitally important, but the ultimate holding power is a combination of the anchor interacting with the substrate. A larger anchor will have a higher ultimate holding power than a smaller anchor of the same design and construction material. Look at the table in post #29. The substrate stays constant but the ultimate holding power increases as the anchor size increases, despite the anchor design and construction material staying constant.

A higher ultimate holding ability in any particular substrate means the anchor can withstand a greater force before it drags In that seabed.

 

[Daverw]

The last few years have had may YouTubers going on about their bigger and bigger anchors being needed, many don’t make a big deal about that they have not had to pay for them, but shout loud enough for long enough and people start to believe it, doesn’t them matter what the data actually shows. It seemed to become the Sailing equivalent of a willy wanging contest