Fortress scaling plot

[Neeves]

The seabed for the Chesapeake tests was specifically chosen by Fortress, other tests, for example those in San Francisco, were chosen by the US Navy.

The Chesapeake tests were indicative of the potential hold developed by a range of anchors - but Fortress with the flukes set at 45 degrees stood head and shoulders above the rest, surprise, surprise. The anchors were chosen on the basis of being of the size recommended for a specific yacht size. With the fluke set at 30 degrees, Fortress is as good, or bad, as a Danforth.

For those who worship at the alter of weight:

The Fortress FX37 weighing 21 lb had an average hold of 1862lb at 45 degrees and 995 lbs at 30 degrees'

A similarly sized Danforth HT, 35lb (flukes I believe are at about 30 degrees) had an average hold of 990lb (and Fortress with the flukes set at 30 degrees returned an average hold of 995lb). So much for the religion of weight.

I don't criticise in any way Fortress choice of location - in soupy mud we would, exclusively, use a Fortress set at 45 degrees, and we kept our Fortress FX 23 with the flukes set at 45 degrees in our bow locker always ready to deploy. In sand we would use either our 8kg aluminium Spade or Excel. For comparison a 45lb Spade developed an average hold of 546lb (vs the 30 degree, 21lb, Fortress of 995lbs), Excel was not tested.

We do try to avoid soupy mud

Jonathan

 

[Neeves]

It would be dangerous to assume the scaling of a Fortress is indicative of any other other anchor. Using the data might favour a childish fascination but has little value otherwise. The testing was conducted in a seabed specifically suited to Fortress. If we look at other results and suggest the tests are indicative of performance other anchors - no-one would buy anything but Fortress. Fortress is basically a fluke, nothing extra, except the shank. Other anchors have other appendages, roll bars and ballast chambers. Some anchors have thick fluke plates to engender strength we have no idea how these might impact scaling. Oil rig anchors based on a significant amount of investment scale at 0.8 - 0.9 (and the original Bruce 0.7). I don't quite think our anchors have enjoyed the same level of investment.

Both Fortress and Danforth have serious defects in weed and stoney seabeds - they have high hold in ideal seabed - but are compromises. Other anchors have different compromises - no anchor is perfect - and based on oil rig and Bruce anchor I, personally, don't believe (and have seen no evidence) that 'our' anchors (excluding Fortress scale at double, weight, double hold - I prefer to scale somewhere between Bruce and modern oil rig anchors).

Anyone reading this thread can make their own decisions. I knew the results before I posted - I don't hide data to suit my suggestions. I'm not here to score points - I provide complete access to data, unedited and as accurately as possible.

In the fullness of time when 'our' anchors enjoy extended reputable testing I might need to change my mind - in the meantime I am cautious and err on the side of safety.

In the meantime I have another bottle of red to finish

Jonathan

 

[thinwater]

... an anchor’s holding ability is not just influenced by weight, but also significantly by the depth of bury (as well as other factors). The maximum depth of bury a specific anchor design is capable of achieving in any particular substrate increases as the size increases. This plays a significant role in increasing the holding power of larger anchors....
.

There are several reasons depth of bury matters, but the most important is that the shear strength of soils, particularly underwater,increases dramatically as you go deeper. Even in loose sand (or a pile of marbles) this happens due to increase friction between particles cause by the mass of the overburden. In mud it can be even more significant, as many (even thousands) of years of consolidation and forcing water out, change it into a different material. An anchor of double the weight does not have double the area, but it is working in firmer soil.

 

[boomerangben]

There are several reasons depth of bury matters, but the most important is that the shear strength of soils, particularly underwater,increases dramatically as you go deeper. Even in loose sand (or a pile of marbles) this happens due to increase friction between particles cause by the mass of the overburden. In mud it can be even more significant, as many (even thousands) of years of consolidation and forcing water out, change it into a different material. An anchor of double the weight does not have double the area, but it is working in firmer soil.

I’m in agreement regarding the importance of soil structure and its relevance to holding capacity. But I am interested in your thoughts about heavier anchors penetrating deeper. I would have thought a smaller anchor would penetrate deeper, especially ones without appendages such as roll bars and stocks. Whilst I get that penetration is (in part at least) a function of tip weight, most of the pressure on the tip of a well designed anchor would be down to the pull on the rode? Or am I missing something

 

[thinwater]

I’m in agreement regarding the importance of soil structure and its relevance to holding capacity. But I am interested in your thoughts about heavier anchors penetrating deeper. I would have thought a smaller anchor would penetrate deeper, especially ones without appendages such as roll bars and stocks. Whilst I get that penetration is (in part at least) a function of tip weight, most of the pressure on the tip of a well designed anchor would be down to the pull on the rode? Or am I missing something

You are not wrong. But you are going deep into the topic!

A smaller anchor may (often will) penetrate deeper at a given force than a larger model. But all things being equal, the larger anchor penetrate deeper at its maximum hold than the smaller anchor will at its maximum hold. In fact a smaller anchor can be more stable in some cases, at a given rode tension. because it is deeper than a larger anchor. For example, and oversized Fortress in firm sand will barely have it's tips in, while a smaller, slightly undersized Fortress might be 3 feet under ground and a bugger to remove. This topic can start a fist fight).

 

[AntarcticPilot]

There are several reasons depth of bury matters, but the most important is that the shear strength of soils, particularly underwater,increases dramatically as you go deeper. Even in loose sand (or a pile of marbles) this happens due to increase friction between particles cause by the mass of the overburden. In mud it can be even more significant, as many (even thousands) of years of consolidation and forcing water out, change it into a different material. An anchor of double the weight does not have double the area, but it is working in firmer soil.

Of course, I agree that's true in general. But others have noted that the Chesapeake mud DOESN'T get harder or stronger with depth - at least, not at depths relevant to anchoring. I presume that's because there is some sort of upward flow through the mud, or perhaps the chemistry makes the clay minerals comprising the mud repel each other (I can imagine mechanisms for this). A further thing is that clay-rich muds can be thixotropic, solid to a steady pull but liquefying on a shock load. I briefly worked in a soil engineering laboratory in London and samples of London Clay - a well-consolidated geological deposit - varied enormously in their strength from place to place, despite being tens of millions of years old! Once pore closure occurs in a clay-based deposit, water remains trapped in the structure, and any disturbance to the structure can result in the deposit reliquefying. This is a common mechanism for landslides in some parts of the world.

 

[boomerangben]

You are not wrong. But you are going deep into the topic!

A smaller anchor may (often will) penetrate deeper at a given force than a larger model. But all things being equal, the larger anchor penetrate deeper at its maximum hold than the smaller anchor will at its maximum hold. In fact a smaller anchor can be more stable in some cases, at a given rode tension. because it is deeper than a larger anchor. For example, and oversized Fortress in firm sand will barely have it's tips in, while a smaller, slightly undersized Fortress might be 3 feet under ground and a bugger to remove. This topic can start a fist fight).

No desire to take this “outside” . It’s nice to think academically about anchors, but the bottom line is that anchoring can be a reasonably accurate science with money and surveys and anchor handling tugs, but the reality for the rest of us is a one anchor compromise and that is the only answer, science or no science. Picking one to buy would be a mine field but the only conclusion I can draw from this buy the anchor recommended, choose your location carefully and set it correctly.

 

[noelex]

But all things being equal, the larger anchor penetrate deeper at its maximum hold than the smaller anchor will at its maximum hold.

Absolutely.

At maximum holding capacity (just before it breaks out) a larger anchor will have a greater maximum penetration depth than a smaller anchor of the same design in the same substrate.

This produces an increase in holding power greater than may be expected from just when looking at the increased fluke area of the larger anchor.

A greater maximum diving depth produces a greater maximum hold via two mechanisms:

1. The size of the substrate failure wedge that is above the anchor (at about right angles to the fluke) increases
2. The substrate usually becomes "firmer" or more cohesive with depth.

The above mechanisms are in addition to contribution of the larger fluke area of the larger anchor.

Large anchors can ultimately dive deeper than an equivalent smaller anchor of the same design.

 

[thinwater]

Of course, I agree that's true in general. But others have noted that the Chesapeake mud DOESN'T get harder or stronger with depth - at least, not at depths relevant to anchoring. Not at all true. I live there. In fact, at 2-4 feet the is commonly a layer of shell below which it gets quite firm. Good anchors, properly wet, easily penetrate that far. This is why the Fortress at 45 degrees was able to hold over 5000 pounds--it reached firm soil. There are also areas where it is underlain by mudstone that is effectively impenetrable. It's complicated. I presume that's because there is some sort of upward flow through the mud (not true--no up-welling flow), or perhaps the chemistry makes the clay minerals comprising the mud repel each other (I can imagine mechanisms for this Possibly--also possibly a lot of organic material exuding slime). A further thing is that clay-rich muds can be thixotropic, solid to a steady pull but liquefying on a shock load Yup, but not the main issue. Most of this mud drains freely and de-waters well. Lots of dredge spoil handling confirms this. I briefly worked in a soil engineering laboratory in London and samples of London Clay - a well-consolidated geological deposit - varied enormously in their strength from place to place, despite being tens of millions of years old! Once pore closure occurs in a clay-based deposit, water remains trapped in the structure, and any disturbance to the structure can result in the deposit reliquefying. This is a common mechanism for landslides in some parts of the world.

One of the differences in the Chesapeake vs. England is that the water temperatures are MUCH warmer in the summer, often reaching 27F at the bottom of harbors (even warmer at the surface). They are also fed by long river systems that bring down a lot of sediment. The drainage basin is large relative to English waters, and thus the amount of fresh sediment is larger. The upper layers of the mud are much younger, I suspect. The drainage basin of the Chesapeake Bay alone is larger than England, thus the epic mud in some areas. Parts of Louisiana are even worse (Mississippi river basin and warmer temperatures).

Often and anchor that is let to set for a few hours will settle into the mud, and then can be pulled deep enough to reach firm layers deeper down. We refer to this as "soaking the anchor." It is a real thing in the Chesapeake. Waiting 15-60 minutes before setting can really help. I've the got data to prove it. (In Practical Sailor)Soaking Anchors

Every area is different. Results in the UK will be different.

 

[boomerangben]

Absolutely.

At maximum holding capacity (just before it breaks out) a larger anchor will have a greater maximum penetration depth than a smaller anchor of the same design in the same substrate.

…..,

on what are you basing your assertion that a larger anchor will go deeper than a smaller anchor?

 

[Neeves]

I was pondering further, as I do.

I find the Plot of Fortress in different locations quite robust (Chesapeake and San Francisco), in terms of the anchor data available (its better than nothing). But at the end of the day the information has no value except of Fortress, and I'm more than happy to accept that, in soft mud, double size and you will double hold. I cannot argue with the data. I have wondered if the weights might correlate well with surface area - I simply don't know and cannot check as I don't have Fortress anchors to check against.

However the plot is only for soft mud and it would be childish and dangerous to extrapolate to sand or to other anchors. Clutching at straws and suggesting that all anchors have the same multiplier is against all the results of oil rig anchors, that have been extensively tested, and they have different multipliers to each other and different multipliers in sand compared with mud.

If you look at the reports on Fortress testing in Chesapeake they have some pictures of the anchors on retrieval - the mud is compressed and 'dry' - all the moisture has been squeezed out. The anchors, or some of them specifically Fortress, reached a point in the substrate where the mud was no longer soupy. Most other anchors obviously did not dive to the same depth - as they developed no hold at all, unless you consider 150kg reasonable hold .

Jonathan

 

[Neeves]

Whilst I get that penetration is (in part at least) a function of tip weight, most of the pressure on the tip of a well designed anchor would be down to the pull on the rode? Or am I missing something

I have been working on the concept that shear strength increases with the square of depth - but maybe that's too simple and it varies with the seabed.

I think tip weight is a red herring. An aluminium Spade or an Excel sets as easily as the steel counterparts. Fortress must have an appalling tip weight - but it sets, usually with ease. Bugel, Viking, Britany, Mantus engage with ease and they have no ballast at all.

I have found its design that allows the anchor toe, or tip, to engage and once it has engaged the fluke starts to bury - the burial is because of the tension in the rode. Setting an anchor too quickly will allow an engaged toe to 'disengage' as might a piece of shell. If you pull a Fortress too quickly the toe will simply rip out - you need to treat the initial set with a little care.

Once the toe has engaged its all about tension - those first few seconds its all about design (and why Ultra and Excel have 'turned down toes' and why some anchors have sharp toes).

Jonathan

 

[Neeves]

I’m in agreement regarding the importance of soil structure and its relevance to holding capacity. But I am interested in your thoughts about heavier anchors penetrating deeper. I would have thought a smaller anchor would penetrate deeper, especially ones without appendages such as roll bars and stocks. Whilst I get that penetration is (in part at least) a function of tip weight, most of the pressure on the tip of a well designed anchor would be down to the pull on the rode? Or am I missing something

An anchor buries because of design, specifically the tension on the rode forces the fluke to bury. However as the anchor buries it also buries the rode and the rode is resistant to burial. At some point the ability of the anchor to dive more deeply is balanced by the rode resisting further burial. The bigger the rode (bigger chain or thicker wire) the quicker the rode will balance the anchor's ability to dive further. The shank also resists burial. The depth of burial will be determined by the size of the components resisting burial vs the design of the fluke forcing it to dive. Roll bars will resist burial.

Most anchors set with the toe and the shackle end of the shank burying together - Fortress does not start to bury its shank until all the fluke is buried - so initially a Fortress is 'all fluke' until the stock (and shank) start to bury (I assume Danforth is similar)

Bigger chain will result in a shallower set.

This makes anchors testing difficult. Really the rode should be constant, same uplift forces, but people using small anchors, or even large anchors might use smaller chain or smaller chain with more strength.

As the resistance to burial of the rode increases the resistance burial 'lifts' the shank gradually until the fluke is lying at about 10 degrees to the horizontal at which point it will drag at a constant tension (its ultimate hold) - and eventually it will break out, clogged fluke or it catches a contaminant.

This is why wire or dyneema might be used for anchoring, it is thinner than chain. Its why HT steels are used in the shank they can be thinner.

Jonathan

 

[noelex]

on what are you basing your assertion that a larger anchor will go deeper than a smaller anchor?

I will show the results from a small Guardian G-5 as an example. This is the slightly simplified version of the Fortress anchor. It is also made by Fortress so its inclusion is relevant to the thread. It is an excellent anchor for the small size, but in the sand I typically encounter the deepest I can usually force the anchor to bury before breaking out is shown in the photo below. Even this result can only be achieved after multiple tries, using a long scope and some careful coaxing.

The anchor is almost disappearing (that is just the mud palm you can see in the photo), but because of the small overall dimensions the tip of the fluke is only 5-6 inches below the top of the substrate. The anchor will break out in this substrate if any more force is applied so this is the anchor’s maximum bury in this seabed.

Contrast this with a large anchor. You would rightfully be very disappointed if the anchor broke out when the tip of the fluke was only 5-6 inches below the top of the sand. Larger anchors can consistently bury far deeper than this even with a normal setting force, let alone when they have dived to their maximum depth and are on the verge of breaking out.

At maximum holding capacity (just before it breaks out) a larger anchor will have a greater maximum penetration depth than a smaller anchor of the same design in the same substrate.

 

[geem]

I I will show the results from a small Guardian G-5 as an example. This is the slightly simplified version of the Fortress anchor so its inclusion is relevant to the thread. It is an excellent anchor for the small size, but in the sand I typically encounter the deepest I can usually force the anchor to bury before breaking out is shown in the photo below. Even this result can only be achieved after multiple tries, using a very long scope and some careful coaxing.

The anchor is almost disappearing, but because of the small overall dimensions the tip of the fluke is only 5-6 inches below the top of the substrate. The anchor will break out in this substrate if any more force is applied so this is the anchor’s maximum bury in this substrate.

Contrast this with a large anchor. You would rightfully be very disappointed if the anchor broke out when the tip of the fluke was only 5-6 inches below the top of the sand. Larger anchors can consistently bury far deeper than this even with a normal setting force, let alone when they have dived to their maximum depth and are on the verge of breaking out.

At maximum holding capacity (just before it breaks out) a larger anchor will have a greater maximum penetration depth than a smaller anchor of the same design in the same substrate.

View attachment 183204

I have an FX55. It's huge but also doesn't set deeply in hard substrate. I can't get enough rode tension to bury it with my engine in reverse. It might be a different story in soft mud. By comparison my 30kg Spade buries flush with the seabed. The FX55 is unstable when only half set. For this reason, it's easily pulled out in a slight wind shift.

 

[noelex]

By comparison my 30kg Spade buries flush with the seabed.

Fortress produce a great anchor, but they don’t set well (or sometimes at all) in hard substrates and are poor at responding to changes in the direction of pull. Unfortunately, they often develop a high list and break out as they try and rotate. These reasons are why they are a poor primary anchor (although they are an excellent kedge anchor).

Your Spade will work much better in this situation and a 30 kg model is a decent size for your Trintella 44. On my reading this 30 kg Spade is oversized for your 44 foot 32,000 lb yacht. (Spade rate your 30 kg model as suitable for boats up to 65 feet) so perhaps you are member of the BIB (Big Is Better) club without knowing it .

I have never had any trouble setting my oversized Mantus M1 anchor, or before this my identically sized Rocna anchor. The Mantus M1 in particular is invariably the best set anchor in the anchorage . You can see photos in the link below showing this anchor together with the anchors from all the other boats nearby and make up your own mind.

I have used an oversized Rocna or Mantus M1 over 16 years of full time cruising, anchoring almost every day. I often dive to observe how the anchor is performing. These anchors have always "shuffled" ie remained buried and engaged with the seabed when rotating around in response to a change in wind direction. Once again, there are many photos of this happening in the link below.

In short, I don’t believe there is any risk of the issues you have been having with your Fortress anchor if you oversize a good primary anchor. The fact that you have observed a deep setting depth from your relatively large Spade anchor reinforces this view.

Photos of Anchors Setting - Page 111 - Cruisers & Sailing Forums

 

[Metalicmike]

I've now got a headache.

 

[thinwater]

I've now got a headache.

You read an anchor thread. That is how they end.

 

[AntarcticPilot]

You read an anchor thread. That is how they end.

I thought anchor threads only ended with the heat death of the universe and then only maybe!

 

[doug748]

I thought anchor threads only ended with the heat death of the universe and then only maybe!

Just so. When perfect Universal equilibrium is attained, there is nothing to suggest it can't kick off all over again.