Big anchors

[noelex]

You still miss the point. I am not asking about ultimate holding power - how many more times do i have to say it - that is obvious. I am asking for evidence that FOR THE SAME PULL a larger anchor generates greater holding power. That table simply shows a bigger anchor has higher POTENTIAL holding power.

I really cannot see why you have so much difficulty with this question - because you are claiming that it does.

Like Thinwater, I am struggling to understand your logic.

If we take the Spade anchor as an example, the table shows that in the test substrate the UHC (ultimate holding capacity) of the 6kg Spade was 120 kgf while the UHC of the 15kg Spade was 420 kgf.

This means if a boat is exerting a pull of less than 120 kgf either the 6kg or the 15 kg Spade would hold. If the pull exceeds 120 kgf, the 6kg would no longer hold and the boat would be dragging while the 15 kg would still be holding providing the force does not exceed 420 kgf. Simple.

So, for example, if the boat exerted a pull of 300 kgf and had a 15kg Spade the anchor would be generating more holding power (at least 300 kgf) than if it had a 6kg Spade (this would be generating less than 120 kgf, probably a lot less because the anchor is moving and not digging in correctly).

This is a simple example where FOR THE SAME PULL (or more correctly the same force on the boat) a larger anchor generates greater holding power.

 

[geem]

Thank goodness.


The ”small is always adequate” argument because you have ”all the holding capacity you need" would not go down well with the boats that dragged (including the boat directly in front of us) during the recent Tropical Storm Bret, which was the catalyst that started this thread.

Is that it? No data? They dragged because? It's not really a strong argument for over sizing your anchor. We didn't drag either. Nor the boat next to us. What does that prove?

 

[Tranona]

Like Thinwater, I am struggling to understand your logic.

If we take the Spade anchor as an example, the table shows that in the test substrate the UHC (ultimate holding capacity) of the 6kg Spade was 120 kgf while the UHC of the 15kg Spade was 420 kgf.

This means if a boat is exerting a pull of less than 120 kgf either the 6kg or the 15 kg Spade would hold. If the pull exceeds 120 kgf, the 6kg would no longer hold and the boat would be dragging while the 15 kg would still be holding providing the force does not exceed 420 kgf. Simple.

So, for example, if the boat exerted a pull of 300 kgf and had a 15kg Spade the anchor would be generating more holding power (at least 300 kgf) than if it had a 6kg Spade (this would be generating less than 120 kgf, probably a lot less because the anchor is moving).

This is a simple example where FOR THE SAME PULL a larger anchor generates greater holding power.

But we are not talking about the difference between a 6 and a 15kg, but between say a 15 and a 20kg, nor are we talking about UHC but about "normal" anchoring where as Neeves points out we never get anywhere near UHC. The argument is about whether an oversized (next anchor size up) offers any advantages NOT whether larger anchors have higher ultimate holding capacity which they clearly do.

What I would like to see is a graph similar to that you posted for hold against size, but plotting hold against load to show where the load point is that the larger anchor takes over and whether this can be related to the sorts of loads a particular boat can generate.

Many (including designers and manufacturers) question the need to oversize on the basis that their recommendations are already conservative and there is no advantage in having a larger anchor - for exactly the reasons I am suggesting. Yet you persist in saying there is advantage but do not give any convincing evidence beyond saying larger anchors have potentially higher holding power. If it really were that simple, why do designers spend time and effort making anchors that are smaller but more effective.

 

[noelex]

What does that prove?

The point is, that contrary to the often repeated advice on this forum, boats do drag even with modern anchors and good technique. The statement below was not accurate at least in this case.

What is being stated is that there is no need for the bigger anchor - as even an anchor of the recommended size has a holding capacity greater than the hold you will ever need.

Jonathan

I know the owner of the boat that dragged in front of us. This boat had a Rocna of the recommended size. He had a 6:1 scope in 9m of water, a good snubber and he set the anchor very carefully.

The owner indicated he began to drag in a gust of over 50 knots, which if accurate makes sense. The Rocna tables only size for a maximum of 50 knots so the anchor would have needed to be oversized, even assuming the holding ground was good and Rocna’s claims are accurate.

While this was an example of high wind speed exceeding the anchor’s holding ability in a reasonable substrate, far more cases of dragging are due to areas of marginal holding ability with more modest winds. Being able to more often safely use these areas is one of the major advantages of oversizing your anchor if you can comfortably manage the larger size.

 

[geem]

The point is, that contrary to the often repeated advice on this forum, boats do drag even with modern anchors and good technique. The statement below was not accurate at least in this case.

I know the owner of the boat that dragged in front of us. This boat had a Rocna of the recommended size. He had a 6:1 scope, a good snubber and he set the anchor very carefully.

The owner indicated he began to drag in a gust of over 50 knots, which if accurate makes sense. The Rocna tables only size for a maximum of 50 knots so the anchor would have needed to be oversized, even assuming the holding ground was good and Rocna’s claims are accurate.

While this was an example of high wind speed exceeding the anchor’s holding ability in a reasonable substrate, far more cases of dragging are due to areas of marginal holding ability with more modest winds. Being able to more often safely use these areas is one of the major advantages of oversizing your anchor if you can comfortably manage the larger size.

I have been in Falmouth Harbour in Antigua. With a squall of over 50kts. About 70 yachts at anchor using every conceivable type of anchor, not just NG anchors. Nobody dragged. Not a single boat.
One anchor dragging proves nothing. We don't know how well set the anchor was or what kind of holding the anchor was set in. We certainly don't know if a bigger anchor wouldn't of dragged. The evidence of a single boat is not evidence.
I was once anchored in the Bahmas next to a friend using a Rocna. We had been anchored for 3 days in moderate winds, all blowing from the same direction. On day 3, his anchor dragged. Again it proves nothing. It just happened. I not using that as evidence for anything and neither should you

 

[srm]

A second anchor can be sensible for a variety of reasons, but if regularly needed to reinforce the holding power in only F7-8 then a larger primary anchor is perhaps more sensible and much easier than deploying multiple anchors.

The larger anchor will cover those unexpected situations when unforecast strong winds arise in the early hours of the morning.

You have missed my point and ignored the key part of my statement.
"winds of F7-8 plus expected".
I prefer to have appropriate ground tackle set prior to the event as it makes life a lot easier. If the expected conditions do not materialise there is no problem. The second anchor was often set towards an expected wind shift.

 

[geem]

You have missed my point and ignored the key part of my statement.
"winds of F7-8 plus expected".
I prefer to have appropriate ground tackle set prior to the event as it makes life a lot easier. If the expected conditions do not materialise there is no problem. The second anchor was often set towards an expected wind shift.

We added a second anchor for tropical storm Brett for similar reasons. The forecast is just a forecast. We are just buying insurance. We did the same and set the second anchor for the wind shift. It turns out we didn't need it. We would have been fine on a single anchor

 

[rogerthebodger]

We added a second anchor for tropical storm Brett for similar reasons. The forecast is just a forecast. We are just buying insurance. We did the same and set the second anchor for the wind shift. It turns out we didn't need it. We would have been fine on a single anchor

As the saying goes

Better to be safe than sorry

 

[[167227]]

We added a second anchor for tropical storm Brett for similar reasons. The forecast is just a forecast. We are just buying insurance. We did the same and set the second anchor for the wind shift. It turns out we didn't need it. We would have been fine on a single anchor

If you have a properly sized anchor that you trust and is sized according to the boat you have, why would you set out a second?

What im getting from this debate is an anchor sized according to manufacturer spec is perfectly adequate for all weather. And you have a spade which is renowned for its resetting ability in 180 shifts. If the fortress didn't hold, would you not have ended up potentially fowling your Spade/chain?

I didn't anchor for Bret, i tied myself to some trees... I watched as people anchored upwind of me with small, undersized anchors and too little chain and decided it was best to go hide in the lagoon.

 

[noelex]

You have missed my point and ignored the key part of my statement.
"winds of F7-8 plus expected".
I prefer to have appropriate ground tackle set prior to the event as it makes life a lot easier. If the expected conditions do not materialise there is no problem. The second anchor was often set towards an expected wind shift.

Absolutely, “having appropriate ground tackle set prior to the event as it makes life a lot easier” is sensible advice, but with the better modern anchors of adequate size this does not have to include multiple anchors to provide enough holding power in that strength wind. Modern anchors should rotate around to a new wind direction without any problem.

There is lot of people advocating small anchors on this forum even when a larger model could be comfortably carried. To compensate, they also promote the routine use of multiple anchors in quite modest wind strengths. Some on this forum suggest two anchors are necessary in only 30 knots.

Obviously you should do what you feel is necessary, but this seems an unwarranted complication in my view, and of course there are times when multiple anchors are impractical, or when stronger wind arrives unexpectedly, so I think it is important to emphasise that this is not a good tactic to always depend on.

There are also drawbacks of deploying multiple anchors. It often creates a "net" in front of the boat that will catch the anchor of boats that drag in front to mention just one drawback that is often overlooked. Personally, other boats dragging are always my greatest fear in bad conditions.

 

[geem]

If you have a properly sized anchor that you trust and is sized according to the boat you have, why would you set out a second?

What im getting from this debate is an anchor sized according to manufacturer spec is perfectly adequate for all weather. And you have a spade which is renowned for its resetting ability in 180 shifts. If the fortress didn't hold, would you not have ended up potentially fowling your Spade/chain?

I didn't anchor for Bret, i tied myself to some trees... I watched as people anchored upwind of me with small, undersized anchors and too little chain and decided it was best to go hide in the lagoon.

The forecast winds were such that if the wind had gone further north than the forecast NE winds, we would have had a leeshore very close behind us, since we were anchored in a very small bay. The second anchor was set such that it gave a favourable direction to hold us off the leeshore. Its something I have done before as a precaution and wouldn't hesitate to do again.

 

[vyv_cox]

If you have a properly sized anchor that you trust and is sized according to the boat you have, why would you set out a second?

What im getting from this debate is an anchor sized according to manufacturer spec is perfectly adequate for all weather. And you have a spade which is renowned for its resetting ability in 180 shifts. If the fortress didn't hold, would you not have ended up potentially fowling your Spade/chain?

I didn't anchor for Bret, i tied myself to some trees... I watched as people anchored upwind of me with small, undersized anchors and too little chain and decided it was best to go hide in the lagoon.

As shown in the video on my website link, our reason for deploying a second anchor in a V is to reduce yawing angle, mainly for improvement in comfort aboard. Adding a second anchor does not reduce the loading on either of them: the load alternates 100% between them.

 

[geem]

As shown in the video on my website link, our reason for deploying a second anchor in a V is to reduce yawing angle, mainly for improvement in comfort aboard. Adding a second anchor does not reduce the loading on either of them: the load alternates 100% between them.

If you set the main anchor on chain and the second anchor on a short length of chain and a long length of nylon, it is possible to load both rodes. We have done this very effectively a couple of times in the past.
I reckon with 50m of 3/4" nylon deployed on the second anchor we see something like 5m of stretch very easily. When this happens, both rodes take some load. It obviously doesn't happen all the time as it depends on the wind direction.

 

[thinwater]

In thunderstorm season, winds of force 9-10 are commonplace and micro bursts for force 12 are not unheard of. IME, anchoring for 60 knots can be a daily affair in nice weather. It feels weird anchoring for 60 knots when the wind is at 0-5 knots and boat isn't even drifting back on the rode, with bridles and two anchors for soft mud, but when the wind suddenly rotates 120 degrees and the hails starts you feel much, much better. When it takes you 10-15 minutes to get the Fortress out of the bottom in the moring you feel OK. Again, it depends on where you cruise and the nature of local weather. (note: if you rig two anchors you have to learn how to do that such that the rodes do not twist together--just a rigging detail.

But to the point ...

Anchors drag for a variety of reasons. These I have experienced:

• Too small in soft mud. A 25-pound Delta on a 34' cat is not enough, based on two experiences. I changed up to a 35-pound NG and never dragged in soft mud again. The Delta is known locally as the drag-o-matic, because it does not hold well in soft mud.
• Excessive yawing. I didn't have a bridle on my first cat and it sail an anchor out. I added a bridle and never experienced that again. That said, I did a good bit of testing on yawing, and if your boat yaws more than +/- 20 degrees (bearing) you are better off working on that than up-sizing. And just because your boat does not yaw in light/moderate winds dies NOT mean it won't start when the chain lifts off the bottom. Test with all-rope rode.
• Fortress or Danforth in very hard sand. It did not actually drag, but only the tip was in, there was a 180 reversal, and it broke out the moment I started to haul rode, while the scope was still 5:1. The anchor that came out due to yawing was a Danforth in hard sand.
• Palm fronds. A close friend lost their boat when they anchored over good sand they have used before. A huricane had littered the bottom. It was a delivery crew that blew it.

Anchors that have failed at very low values during my testing. Size helps, but often the hold is so reduced it would fail anyway.

• Fouled. If the anchor does not set within a shank length (and it will take longer in soft mud) is likely to foul on some manner of trash. A common occurrence in the Chesapeake is stick that prevents the chain and shank from burying, and in soft mud, unless the anchor is several feet under, where the mud gets better, you don't have much hold.
• Sand over hardpan. Locally, it is 2-8 inches of sand over mudstone. Feels good, but the anchor can't dig. Other places it is coral rock. Hard to detect without power setting HARD, or excavating with a shovel.
• Weeds. Obviously. Some anchors are better than others, but if the weed is so dense that the anchor is only hooking the roots, then root strength is the limit of hold for any size anchor. Bigger helps some, sharp helps more.
• Rock. If you are hooking pockets, size matters little; you either found a good hole or you didn't. A 2-pound anchor can hold. If it is friction on shingles and flat stone, then hold ~ mass. This is the one time when two anchors in a row can help, but this requires special rigging and practice, and mostly it doesn't make bad into good. In both cases, very long scope helps by eliminating up loft, but the best answer is to move.
• Very soft mud. But this is never a surprise. Bigger is always better in this case.

So no, with rare exceptions, if the anchor dragged it was not because it was too small. In the case of pivoting fluke anchors, I have multiple experiences and tests that tell me that you only want to oversize for very soft mud, and probably not then, because Fortress is so good in mud. Amazingly good. With NG I don't have a strong opinion and I just follow the threads for information. IME, the NG sizing guides seem pretty conservative. Bottom line: there is no substitute for knowing the bottom ... which is not always practical. That's a bugger.

 

[Neeves]

which demand quick and reliable setting, high holding power at low loads, resistance to breakout following changes in angle of load and quick resetting. NG anchors in general are much better in these situations plus have the bonus of higher holding power (if the substrate will take it!) in extreme conditions.

I don't want to be picky, but I will be picky.

You cannot have high holding power at low loads.

The only way to engender high holding power is to have a high load - and the maximum hold (the holding power) is the maximum tension, which in real life will be much lower than the ultimate hold, in the rode.

You can have a high potential holding power (but all anchors have that). Your Epsilon, if it is 15kg, if you set it with your engine to 300kgs has about 1,700kg (and a 15kg Delta might have 700kg in reserve) of holding capacity in reserve - assuming you have the wind to generate that tension in your rode (which is physically (or weather wise) impossible). Accessing that potential 1,700kg also demands that the seabed has a high enough sheer strength - which you will never know - but oddly all anchors classified as SHHP seem to find seabeds easily with sufficient shear strength - they seem to be quite common.

I have made much of veering as it can reduce hold.

Chop will do the same thing though Vyv's photos seem to indicate that if your rode is thrashing the seabed then anchors can simply set more deeply (and increase their hold even in the absence of higher tension). The mechanism seems to be the thrashing rode jiggles the anchor, the seabed around the anchor liquifies, the shear strength collapses and the anchor settles or is tensioned into the lower, and deeper, shear strength environment - and when the seabed reconsolidates - you are back to the original shear strength - but higher hold (without an increase in tension) - because the anchor has settled more deeply. Recall the anchor will always be under tension so there are a number of mechanisms all reinforcing each other.

Testing for all of this is, to my mind, an insurmountable challenge.

What is interesting is that a Delta, CQR et al have a high reserve of hold - but they do drag. As mentioned there is a, slightly, unhealthy, focus on hold. Its more than hold. A Delta drags, despite that 700kg of reserve but a Rocna with 1,700kg of reserve does not drag (or not with the same frequency). It appears that hold, or high hold, is also indicative of other characteristics that are important..... ease of setting being but one. Maybe the lower hold anchors, or those that have not dived sufficiently (like a big anchor do not settle into the substrate when jiggled....???

Extending this thought process - my belief is anchors do not drag as a result of an absence of hold. There are other mechanisms at work and I focus on veering and chop.

There is, still, an act of faith in all anchoring - you can set your anchor, big or recommended size, at maximum revs and develop that high hold. Maybe if you have a really big engine you could set to a tension equivalent to a 40 knot wind. If the wind increases to 50 knot the anchor will set more deeply - but if there is some 'trash' (using Thinwater's word) which might be a beer can, lost swimsuit, bit of seaweed or a shell fragment and the toe of your anchor catches that trash - the anchor is very likely to drag. It does not matter the anchor is big or small - it will drag.

I have seen this during testing - you know roughly what hold the anchor will develop. You increase the tension on the rode - the anchor moves and slowly dives - but suddenly the anchor starts to move more quickly and starts to surface. If you release the tension and dig the anchor out, carefully - you find something, often small and insignificant impailed by the toe of the anchor. I have found oyster shell and small pieces of water logged wood (never a beer can nor swimsuit. ).

The danger is - the mantra is - buy a bigger anchor - forgetting that trash will negate any benefits of any anchor - complacency can be an issue. I don't need to power set at such high revs - I have a BIG anchor - stands to reason it is BETTER.


I see mention of small anchors, repetitively. As far as I know no-one, not a single person is advocating nor recommending small anchors (we use small anchors but I don't recommend, nor discourage use - I mention as a matter of fact). The anchors recommended and the subject of this and other similar threads are the anchors suggested by the anchor makers, they are not small they are correctly sized.


Its interesting people choose an anchor because of the design developed by the 'inventor'. The designer obviously knew what he was about - though in some isolated cases I think the designers are charlatans (but if I mention 'who' I'll then become the subject to be considered by the Mods ) - but when it comes to size of anchor the advice of the expert who developed the anchor is discarded and the owner says to himself - I know better. Makes no sense.


Its a bit like the new Defender parked outside Harrods - makes you feel better, even if you never use it to its full potential - But the choice of the new Defender is indefensible with any data - like the reasoning behind a bigger anchor. - but the new Defenders are all down Kensington High Street (and Kings Cross in Sydney).

For those of us who accept the logic to using an anchor of the recommended size we have to accept there will always be people who are unable to understand - sad really - especially when they perpetuate and repetitively disseminate indefensible myths for which they cannot provide data. Oddly the people who perpetuate such myths cannot realise that their indefensible views reduce their overall credibility.


Jonathan

 

[thinwater]

I suppose there are parallels in rock climbing:

• There is no point in using a rope stronger or fatter than a UIAA single rope. It does not add safety because a standard rope is strong enough for any actual load. More strength is wasted.
• A stronger rope increases the impact force on the anchors. That's not a good thing.
• Any anchor that is rated for >2000 pounds is strong enough, since it is well known they do not break. But smaller gear sometimes causes the rock to break because of higher point loadings (problem with some soft desert sandstone). But bigger gear can be more difficult to "set" soundly (similar to power setting a boat anchor) in hard rock and thus may be more likely to pop out in a swinging fall or due to rope whip or outward pull. All around, the mid-sizes are often most trustworthy.
• Careful placement is often the most important factor.
• Sometimes anchors are placed in groups of 2 or 3. Most often, this is for belays, but it is sometimes done if individual placements are either substandard, small, the rock is weak, of the potential fall either swinging (like yawing) or very long. But a single, well-placed anchor (running belay) is the standard because if done well it is enough, and setting up multiples takes time (during which you are getting tired).

Climbers carry smaller anchors that cannot hold a severe fall. The anchor has to fit the crack. Fortunately, not all falls are very high load. Not really a direct parallel.

The lessons are, in fact, similar. That the right size/strength is enough, that the anchor is never better than the substrate, and that in weak substrate, bigger can help.

 

[Neeves]

I had another look at Vyv's treatise on oversizing anchors.

In Vyv's article on over sized anchors Oversize anchors – necessary? the second and fourth pictures, excellent pictures, show his Rocna after some strongish winds in which there has been a yaw, or yawing and the chain has been 'off' the seabed and looks also to have been thrashing the seabed. These seem to be ideal conditions for the seabed to have lost some shear strength as a result of the constant movement of the anchor. I, and others, have touched anchors when the chain is off the seabed (but not thrashing it) and the anchor is constantly moving, not much - but you can feel the movement - ideal to reduce shear strength. The effect I 'describe' or 'consider' is the same as when you twitch your feet in shallow water at the beach or jiggle a trowel in wet cement. The best example is that of the trowel - when you do the jiggling the cement adjacent to the trowel becomes wet (or when you jiggle your feet - you sink into the seabed).

As I mentioned I consider yawing and chop are strong candidates to engender dragging especially as early gen anchors drag in conditions where their holding capacity will not have been tested (who has experienced tensions of 1,000kg on a 15 kg Delta anchor in good sand?).

Vyv's anchor has obviously not dragged yet it has been subject to the conditions where I would expect an anchor to struggle.

I accept that many will consider jiggling or twitching anchors to be a flight of my imagination.......the US Navy had a research project on exactly this topic - how a loss of shear strength would impact their fleet moorings.

We do know many modern anchors will set easily, deploy, drift back and the anchors will set (and hold) at what seems like very questionable scopes, less than 3:1 and with minimal attention to the set - it sets by itself.

The suggestion might seriously be that modern anchors (or some at least) set further when jiggled (under the conditions as I have analysed). Interestingly - for those whose memories might be tweaked - this is what Jumbleduck suggested (that the anchor would set further) in a similar thread a long time ago (so all credit to JD). It might not only be about holding capacity -but that indefinable characteristic 'to be able to be so forgiving of 'setting' conditions'.

 

[srm]

but with the better modern anchors of adequate size this does not have to include multiple anchors to provide enough holding power in that strength wind.

Sailing in northern waters my experience anchoring in winds of F7-8 plus has included a significant number of hours in winds of 60 to 85 knots. On one occasion the second anchor prevented a serious situation developing. I prefer two anchors as should one drag when subject to severe shock loads, sea bed composition or a storm surge, the second anchor can check the boat and allows the first to reset.

As I said before, "anchoring is an act of faith" and sea bed sediment can not be relied upon to be consistent. Just because your anchor has held you reliably in the last 99 locations does not mean that the sea bed will allow it to perform the same way in the hundredth.

As always, its the skippers choice.

 

[GHB]

I have a 50hp engine. 14 tonne, 38 foot boat and a 30kg spade. I struggle to get it to set well in hard sand. How has your experience been? I also have a reversing pitch propeller.

don't frequent here anymore but popped in to send a message & had to have a look...
Vaguely similar, 10m steel boat cruising & donkeys years if any real wind was forecast have cleated the chain off and made sure there's a good amount of slack then got a load of way on in reverse, then invariably it goes bang, few bits might fall of shelves as boat spins round & bow dips down as the hook digs deep. For many years this was with an elderly 20Hp bukh which was probably a lot less than 20. But never had a problem getting to hook well dug in. For rocky of hard bottoms seems better to gently knock it in first to get it started. Going towards 2 decades full time onboard mostly on the hook, can only remember dragging badly once, up a muddy river in Brazil with tiller tied off to the side.
That was 20Kg rocna, now main hook is 25Kg spade. US site says 25Kg is up to 60' so oversize. ISTR sizing used to say only up to 50Kts which seems to have gone. 50Kts really gets your attention but not totally mental, been in worse. Though wind isn't really so much of a worry, it's huge gusts from all directions that really get the heart racing. That and any boat upwind.
Never really thought about it until a couple of posters started banging on about anchor weight should somehow be related to engine size on here, met long term off the beaten track cruisers who do the same. Very reassuring knowing the hook can provide such massive loads in an unknown bottom.
Recently welded on some stainless plate with a slot as a chain grab in front of the windlass which works very well, just takes a tap with a big toe to bypass the windlass. Opencpn gets used a lot to keep an eye on things, using a waypoint with a range ring, very useful to see what's going on. Blue line is wind arrow.

Recent Halo+ radar is *so* useful as well, both for finding a spot in the anchorage & watching if other boats are dragging. New LiFePo4 means all the toys can stay powered up without worrying about getting batts back up to 100%

Not advice, each to their own, I really don't care. just what happens on another boat. If outfitting again would not hesitate for a millisecond going up a size, yet to find any downside.

 

[boomerangben]

Assuming that in the same conditions your small anchor would be completely buried, with just the stock still visible, then the whole fluke embedded, I would expect the larger anchor to bury, very roughly, the same area of fluke. So if your small anchor was fully buried I'd expect the larger anchor to bury that same area. Hold is roughly a function of area.

There will be differences - the larger anchor will be made from aluminium that is thicker - and this will resist burial (more than the smaller anchor, made from thinner plate) - but variations in seabed shear strength might mask (or enhance this).

We have a FX 16, FX23 and FX 55 and I marked the fluke of the FX 55 to indicate the area of the FX 16, simple straight line across the 'toe' of the FX 55. I then set the FX 16 to bury it, with the stock lying on the seabed, fluke completely buried and shank lying 'touching' the seabed. I then set the FX 55 alongside to the same tension in the same rode - the flukes buried roughly as expected. Hold is a function of area.

The larger anchor should be easier to trip as the larger anchor has a longer shank and thus offers you greater support of the lever arm effect.

In the conditions you describe I would use the bigger anchor simply because it would be easier to retrieve. You don't know how bad it might get and retrieving a deep set smaller Fortress by hand would be a devil of a job (and might be irretrievable). In the annals of Fortress history anchors lost simply because they cannot be retrieved is not uncommon.

An interesting observation, totally expected.

Hold as a function of area, or a function of weight in the case of a Fortress, should not be a difficult concept to grasp. With 'other' anchors it can be a bit more complex as steel is made to specific thicknesses and when the anchor designer scales up he might want to use, say 8mm plate and if it is unavailable he then has to decide if he will use, say, 6mm or 10mm....Bigger anchors also are deployed using bigger chain and shackles - all of which impede burial (scaled accurately to a greater or lesser effect).

Jonathan

Edit

As long as the smaller anchor still has potential capacity, so its hold could increase if there was more tension (say stronger wind) then the big and small anchor will have the same developed hold. If the wind continues to increase their individual holds will increase, until the small anchor reaches its maximum hold in that seabed. Even for a small Fortress that hold can be huge, more than the tension you are ever likely to impose on the anchor.

The reason the big anchor's hold is 'questionable' is that if the wind veers, or reverses then the larger (longer) lever arm of the shank will allow a sideways pull to cause the anchor to move, laterally - and the hold may then be compromised. This is exacerbated as only part of the fluke is embedded, the section not buried simply increases the lever arm effect. In the same way the lever arm allows an easier retrievals of a deep set anchor it also 'encourages' the anchor to move.(so ease of retrieval does have side effects).

Fortress are listing the ultimate hold of a FX 16 as 5,000lb in hard sand (much less in mud - where size does matter, as does using a Fortress) - I hope no-one reading this ever experiences even 25% of that tension in their rode.

J

Just a quick thought……. You’ve made a clear correlation between fluke area and holding. My question is this: how did the perimeter of the anchor fluke correlate to area buried and holding? I would propose that anchor holding is a function of the shear plane within the substrate rather than the area of the buried fluke. Of course the geometry of flukes might mean that buried area and perimeter of that buried fluke might also be linear. Will need to think about the properties of a triangle…