Big anchors

[Kelpie]

Sailing in northern waters... I prefer two anchors...
As always, its the skippers choice.

This is very true. What works best in one place may not work best in another.
We cruised around Scotland for ten years and usually had anchorages to ourselves. You could do what you liked because the biggest danger was your lee shore.
In the Med, and to some extent the Caribbean, the biggest danger is probably that charter yacht lying upwind of you with insufficient scope and a delta copy lying on its side in a weed patch.
I've stopped using a second anchor in any situation where there is a risk that I have to move in a hurry.

 

[geem]

This is very true. What works best in one place may not work best in another.
We cruised around Scotland for ten years and usually had anchorages to ourselves. You could do what you liked because the biggest danger was your lee shore.
In the Med, and to some extent the Caribbean, the biggest danger is probably that charter yacht lying upwind of you with insufficient scope and a delta copy lying on its side in a weed patch.
I've stopped using a second anchor in any situation where there is a risk that I have to move in a hurry.

Yep, we rode Brett in an anchorage with just two boats. We were not far from the beach. We both deployed two anchors. Nobody in front of us. My biggest concern is other boats not the size of my anchor. I big anchor doesn't save you from a dragging boat.
We will be heading South soon for the summer hurricane season. My biggest concern is finding a spot without dragging boats. In the event of any strong winds, we are likely to leave and go elsewhere. Trini, ABCs or out to sea

 

[thinwater]

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…

True. To an extend, area is a surrogate for perimeter. However, the mass of the triangular prism also matters, so probably both.

Another factor is the shape of the plate. In the case of a slightly convex anchor (Excel), depending on soil mechanics, the anchor compresses the soild to the sides and the sheer plane may extend farther outwards than a flat plate. Many soil anchors exploit this. Too sharp a convex angle, of course, just plows through the bottom.

Most important, most of the time, is how deep the anchor can dig, and this is dependent on many factors. The deeper the anchor, the greater the strength of the soil. In sand it may be only increased overburden pressure. In mud, there is often a considerable transition from soup, to mush, to goo, to firm mud 3-4 feet down. This varies with the substrate and has been vexing anchor designers for generations.

 

[srm]

In mud, there is often a considerable transition from soup, to mush, to goo, to firm mud 3-4 feet down.

Very true, early in my career I was working as a surveyor with geophysicists in the Bristol Channel monitoring the flow and build up of fluid mud. Shallow seismic profiles clearly showed increasing density layers with depth.

 

[boomerangben]

Quite frankly despite advances in technology, anchoring remains a dark art, or rather a game of chance. The choice of anchor is a means of hedging those bets. The interesting fact in this is that a relatively modest anchor will generate 2 tonnes of ultimate holding capacity. How many people would be happy hanging a Range Rover or 2 big bags of sand off their bow cleat?

To my mind, the next move forward in anchoring technology in the leisure market is the development of sonar into a tool that can provide more information about the soil we are trying to anchor in.

 

[Neeves]

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…

As Thinwater says, post 83, once you move from the simple (with apologies) of the Danforth/Brittany to more complex shapes, like the convex Excel and concave/convex of the Epsilon it all become more difficult. This is illustrated by a comparison of an Excel with Delta - superficially similar geometry but the Excel (for the same weight) has twice the hold. Or compare a Mantus with Rocna, ostensibly one is the copy of the other - but Rocna of the same weight as a Mantus has twice the hold - because it dives more deeply. Equally complex if you take 15kg anchors, or anchors of the same weight, from an assortment of Rocna, Excel, Spade, Epsilon, Ultra - all very different geometries - they will all have a similar hold in a similar seabed. Weight is simply not relevant. A Fortress with a similar area to a steel anchor will have the same, or similar, hold but be only half the weight (and a Viking somewhere in between). You get better value from your area with an aluminium or HT steel anchor.

It is interesting that Lewmar decided to hedge their bets with design for the Epsilon, a concave skirt, giving area, and a convex central section.

On shear strength - I stand to be corrected (and there are always exceptions) but shear strength increases with the square of depth. The deeper you can bury your anchor - the more secure (which is reinforced experimentally by Thinwater's yawing tests) - shallow set anchors are more prone to tripping than deep set anchors. Manage yawing, 2 anchors in a fork or take your dinghy off the bow (lots of other remedies) and yawing becomes less of an issue. However yawing not only occurs because your yacht is poorly balanced and forecast are just forecasts - it seems prudent to deploy that second anchor - when you have a rough idea how the wind oscillates - to manage an unstable wind. To constantly denigrate those who deploy a second anchor is arrogant and dangerous - how many in the UK's northern offshore islands have been buffeted by bullets, from unexpected directions, once the wind gets up. In our naivety, before we learnt about the value of a Boomerang and decent snubbers, we destroyed a swivel, bent one side of the fork and sheared the sex bolt holding it all together - bullets from unstable 50 knot average wind and an anchorage too restricted to deploy more rode = big snatches. No fault of the anchor, it was immovable (hence the snatches). You learn very quickly from your own mistakes - we dumped swivels, designed the Boomerang and had a long term focus on the rode, viz new ideas on snubber deployment.


Jonathan

 

[thinwater]

The subjects that interest me the most at this point, are ...

• How do we detect bad bottoms? Power setting can reveal some (thin sand over hard pan, for starters). Some we can guess by pulling up the anchor after a poor set and seeing what is on it. You can dive, sometimes you can see, and good fish finders can give some information (you see the echo).
• Holding in good bottoms is not the problem. Manage yawing, use enough scope, and a decent anchor of reasonable size will hold. What designs are best on the widest range of bottoms? Panope has contributed some interesting stuff, but you really need to watch a lot of video to see the details of how the misbehave. He tries with ratings, but they don't tell the whole story. Some anchors are brilliant on one test run and not on the next--I want consistency.

 

[[167227]]

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.

Nice man! Thanks for the advice, I also use opencpn. I have decided life is too short to worry about anchors and went to the beach bar to get drunk with the wife. Good result.

 

[noelex]

The subjects that interest me the most at this point, are ...

• How do we detect bad bottoms? Power setting can reveal some (thin sand over hard pan, for starters). Some we can guess by pulling up the anchor after a poor set and seeing what is on it. You can dive, sometimes you can see, and good fish finders can give some information (you see the echo).
• Holding in good bottoms is not the problem. Manage yawing, use enough scope, and a decent anchor of reasonable size will hold. What designs are best on the widest range of bottoms? Panope has contributed some interesting stuff, but you really need to watch a lot of video to see the details of how the misbehave. He tries with ratings, but they don't tell the whole story. Some anchors are brilliant on one test run and not on the next--I want consistency.

I think these are good points. Some poor substrates such as rock are usually easily detected, but others can be very difficult or even impossible to pick. I also feel that poor substrates are the most common reason why most boats drag.

A versatile anchor design that works well in a wide range of substrates is important and this should influence what primary anchor model you select. In keeping with the title of this thread, it is also another area where selecting the largest anchor you can comfortably manage is a help. A larger anchor can lose a higher percentage of its holding power (because the substrate is poor) than a smaller anchor (of identical design and construction material) and still deliver adequate performance.

 

[thinwater]

I think these are good points. Some poor substrates such as rock are usually easily detected, but others can be very difficult or even impossible to pick. I also feel that poor substrates are the most common reason why most boats drag.

A versatile anchor design that works well in a wide range of substrates is important and this should influence what primary anchor model you select. In keeping with the title of this thread, it is also another area where selecting the largest anchor you can comfortably manage is a help. A larger anchor can lose a higher percentage of its holding power (because the substrate is poor) than a smaller anchor (of identical design and construction material) and still deliver adequate performance.

Yes.

But we're typically talking about going up 20-30% is size, but a poor substrate will decrease hold 70-90%. The increase in size is far less important than the loss. But the reason going up a size or two helps, is that much of the time we are operating at only 10% of what is available in good sand. If that turns out to be 12%, and the loss in hold was 90%, and we upsized by one size ... then it was enough.

Makes you wonder why we don't drag more often, and draging really is rare. It's all about the safety factors.

 

[Neeves]

Makes you wonder why we don't drag more often, and draging really is rare. It's all about the safety factors.

But the march of time has helped. We used to regularly drag with our copy CQR, well overweight. Previously we never dragged with our genuine CQR. It all changed with the 2006 YM report on the West Marine anchor test - oddly about 15 years after Spade and Fortress were introduced - both of which slipped well under the radar, until 2006 (don't knock the media! ).


Ah! You mean the safety factors the manufacturers have already accommodated, wisely, within their recommendations - as they will not want to be taken to the cleaners (or found on a lee shore) by a tort lawyer. I have always thought they must finish work each day with a smile on their face as their prediction on the size of their pensions is again an underestimate. Who needs salesmen with people recommending (without an iota of data) even bigger anchors on every relevant thread - they don't need to sell more anchors - when people are buying bigger and bigger.

It does make you wonder why people think owners are parsimonious.


It does seem inconceivable that the anchor spreadsheets do not already incorporate a safety factor


Jonathan

 

[noelex]

Yes.

But we're typically talking about going up 20-30% is size, but a poor substrate will decrease hold 70-90%. The increase in size is far less important than the loss. But the reason going up a size or two helps, is that much of the time we are operating at only 10% of what is available in good sand. If that turns out to be 12%, and the loss in hold was 90%, and we upsized by one size ... then it was enough.

Precisely. We are continually promised in this thread that a 15kg anchor will hold 2000kg. It may in an ideal substrate, but even in many ordinary common substrates the results will be much less. Once we consider difficult substrates, the results will be a small fraction of the readings found in the perfect hard sand. The substrate is of critical importance.

If we look at the results of Professor Knox listed in post #29. This test was in sand/mud rather than the perfect hard sand. Here a 15kg Spade held 420 kg. A 6kg Spade only held 120kg.

The greater holding power of a larger anchor will enable the safe use of a larger range of substrates while keeping the anchor’s ultimate holding ability in that substrate adequate. The ability to extend the range of usable substrates is one of the major advantages of fitting a larger anchor.

 

[geem]

Precisely. We are continually promised in this thread that a 15kg anchor will hold 2000kg. It may in an ideal substrate, but even in many ordinary common substrates the results will be much less. Once we consider difficult substrates, the results will be a small fraction of the readings found in the perfect hard sand. The substrate is of critical importance.

If we look at the results of Professor Knox listed in post #29. This test was in sand/mud rather than the perfect hard sand. Here a 15kg Spade held 420 kg. A 6kg Spade only held 120kg.

The greater holding power of a larger anchor will enable the safe use of a larger range of substrates while keeping the anchor’s ultimate holding ability in that substrate adequate. The ability to extend the range of usable substrates is one of the major advantages of fitting a larger anchor.

Only if you can fully set it in the difficult substrate. If it's only partly penetrating the difficult substrate due to broken coral debris, as is very common in the Caribbean, it will only develope the same hold as a smaller anchor that is fully buried. If the bigger anchor can fully bury itself then presumably the smaller anchor can easily bury itself, since it will have the same load applied by the yacht. In my experience of anchoring in such seabeds, a fully buried anchor doesn't drag.
I think when you use a bigger anchor in muddy sand then you have an advantage. One place in the Caribbean where the holding is poor is the anchorage at Fort de France. I can drag my Spade anchor all around that anchorage with my engine in reverse. The Spade has a relatively small surface area of fluke for its weight. The lead ballast in the tip is fantastic at aiding the setting process and the sharp tip penetrates extremely well but in mud, these features are really of no great benefit. At that point, it is far better to use an anchor more suited to mud. A Fortress or similar will be more effective than any NG anchor.

 

[noelex]

It does seem inconceivable that the anchor spreadsheets do not already incorporate a safety factor


Jonathan

The better anchor manufacturers list the wind strength that was used when formulating the table. This is often ridiculously low. For example, in the fine print the Fortress table uses 30 knots of wind. Some manufacturers are more realistic, Rocna for example use 50 knots. This is why the recomended Rocna size is much larger than lower performing anchors such as the Delta and Kobra. The manufacturers of these anchors are using a much lower (but unfortunately often unspecified) windspeed.

Even the Rocna tables would hardly be described as using a conservative criterion for those who anchor frequently. Gusts over 50 knots are by no means exceptional.

Manufacturers want to promote small anchor sizes. It sells anchors. If a smaller anchor is recommended, “the model must be better” is the usual thinking of the anchoring buying public. In particular, boat manufacturers want to buy anchors from companies that are prepared to recommend small anchor sizes.

It is unfortunate that some people on this forum want to promote these tables as the gospel truth to be blindly followed. The discrepancies in the recommended sizes are glossed over. For example, for my boat Rocna would recommend a 40kg model (Rocna or Vulcan) . Lewmar would recommend a 20kg model (Epsilon, Delta or CQR). Plastimo would recommend a 16kg Kobra and Viking a 12.6 kg anchor. We can argue which is the best model anchor, but this spread makes absolutely no sense.

 

[Supertramp]

I do not want to sit in gusts of 50knts+ (which is not unforeseeable) wondering if the anchor manufacturer did their sums right.

I will shortly buy a new anchor. Or two. Boat is 36ft monohull ketch, 8.5 tonnes but over 10 tonnes fully loaded, high windage (pilothouse, davits).

So far based on this and other threads I am keen on a Fortress (Fx23) as a kedge lying on 10m of 10mm chain then a rope rode. Means I can deal with mud and have a rode light enough to lay from the dinghy if needed. I have used Fortress before and found it limited in weed and shingle. I have a third rope/chain kedge rode with a 25lb CQR or Brittany.

The main anchor on 60m of 10mm chain could be Epsilon or Rocna. Not keen on the complexity and galvanising quality of Spade. Don't think a Knox will fit well. Rocna suggests 20kg, 25 would be oversized. Lewmar Epsilon suggests 16kg, 20 would be oversized. Is that because they are using different wind strengths to recommend sizes?

I want a single anchor which will hold me beyond foreseeable UK weather. And I can't guarantee the substrate quality. When the weather gets beyond 40knts I don't think I could kedge singlehanded!

So after reading this and other threads I think I can see why people oversize but I also see that it might not be a good way to compensate for poorer holding substrates.

Anyone care to recommend a size and type for me?

 

[Tranona]

Precisely. We are continually promised in this thread that a 15kg anchor will hold 2000kg. It may in an ideal substrate, but even in many ordinary common substrates the results will be much less. Once we consider difficult substrates, the results will be a small fraction of the readings found in the perfect hard sand. The substrate is of critical importance.

If we look at the results of Professor Knox listed in post #29. This test was in sand/mud rather than the perfect hard sand. Here a 15kg Spade held 420 kg. A 6kg Spade only held 120kg.

The greater holding power of a larger anchor will enable the safe use of a larger range of substrates while keeping the anchor’s ultimate holding ability in that substrate adequate. The ability to extend the range of usable substrates is one of the major advantages of fitting a larger anchor.

You are still ignoring the direct relationship between load applied and holding power. You have still not shown that for the same load the holding power of a larger anchor is greater - only that it has greater potential holding power. That is exactly what the Knox tests and all the others show, and why I asked for evidence of hold against load.

Now you are saying forget the tested high holding power and recognise that dragging (failure to hold) because of the nature of the seabed. If that is the case for a given load both smaller and larger anchors will drag. The limitation is the seabed NOT the anchor.

Whenever you give example like your most recent one dragging is assumed to happen because the anchor is too small and yet many people report failure to hold even when using oversized anchors.

You can't have it both ways.

 

[vyv_cox]

Precisely. We are continually promised in this thread that a 15kg anchor will hold 2000kg. It may in an ideal substrate, but even in many ordinary common substrates the results will be much less. Once we consider difficult substrates, the results will be a small fraction of the readings found in the perfect hard sand. The substrate is of critical importance.

If we look at the results of Professor Knox listed in post #29. This test was in sand/mud rather than the perfect hard sand. Here a 15kg Spade held 420 kg. A 6kg Spade only held 120kg.

The greater holding power of a larger anchor will enable the safe use of a larger range of substrates while keeping the anchor’s ultimate holding ability in that substrate adequate. The ability to extend the range of usable substrates is one of the major advantages of fitting a larger anchor.

The 2006 YM/Sail/West Marine tests were carried out in three very different seabeds, one selected because it was poor. The concave anchors (only Rocna, Spade and Manson were in production then) plus Fortress held well in all of them. From memory the Rocna and Mantis did not quite make 5000 lbs in the poor one but otherwise all held at above this figure. All were 15 or 16 kg models except Fortress, which was proportionately too big in error.

 

[Tranona]

I do not want to sit in gusts of 50knts+ (which is not unforeseeable) wondering if the anchor manufacturer did their sums right.

I will shortly buy a new anchor. Or two. Boat is 36ft monohull ketch, 8.5 tonnes but over 10 tonnes fully loaded, high windage (pilothouse, davits).

So far based on this and other threads I am keen on a Fortress (Fx23) as a kedge lying on 10m of 10mm chain then a rope rode. Means I can deal with mud and have a rode light enough to lay from the dinghy if needed. I have used Fortress before and found it limited in weed and shingle. I have a third rope/chain kedge rode with a 25lb CQR or Brittany.

The main anchor on 60m of 10mm chain could be Epsilon or Rocna. Not keen on the complexity and galvanising quality of Spade. Don't think a Knox will fit well. Rocna suggests 20kg, 25 would be oversized. Lewmar Epsilon suggests 16kg, 20 would be oversized. Is that because they are using different wind strengths to recommend sizes?

I want a single anchor which will hold me beyond foreseeable UK weather. And I can't guarantee the substrate quality. When the weather gets beyond 40knts I don't think I could kedge singlehanded!

So after reading this and other threads I think I can see why people oversize but I also see that it might not be a good way to compensate for poorer holding substrates.

Anyone care to recommend a size and type for me?

Same issue as I found - the Lewmar guides are baffling as they use essentially the same for all three designs, CQR, Delta and Epsilon which is clearly wrong. They don't give any explanation as to how they arrive at the recommendations.

I chose the recommended size (10kg) on the basis that I have been using that size Delta on a larger boat of the same displacement but higher windage (but within the Delta recommendation) for a number of years without any problems. The Epsilon is claimed to be a SHHP anchor so should be potentially better. The other main consideration was that my style of sailing is undemanding from an anchoring perspective, both in terms of weather conditions and seabed (mostly firm mud and/or sand). I am comfortable with the choice.

However as many know my boat was commonly used for ocean voyaging and if I planned to do the same I would fit a 16kg because of the potential in more demanding conditions. When I bought the boat it had a 35lb CQR which was standard fit in the 70s when the boat was built, and I guess most of the early intrepid voyager managed quite well with it!

So to answer your question, yes, I think the 16kg would be fine, However if you look closely at the bars on the sizing chart your boat would be position at the top end of the solid blue part, rather than at the lower end like mine, suggesting that the specifics of the boat might lead to consideration of the next size up, even if it starts at boats 5' longer than yours! As I said the charts are baffling.

 

[Supertramp]

Same issue as I found - the Lewmar guides are baffling as they use essentially the same for all three designs, CQR, Delta and Epsilon which is clearly wrong. They don't give any explanation as to how they arrive at the recommendations.

I chose the recommended size (10kg) on the basis that I have been using that size Delta on a larger boat of the same displacement but higher windage (but within the Delta recommendation) for a number of years without any problems. The Epsilon is claimed to be a SHHP anchor so should be potentially better. The other main consideration was that my style of sailing is undemanding from an anchoring perspective, both in terms of weather conditions and seabed (mostly firm mud and/or sand). I am comfortable with the choice.

However as many know my boat was commonly used for ocean voyaging and if I planned to do the same I would fit a 16kg because of the potential in more demanding conditions. When I bought the boat it had a 35lb CQR which was standard fit in the 70s when the boat was built, and I guess most of the early intrepid voyager managed quite well with it!

So to answer your question, yes, I think the 16kg would be fine, However if you look closely at the bars on the sizing chart your boat would be position at the top end of the solid blue part, rather than at the lower end like mine, suggesting that the specifics of the boat might lead to consideration of the next size up, even if it starts at boats 5' longer than yours! As I said the charts are baffling.

Thanks. Your reasoning matches mine. I find it hard to trust the charts but replacing a longstanding 20kg CQR with a 20kg Epsilon should add security without going OTT in sizing.

 

[boomerangben]

….

The greater holding power of a larger anchor will enable the safe use of a larger range of substrates while keeping the anchor’s ultimate holding ability in that substrate adequate. The ability to extend the range of usable substrates is one of the major advantages of fitting a larger anchor.

Can you prove that and can a sue you if I follow your recommendation on oversizing my next anchor and end up wrecked??

Of course I am being obtuse. The whole issue with anchoring is the multiple unknowns and the compromises we are willing to make as users. I would suggest having a two or maybe even three anchors of the correct size rather than one oversized. May I ask if your kedge and or spare is oversized?. If it isn’t, then your argument falls flat since in my book losing your bower is a significant risk to a live aboard.