Anchor design and misnomers

[Kettlewell]

Snowpetrel, you may be on to something there. We now need someone to come up with a Java widget you can plug your information into to come up with a number. Clever to think of the roller furling headsails--I think that is a big factor. I can detect significantly less force when I drop the furled jib for hurricanes and such. Still, I think it is overkill for most of us, as the rules of thumb seem to come up with the same result--at least for me!

[Delfin]

Originally Posted by JonJo

Thanks Delfin,

The question over a second anchor is thus,

It actually does not need to be a big anchor, but if you follow BIB then your anchor might demand more than the direct muscle power of a cruising couple and thus becomes more of an issue.

Scenario

Your anchor becomes stuck, you needs abandon it (you might buoy it for later retrieval) but you now need a second anchor. Or you find your primary anchor does not work in the gravel bed, thin mud, hard substrate, in which you desire refuge (you know that because you have tried it 3 times). The final horror would be, in your case, the hydraulics fail. I understand you use, or carry, a Guardian - I assume that it is your fallback - and being a Guardian will be relatively easy to attach to your rode.

My thought was that if people were to say, carry a 55kg Rocna, they might also carry say a 55kg Spade, or Delta as a second and the Guardian/Fortress carried as a hurricane anchor. So I was really thinking of how to handle an equally large steel model (as opposed to an alloy as the only second anchor). I also wondered where they might store the second heavy anchor and whether they had 2 rodes.

People are carrying anchors, BIB, for worst case scenarios - your worst case scenario must be loss of hydraulics as your anchor is certainly large. As Evan said anyone with any modicum of seaman ship should be able to arrange use of a sheet or halyard winch but I suspect you have neither. So what is the fall back - as whatever you do might be applicable in other situations for other people

I had hoped to ascertain a good cross section of replies, but apparently they have already all replied with an equal clarity to your post and certainly cannot afford the time to repeat, even if they have the time to show frustration, whatever it was they said.

Chastened,

Jonathan

The fall back is a come along attached to the mast. Since I have two sources of hydraulics and two windlasses the chances of not having flow isn't that great, but I suppose it could happen which is why I carry the come along and a handy billy.

If a 176# Claw won't isn't right for the seabed, I don't know what would be so I'd go someplace else. Gravel, no problem. Thin mud, it's going to be sinking deep and there must be some thicker mud someplace. Hard substrate, go elsewhere.

The secondary anchor is a Guardian because it is big, will hold the boat, has its own rode and can be handled by moi by hand.

[JonJo]

SnowPetrel,

We looked at a comparison of a catamaran to cruiser yacht based on cross section area of a maximum beam section and maximum LOA section.

We did this from the design drawings, of a Bav 45 (about 5 years old model now) and our own catamaran. Our cat is actually 35' with 3' transom platforms, which are only about 3 inches above the waterline (so in terms of windage we discounted them, or they make no difference).

A 35' cat has the same windage (beam + length) as a 45' standard production yacht (beam + length). (Ignores bimini, boom cover, mast etc). The 45' yacht will weight about 2 times the cat, both in full cruising mode. The underwater profile is also different, but have not looked at it. Our cat tends to be a bit more streamlined than say a Lagoon and a Bav 45 a bit more streamlined that the new Moody deck saloon. We would use a bridle, not sure how that impacts.

Others who have looked at windage have taken frontal area and, say, 50% of length. In terms of worst case scenarios then full side area is probably more relevant. For a mono - length looks sensibly simple.

Individuals might need to consider how much extra windage they have added, dinghy on davits, biminis, dodgers etc (which can be quite large, especially on monohulls)

How this fits into your formula I'm not sure?

maybe multihulls:

2 times (1.3 times length overall in feet) divided by anchor weight, lbs?

But I'm not sure where weight of vessel fits into the equation?

I'd agree with using 2 as the alloy anchor factor weight,

And 1.25 looks an uncontentious factor for next G anchors (its used by the Classification Societies) and anything to keep ideas uncontentious?. I'd actually prefer 2 separate polls, one for Next G and one for older - with the 2 types listed (too complex?)

Jonathan

[JonJo]

SP, you had moved on as I was slowly drafting - some of my comments you have covered, but 1.3 for multis - unless there is a weight issue?

Jonathan

[nigel1]

Originally Posted by Snowpetrel

By the way Nigel1, I noticed anchor handlers have exceptionally large anchors for there size, is this due to salvage and towing requirements. Their anchor factor falls in at around 8 to 16?


Cheers

Ben

I believe this is the case. When I worked for Smit Tak, there were occasions when we would lay out both anchors after connecting the tow wire to a grounded ship and then use winch and windlass in an attempt to free them.
One advantage was to avoid using engines in typically very shallow water with the risk of pulling mud/sand into the cooling system.

In the past, it was common for anchor handlers to moor to rigs. Typical method was to steam towards the rig, let go one anchor on the way in, when close to the rig (really close), turn the boat hard over, steam ahead, drop second anchor, then back down and make fast two stern lines to the rig. (Very rarely done nowadays). It was also common for the tugs to carry much longer lengths of chain. Smit tugs generally carried 26 shackles on each side (over 700 meters).
Its also useful to have big anchors on this type of boat,as whern nat anchor, they behave like a typical sail boat, and sail around the anchor due to the windage forward. Bigger anchor sets better and less likely to pull out as the boat sails about.

[Dockhead]

Originally Posted by cwyckham

Where did you find this? You have presented a lot of material in this thread as fact, but I don't know if it represents actual soil science or your (or a biased anchor manufacturer's) attempt to understand.

As others have said, I think that we can basically call any anchoring substrate incompressible. However, it is absolutely correct to talk about "compressive forces" if you want to use that term. Shear forces may also be important. However, I am not a soil scientist, so I really don't know.

My gut feel is that convex anchors may actually be more effective if they can be convinced to dive, while concave anchors may be more effective if only skimming the surface.

The only direction that the substrate can move is upwards. Sideways force must be absorbed into the effectively infinite, incompressible substrate. If one could somehow take away the surface effect and bury a wedge shape very deeply into the substrate, then the convex wedge shape will put large compressive forces on the substrate on either side of it. The forces will be smoothly distributed in a bulb shaped area on either side of the wedge. The substrate can't move out of the way as it is incompressible and the holding will be excellent.

If we do the same experiement with a concave anchor, then there will be no compressive forces out to the side, only on a thin column the same width as the anchor itself. Again, the holding will be good since the substrate is incompressible, but if we assume minimal shear strength, then the shape and distribution of the substrate providing the resistance is very different than for the wedge. It is a narrow area directly in front of the anchor with very distinct edges.

Once we take this experiement closer to the surface, then you have the more realistic case and can see the effect. The convex wedge shape will push both sideways and upwards. If it doesn't dive, it will just lift up the substrate and plough. If it dives, then it will hold very well.

The convex anchor will either lift up a divet of substrate or it will dive. If it dives, then it will hold well. If it stays near the surface, then it is relying on the size of the column of substrate in front of it and on the shear strength of the substrate preventing a divet from being pulled out. Surface area will be key.

Anyway, I think it's safe to say that nobody on this thread, and maybe not very many people actually in the anchor industry are doing realistic models of actual substrate behaviour. It is a tough thing to model. Simplified diagrams showing which way the forces are pointing on the anchor itself probably don't help much since it is the behaviour of the substrate that determines whether the anchor tears through or stays fast.

I think diving ability is probably much more important than other shape characteristics in the end, and I'm not sure what would determine which anchor dives well and which doesn't. I do agree that a hoop would often limit the depth of dive, though.

You've just given everyone a license to play amateur soil engineer Thanks!

If you are correct about the soil being "essentially incompressible" then once an anchor is fully buried, then the shape doesn't matter so very much, does it? As the soil qualities approach actual total incompressibility, then differences in holding power disappear, become a pure function of surface area presented towards the direction of pull, not so?

I actually think (based on pure, uninformed, dilletantish, amateur speculation ) that the shape of an anchor is actually important exactly because of the way it affects the way an anchor sets, not nearly as much because of how it holds once fully buried. My speculation is not worth anything, as I totally lack any professional knowledge, but "my gut" (as you called it), is that probably most convex anchors will be plowing, pushing aside the seabed, at a certain stage where most concave anchors are probably digging in.

The concave anchor gets into a positive feedback loop -- a virtuous cycle -- when it gets just a bit of its tip into the seabed -- the soil gets heaped into t he concave part instead of being pushed aside, and all the forces combine to point the tip of the anchor down into the seabed, so that it sets. This would account for the strong "jerk" that people notice when they first try out concave anchors.

Convex anchors, most of them, probably, in most cases, need to be deeper in the seabed before they start to set, because the first thing they start to do is push aside the soil, which does not draw the point into the seabed. Eventually, if you keep pulling on it, you overcome this effect and the point goes in.

Certainly, this is that it feels like. I have more than a decade of experience with various convex anchors, and all of the ones I have used feel like they drag across the seabed as you first starting setting them, and then if everything goes right, they start to gradually sink in as they plow gradually deeper and deeper. Whereas concave anchors tend to either set immediately with a pronounced "jerk", or else for whatever reason they don't set at all. But not setting at all is much rarer, with all the concave anchors I've used, than with any of the convex anchors I have used.

Once really set well, I doubt that there's much difference between concave and convex anchors. It's how you get to the point of being really well set is what makes the difference.

At least, that's what my totally uninformed speculation tells me.

As others have said, not every convex anchors works the same way; likewise with concave ones. Fairly subtle differences in geometry or balance can make a big difference. So I also don't think you can lump all anchors of a given type together, either.

[Kettlewell]

Certainly, this is that it feels like. I have more than a decade of experience with various convex anchors, and all of the ones I have used feel like they drag across the seabed as you first starting setting them, and then if everything goes right, they start to gradually sink in as they plow gradually deeper and deeper. Whereas concave anchors tend to either set immediately with a pronounced "jerk", or else for whatever reason they don't set at all. But not setting at all is much rarer, with all the concave anchors I've used, than with any of the convex anchors I have used.

Once really set well, I doubt that there's much difference between concave and convex anchors. It's how you get to the point of being really well set is what makes the difference.

I think it is highly dependent on what type of bottom you've got. What you describe above sounds about like a nice firm mud/clay bottom to me, but it won't work that way in say firm sand, soft sand, very oozy mud, very hard bottoms, etc. For example, I have many times observed down in the Bahamas and Caribbean (where the water is warm and clear) my various anchors only partially buried with significant parts of the anchor still on the surface, yet supplying enough holding power for 30-40 knots or more of wind. In these trickier holding grounds anchor shape, weight, rode, and other factors are very important and can make a huge difference in holding. Look at Evans' report when he tested anchors down in Chile. From his experience there you need an anchor with multiple gripping points, lots of weight, and surface area to weight ratio is not as important as in a nice muddy bottom.

One thing we have to be careful of when looking at the oil rig anchor information is that I am pretty sure most of these rigs are anchored where there are deep mud bottom sediments. In other words, perfect holding ground. Their anchors are optimized for that--no weed, no rock, as much scope as they want to put out, one direction of pull.

[Factor]

Originally Posted by Dockhead

The concave anchor gets into a positive feedback loop -- a virtuous cycle -- when it gets just a bit of its tip into the seabed -- the soil gets heaped into t he concave part instead of being pushed aside, and all the forces combine to point the tip of the anchor down into the seabed, so that it sets. This would account for the strong "jerk" that people notice when they first try out concave anchors.

Convex anchors, most of them, probably, in most cases, need to be deeper in the seabed before they start to set, because the first thing they start to do is push aside the soil, which does not draw the point into the seabed. Eventually, if you keep pulling on it, you overcome this effect and the point goes in.

.

To prove your point about all being different, if I dont get a quite significant jerk, and sudden coming to a halt then I re-anchor, so Convex - good ones, do offer similar behaviour

[Kettlewell]

The question is does the initial good set and "jerk" mean your boat will hold during the 2am thundersquall? From all accounts it does seem that the new roll bar, concave anchors dig right in and provide a good initial set that holds well while backing down. The question some are asking is whether or not the roll bar and the concave shape resist really deep burying in appropriate bottoms, which is what gives ultimate holding power. For example, you will feel some give backing down hard on a Fortress anchor in a mud bottom because it is diving to Australia (watch out down there!). We also have some areas over here, most notably in the Chesapeake, where the bottom is extremely soft mud that requires a lot of diving before the anchor gets down to holding ground. In the past with CQRs I used to drop the book and then let it just sit as long as possible before backing down, and then it would only be a little, wait some more, then back down a little, etc. If you just drop the hook, let out scope, and back down you will pull the anchor right out of the harbor. A lot of people use Danforths and Fortress anchors in these bottoms because they max out the surface area and really dive deep too.

[Andrew Troup]

Originally Posted by nigel1

..... When I worked for Smit Tak, there were occasions when we would lay out both anchors after connecting the tow wire to a grounded ship and then use winch and windlass in an attempt to free them.....

Very interesting post, Nigel

Is there anything you can tell us about the anchor design families favoured by anchor handling vessels?

Do you yourself have any preferences?

[Snowpetrel]

Originally Posted by nigel1

I believe this is the case. When I worked for Smit Tak, there were occasions when we would lay out both anchors after connecting the tow wire to a grounded ship and then use winch and windlass in an attempt to free them.
One advantage was to avoid using engines in typically very shallow water with the risk of pulling mud/sand into the cooling system.

In the past, it was common for anchor handlers to moor to rigs. Typical method was to steam towards the rig, let go one anchor on the way in, when close to the rig (really close), turn the boat hard over, steam ahead, drop second anchor, then back down and make fast two stern lines to the rig. (Very rarely done nowadays). It was also common for the tugs to carry much longer lengths of chain. Smit tugs generally carried 26 shackles on each side (over 700 meters).
Its also useful to have big anchors on this type of boat,as whern nat anchor, they behave like a typical sail boat, and sail around the anchor due to the windage forward. Bigger anchor sets better and less likely to pull out as the boat sails about.

Wow. Thanks. Thats some serious chain you are carrying.. Most of the ships I worked on only had 12 shackles per side. I did wonder about the windage and sailing about on those things, a few of the tugs here moor stern to partly for that reason.

I used a similar salvage system to pull a boat off the shallows in antarctica. Except we also ran two shore lines to a handy islet due to poor holding. He then ran a line out to us and winched it tight. I had the engine ticking over to take some of the strain as well. Worked a treat.

[nigel1]

Currently use AC14 anchor or similar. I do not get a say in what type of anchors we end up with.
AC14 is pretty good, higher holding than a typical stockless anchor and stows nicely in the pocket.
Heavy anchor and lots of heavy chain is what we tend to rely on to keep us in place.

[Andrew Troup]

Thanks for that, Nigel, interesting as always.

I hadn't thought about it before, but it occurs to me to guess that above a certain size, it's no longer practical for any vessel to use an asymmetric anchor at the bow, because every time it happened to come up to the hawse the wrong way round you would have to put people at risk at the 'wrong end' of the boat --

and (here I'm really going out on a limb) possibly even endanger the vessel, if the reason you are weighing is that the situation has become untenable?

It makes me wonder again about Mirabella V's predicament, when it dragged and went aground in F6-7

I wonder how suitable a Bruce-style anchor of 600kg on a centreline hawse -- with another of 400kg on a hawse above it -- really are, from a handling point of view.

Especially on a vessel which may not have sufficient foredeck crew immediately available 24/7, and whose layout and equipment are presumably hemmed in by the desire of the owner to impress ...

And especially especially, when the anchors are judged (with hindsight) to be only 60% of the size they should have been, considering the windage of the rig: imagine if the anchors were 1000kg and 700kg, and the bulk of the people available for foredeck duty were more qualified to handle profiteroles than Tirfors, pinch bars and club hammers (let alone gascutting torches)?

And while the arrangement looks tidy, I can understand why they would avoid ever laying a second anchor, with an 'over/under' layout (apologies for the shotgun terminology !). If one anchor has problems entering or leaving the hawse, it's surely compromising the other anchor, for just one thing.

... The photo shows the vessel grounded, with the smaller, top anchor still on the bow

The Master was criticised for not taking steps to minimise yawing, which was the judged to be the proximate cause of the dragging (although lack of scope was contributory)

It strikes me, though, that dropping the second anchor, even under the forefoot, would quickly become a serious complicating factor if the action didn't prove to be sufficient.

I'm guessing the designers THOUGHT they were following a BiB, single anchor (at a time) philosophy, perhaps expecting the 400kg anchor to be the day-to-day anchor.

I say that because the bigger anchor was referred to as the 'storm' anchor - but it was actually in use in the days the vessel was anchored, initially in light weather, leading up to the grounding incident. On the face of it, it would be better if the upper anchor were used routinely, and the lower once dropped under the forefoot if needed for yaw control, or as a backup should the other one drag.

I apologise for speculating out loud, and I hope I'm not perpetrating any untruths; I did read the full inquiry report at the time, but I cannot find it on the www now.

It's interesting (although it makes my brain hurt a bit) to puzzle out the ramifications of a crossed-hawse situation with "over/under" layout. Up to 180 degrees of veer, I guess it's actually slightly better than usual big-ship "side-by-side" layout, as (given equal scope) the top chain will simply cross over, staying clear of the bottom chain, but if the boat were to do a full circle or worse, I shudder to think about the ramifications of trying to get both anchors sorted at the same time in a fire-drill situation, even if it wasn't complicated by one anchor coming up arse-about (or, perish the thought, both!) on top of a foul hawse.

I think unless I'm missing something that the vessel might be well served by having its own little anchor handling vessel, with a nice stainless steel Hiab crane, ROV, etc... now that would be a cushy job, most of the time, Nigel !

[Kettlewell]

I don't know all the details of the Mirabella incident, but it seems to me that they would have been much better served with one big main hook on the centerline and then a couple of huge Fortress anchors on mostly rope that could have been taken out in their tenders.

[Andrew Troup]

Yes, Kettlewell, that has appeal on the face of it

but .... should that centreline hook have a swivel? (which should have been the underlying point of my last post, except it was getting too long).

I would not personally like to be the design engineer who would have to sign off on a compact and streamlined swivel which had to routinely survive the angular deflection ("nipped cable", in ship terms) imposed by a bow roller, without compromising its performance under repeated abrasion and corrosion and shock loading on top of design loads of maybe 30T, when it was arguably the most crucial single item on the boat.

And if it is not equipped with a swivel, or if the swivel cannot be relied on not to jam, is there not a problem?

A conventional ship-style hawse, alongside the bow, housed behind a door if the anchor was bulky like a Bruce rather than low-profile like the Danforth or Dreadnought style, or in any case for reasons of vanity... might be a better solution.

If the anchor were asymmetrical, the swivel could end up below the turning point of the chain, in such a layout.

I suppose a similar arrangement could be done on the centreline, perhaps with a miniature version of a RoRo bow - but that definitely puts all the eggs in one basket, (what if you have to slip a fouled main anchor? I doubt if the classification societies would contemplate only one 'permanent installation' anchor for a vessel that size...) and if yaw control is a routine requirement, then maybe two anchors has to be on the regular menu anyway?