A contradiction with bigger anchors

[lw395]

'.... Gusts in the 25-30 kts range in shallow water with 4:1 scope with a well set anchor can be very uncomfortable. .......

Depending on how shallow, that sounds like a short scope in a blow?
If you draw some arbitrary catenary in say 10m of water with say 4:1 scope, then think about extending the same curve upwards as if the tide has come in, the extra chain (or string) added will only be say 1 and a bit times the extra depth of water. Scope ratios go wrong at extremes of deep and shallow. In fact something like 1.5x depth plus 30m might make a lot more sense? (for chain or mixed rodes)

Rest of your post makes my main point. Anchoring is dynamic in the real world.
Snubbers are good.
Nylon rodes are good.
Catenary is good but limited.
Angels/Chums/mid-rode weights can be good (sometimes?)

 

[Robin]

'Fully clear of the bottom in gusts'
That means it has allowed the boat to move back in the gust, absorbing some energy. Maybe it doesn't bode well for Force 10, but it will be reducing the peak loads and making life better onboard in the F6/7.

Not my experience. Gusts in the 25-30 kts range in shallow water with 4:1 scope with a well set anchor can be very uncomfortable. The chain can lift off the bottom very quickly and create a considerable snatch due to the relatively short length of chain and lack of weight. A very long snubber line and an over sized rubber snubber make a huge difference but that 10mm chain is totally straight. We now use a three strand snubber line that is half the length of the boat (44ft boat). The fairly recent addition of the large rubber snubber to this line has made a considerable difference.
If we anchor in deeper water we don't get the snatch in this kind of gusty weather which would suggest we are not straightening the chain at these wind strengths. So deeper water in better for comfort as the weight of chain is useful.. We have found this true up to gusts of circa 40kts. We have not needed to anchor in stronger winds.[/QUOTE]

I have been a fan of rubber snubbers added in thesnubber line for many years. Whilst the chain weight and stretchy nylon snubber line all help the last 'jerk' seems to be better absorbed by the rubber dog bones. We used to have two such, one normal and one heavier duty for really bad conditions. currently we use two separate lines ( joined at the chain connection) set as a bridle, better suited to our foredeck arrangement, with equal sized dog bones in each arm (or is it leg?) of the bridle which we adjust to suit how we want the boat to lie comfortably to any waves/swell.

 

[KellysEye]

Anchor size for boat length is here https://www.jimmygreen.co.uk/p/technical/chandlery-technical-articles/anchor-size-guide it's normal to buy one size bigger. That guide doesn't apply to medium and heavy displacement boats. Bear in mind the length and weight of the chain catenary is important to stop dragging, shallow water needs chain 10 x depth deep water 3 x depth.

 

[NormanS]

Anchor size for boat length is here https://www.jimmygreen.co.uk/p/technical/chandlery-technical-articles/anchor-size-guide it's normal to buy one size bigger. That guide doesn't apply to medium and heavy displacement boats. Bear in mind the length and weight of the chain catenary is important to stop dragging, shallow water needs chain 10 x depth deep water 3 x depth.

What sort of wind conditions is that guide supposed to be for? Some of us who anchor all the time have to be able to take whatever the weather gods send us.

 

[vyv_cox]

The mass of the chain gives it inertia as well as weight.
As it is dragged through the water it gives some damping.

How can inertia come into it when the chain is effectively straight? I accept that there is always a small amount of curve but nowhere near enough to off significant elasticity.
Damping due to dragging through the water? What contribution is that to my seven ton boat yawing about in a force 6? When I dragged my 60 metres of chain down the boatyard several people here suggested that my pulling force was probably 50 kg at most. So dragging it through the water must require considerably less, 10 kg?

That means it has allowed the boat to move back in the gust, absorbing some energy. Maybe it doesn't bode well for Force 10, but it will be reducing the peak loads and making life better onboard in the F6/7

Having spent a lot of time watching this happen I don't think this is the true situation. Almost all of the chain is nearly in a straight line, with a metre or two lying on the seabed. In gusts or at the turning points of yawing the last short length lifts off the bottom. I have not bothered to calculate the horizontal movement represented by this action but I would assume it to be minuscule. Without a snubber the snatching at the boat is considerable and I doubt that any energy is being absorbed. Every time that this takes place, snubber or not, the shank of my Rocna lifts a little.

 

[jimbaerselman]

I have been a fan of rubber snubbers added in thesnubber line for many years. Whilst the chain weight and stretchy nylon snubber line all help the last 'jerk' seems to be better absorbed by the rubber dog bones. We used to have two such, one normal and one heavier duty for really bad conditions. currently we use two separate lines ( joined at the chain connection) set as a bridle, better suited to our foredeck arrangement, with equal sized dog bones in each arm (or is it leg?) of the bridle which we adjust to suit how we want the boat to lie comfortably to any waves/swell.

how long is shaft of your rubber bone? And how many turns do you take around it? Typically, 30 cm, and two turns. That's about 10cm of strain.

2m of nylon rope allows the same strain with a big safety margin - as long as rope is light enough, which it can be when you've got chain back-up. You can then stress it up to about half its breaking strain - giving nearly 10% strain.

If we're matching component stresses (a good concept, the main subject of this thread) we'd go for a max likely load of 2,000 kgf. For rope, try 4,000 kgf breaking strain - 16mm diameter, give or take a bit.

My 20m of 14mm nylon snubber is allowed 2m strain before the chain takes over. The chain occasionally comes taught if it's really windy and I'm only on one rode. That's 10 times more spring than a rubber bone.

My opinion is that those rubber bones are a marketing triumph . . . relying on people not considering lighter nylon lines for a soft ride - with strong backup lines/chains to ensure they're not stretched beyond limits.

Lessons learnt wintering in a surge prone marina . . . heavy slack lines prevent you hitting things, and 10mm lines keep the boat centered with +/- 1m roaming room. Do that with steel springs or rubber bones . . .

 

[rgarside]

...
Having spent a lot of time watching this happen I don't think this is the true situation. Almost all of the chain is nearly in a straight line, with a metre or two lying on the seabed. In gusts or at the turning points of yawing the last short length lifts off the bottom. I have not bothered to calculate the horizontal movement represented by this action but I would assume it to be minuscule. Without a snubber the snatching at the boat is considerable and I doubt that any energy is being absorbed. Every time that this takes place, snubber or not, the shank of my Rocna lifts a little.

Here are some pictures of the static, in-plane, characteristics of an anchor line which might be of interest:
https://www.dropbox.com/sh/kz4fzuzwmovc2p3/AABpzyyMVy_HkdLcdAK6hS7va?dl=0

They show the effect of a nylon section in the rode.

 

[NormanS]

How can inertia come into it when the chain is effectively straight? I accept that there is always a small amount of curve but nowhere near enough to off significant elasticity.
Damping due to dragging through the water? What contribution is that to my seven ton boat yawing about in a force 6? When I dragged my 60 metres of chain down the boatyard several people here suggested that my pulling force was probably 50 kg at most. So dragging it through the water must require considerably less, 10 kg?



Having spent a lot of time watching this happen I don't think this is the true situation. Almost all of the chain is nearly in a straight line, with a metre or two lying on the seabed. In gusts or at the turning points of yawing the last short length lifts off the bottom. I have not bothered to calculate the horizontal movement represented by this action but I would assume it to be minuscule. Without a snubber the snatching at the boat is considerable and I doubt that any energy is being absorbed. Every time that this takes place, snubber or not, the shank of my Rocna lifts a little.

Is it not far better to do something to stop all this yawing and snatching in the first place? See some suggestions in #13.

 

[geem]

My opinion is that those rubber bones are a marketing triumph . . . relying on people not considering lighter nylon lines for a soft ride - with strong backup lines/chains to ensure they're not stretched beyond limits.

I would have agreed with you until somebody gave me one of the big ones! Added it to the very long 16mm snubber line. It becomes the stretch of last resort! We broke the snubber in some windy conditions prior to getting the large black mamber! In similar conditions we just don't feel the snatch now. Quite impressed but I think the secret is a big one! We are hanging a 19 ton boat of ours.

Lessons learnt wintering in a surge prone marina . . . heavy slack lines prevent you hitting things, and 10mm lines keep the boat centered with +/- 1m roaming room. Do that with steel springs or rubber bones . . .

 

[lw395]

How can inertia come into it when the chain is effectively straight? I accept that there is always a small amount of curve but nowhere near enough to off significant elasticity.
Damping due to dragging through the water? What contribution is that to my seven ton boat yawing about in a force 6? When I dragged my 60 metres of chain down the boatyard several people here suggested that my pulling force was probably 50 kg at most. So dragging it through the water must require considerably less, 10 kg?



Having spent a lot of time watching this happen I don't think this is the true situation. Almost all of the chain is nearly in a straight line, with a metre or two lying on the seabed. In gusts or at the turning points of yawing the last short length lifts off the bottom. I have not bothered to calculate the horizontal movement represented by this action but I would assume it to be minuscule. Without a snubber the snatching at the boat is considerable and I doubt that any energy is being absorbed. Every time that this takes place, snubber or not, the shank of my Rocna lifts a little.

Water is a lot more viscous than air.
The rode is not generally in still water.
A lot of assuming and 'not bothering' going on.

 

[Neeves]

A lot of assuming and 'not bothering' going on.

I am not suggesting you are right, or not - but it is the internet

Some, or many, quote as if they have been there and done that - the reality is they are repeating on something that happened to someone else, once, 10 years ago, or something, someone might have said as a guess.

The other classic is to recount a story - but miss half the salient detail out!

As long as it is repeated often enough it thus must be true.

As an example - the informed knowledge is that you cannot re-galvanise G70 chain (I confess to have repeated this 'truth'). The reality is that in fact I have never seen any verification of the statement (so its now on my must 'to do' list) and this example does have some connection to the thread

There are lots of examples - but that would invoke massive thread drift

But there are people on here who give reliable advise, tend not to assume and do bother. You make your own decision on who you think is reliable. In my case I try to add some of the questions to my to do list - and these threads generate interesting aspects.

Jonathan

 

[Robin]

how long is shaft of your rubber bone? And how many turns do you take around it? Typically, 30 cm, and two turns. That's about 10cm of strain.

2m of nylon rope allows the same strain with a big safety margin - as long as rope is light enough, which it can be when you've got chain back-up. You can then stress it up to about half its breaking strain - giving nearly 10% strain.

If we're matching component stresses (a good concept, the main subject of this thread) we'd go for a max likely load of 2,000 kgf. For rope, try 4,000 kgf breaking strain - 16mm diameter, give or take a bit.

My 20m of 14mm nylon snubber is allowed 2m strain before the chain takes over. The chain occasionally comes taught if it's really windy and I'm only on one rode. That's 10 times more spring than a rubber bone.

My opinion is that those rubber bones are a marketing triumph . . . relying on people not considering lighter nylon lines for a soft ride - with strong backup lines/chains to ensure they're not stretched beyond limits.

Lessons learnt wintering in a surge prone marina . . . heavy slack lines prevent you hitting things, and 10mm lines keep the boat centered with +/- 1m roaming room. Do that with steel springs or rubber bones . . .

Long before commercial rubber bone compensators, I and others in my then YC made our own out of multiple lengths of shockcord, bound into one unit, these were hooked in over a slack loop in the chain rode. OK back then we mostly anchored in lighter winds in more sheltered locations. Later as we grew more adventurous I discovered the rubber compensators which are inserted into a nylon snubber line, typically 20ft or more. We used two of these also on the fixed ( bow) lines our home marina berth too, but not on the other lines. I'm not sure how long our current rubber bones are as Im not on board, the size is generally related to the rope size. I do know I have more than a couple of turns around the rubber bone, probably at least 4, whatever seems right for the line size in use when I make them up. If they break the load is still on the nylon line or ultimately of course the chain I can only remember one bone breaking although they do show signs of being distorted by the line after time I think our line on the normal ones is around 12-14mm and on the heavy duty bone the same although that one ( different maker) could take more, like18mm if needs be. In normal winds that one stretches little which is why it is reserved for bad conditions and therefore the softer ones are used normally. my feeling is the chain catenary does some work but the snatch loads from yawing re absorbed first by thenylon snubber lines but that the very last jerk as the yaw ends is negated by the rubber bone going to it's limit within the number of line wraps around it.

 

[Neeves]

I tested these rubber dog bone things and as previously mentioned they are the equivalent to 2m of the relevant sized or matched nylon. To produce the same elasticity as 10m of nylon you would need 5 dogbones. The dogbones are more expensive and much heavier than nylon and I would subscribe to the idea they are well marketed. However if you are using 10m of nylon and find extra length, of nylon, inconvenient then they offer (if you use 2 dogbones), in a 10m length a 14m equivalent. Personally I'd just use 14m of nylon (which is what we have anyway, each side - climbing rope).

I have tried them and having 2 each side on a cat bridle is quite hefty.

I recall from memory that the recommendation is 3 or 4 turns of the cordage round the bone. If you increase the turns the dogbone could stretch further and I suspect have a short lifespan.

They are valuable for mooring lines as you get that 2m of nylon equivalent in about a 500mm length (that's, I think, the smallest).

Jonathan

 

[KellysEye]

>What sort of wind conditions is that guide supposed to be for? Some of us who anchor all the time have to be able to take whatever the weather gods send us.

Any wind less than gale force if one was forecast we put out a second anchor.The other thing we did was use a riding sail it signifcantly cuts down snatch loads by cutting the swing from up to 90 degrees to 35 to 40 degrees.

 

[Robin]

I tested these rubber dog bone things and as previously mentioned they are the equivalent to 2m of the relevant sized or matched nylon. To produce the same elasticity as 10m of nylon you would need 5 dogbones. The dogbones are more expensive and much heavier than nylon and I would subscribe to the idea they are well marketed. However if you are using 10m of nylon and find extra length, of nylon, inconvenient then they offer (if you use 2 dogbones), in a 10m length a 14m equivalent. Personally I'd just use 14m of nylon (which is what we have anyway, each side - climbing rope).

I have tried them and having 2 each side on a cat bridle is quite hefty.

I recall from memory that the recommendation is 3 or 4 turns of the cordage round the bone. If you increase the turns the dogbone could stretch further and I suspect have a short lifespan.

They are valuable for mooring lines as you get that 2m of nylon equivalent in about a 500mm length (that's, I think, the smallest).

Jonathan

Very roughly we have 2 x 5m lengths of nylon each with an inserted dog bone set together in bridle form to spread the load from chain (hook) to both foredeck cleats, on a monohull of course. I remain fan and I'm a very cynical non-believer of marketing guff. As a matter of interest I have never seen dog bones advertised as being for anchor line snubbers, only as mooring compensators for marina based boats. I had them on our then UK marina berth bow lines allowing some boat movement aft in/after wind gusts ( we were berthed stern to the prevailing winds), but movement forward was prevented by tight spring lines This arrangement served us well through many years and some bad storms and for sure made sleeping more restful in the forecabin.

As another illustration of snubber design use, some boats in the UK on shallow ( drying) moorings broke the riser chains in wave action snatch loads. A solution some used was to add an old car tyre into the riser as a extra strong snubber with a slack loop of chain backup. This was even published as a suggestion at the time in PBO magazine. I was a mooring master at the time at my then YC and we occasionally had boats where the mooring riser chains had snapped, notably from shallow water wave action when the local cross Channel Car ferry had passed by just as the boats were getting ready to take the ground on their drying mooring, it's wake used to send a mini tsunami wave through the moorings which if this met the strong ebb current near LW was almost wall like. An old tyre is not feasible for anchoring unless you have a giant boat but the principle is similar.

 

[Neeves]

Very roughly we have 2 x 5m lengths of nylon each with an inserted dog bone set together in bridle form to spread the load from chain (hook) to both foredeck cleats, on a monohull of course. I remain fan and I'm a very cynical non-believer of marketing guff. As a matter of interest I have never seen dog bones advertised as being for anchor line snubbers, only as mooring compensators for marina based boats. I had them on our then UK marina berth bow lines allowing some boat movement aft in/after wind gusts ( we were berthed stern to the prevailing winds), but movement forward was prevented by tight spring lines This arrangement served us well through many years and some bad storms and for sure made sleeping more restful in the forecabin.

/QUOTE]

There is an American company producing something like the dogbone (which are made in a variety of places) called Shockles

http://www.davisnet.com/marine/products/shockles.asp

They do advertise their devices as applicable for anchoring.

They have a similar performance to the dogbones, I've tested them as well, and one is roughly equivalent to the compatible sized 2m of nylon.


All of these devices, including bridles and snubbers reduce the snatch loads on an anchor chain and thus compensate for not upgrading the chain at the same time as the anchor. They do nothing to reduce the load on the chain when you are trying to break the anchor out after a big blow and there are still waves and chop in the anchorage. It is at this point when there is 'no' scope, the chain is near vertical that huge shock loads can develop and when many anchors are bent or lost as the shackle or swivels fails.

It was said that the infamous Venice Lagoon bent shank incident occurred during retrieval.

An anchor's performance is quantitatively defined by its hold, and this is commonly measured by pulling the anchor with a rode at roughly zero degrees to the seabed. The idea being this is roughly equivalent to how the anchor is used. I have never seen a chart comparing this hold data with the amount of hold of an anchor when lifted. Commonly one tightens the chain until it is vertical and allows the roll and seesawing of the yacht to break the anchor out - this is much more difficult to achieve gently in a rough anchorage.

Jonathan

 

[thinwater]

To me, and shock absorber that can only extend less than a foot (dog bone) is pointless. Good for docklines if there is no better answer.

To me, an bungee system that extends at relatively constant force that is no more than a 20 knot wind load is pointless. It will be extended before the real storm even starts.

Regarding the effect of chain inertia and friction through the water, certainly it exists. However, all of the boat-based testing (Practical Sailor and others) takes this in to account, since they used chain in water. Without that friction, the loads would be several times higher and chains would break somewhere every week. Ask a rigger what happens when a heavy load comes up against a tight chain in the air; something breaks.

 

[Birdseye]

Read any sailing forum on anchors and somewhere, someone will advise, buy the biggest modern anchor that you can safely and efficiently carry. Looking at anchor sizing charts suggestions are go one or 2 sizes bigger - some suggest going to twice the size. The thought process is that the bigger anchor will have less chance of dragging. There is an extension to this advise and thought that if you buy an anchor 25% bigger it will give you 25% more hold as bigger is commonly thought to be a measure of weight.

.................................................................................
To make it clear - as most who read my comments will know: I don't believe there is a simple relationship between weight and hold, or not as simple as double weight and you double hold and I am also yet to hear of a modern anchor of the size recommended on the charts dragging in a normal, sand, clay, silt seabed.

Jonathan

I dont for a minute believe that holding power is a function of weight - the Fortress aluminium anchor proves that. Point weight and sharpness are key in terms of digging in but once embedded in the bottom it is area and shape that govern holding power. The traditional advice on weight comes from the reasonable assumption that the buyer will be buying an iron anchor and increasing weight increases area.

Chain weight is a different issue since you are relying on the catenary to absorb movement and prevent shock loading so there isnt an equation that says chain strength must be no more than anchor holding power. Obviously you dont have chain strength less than anchor holding power.

And then there is a third issue - how strong is the fixing of the cleat to the boat deck. Is the modern lightweight AWB with high windage and high wind loadings equipped with sturdy cleat or just deck ornaments fixed through thin grp with no more than small washers underneath?

 

[noelex]

I dont for a minute believe that holding power is a function of weight - the Fortress aluminium anchor proves that. Point weight and sharpness are key in terms of digging in but once embedded in the bottom it is area and shape that govern holding power. The traditional advice on weight comes from the reasonable assumption that the buyer will be buying an iron anchor

+1

Holding power is roughly proportional to anchor weight. As you correctly point out (and I think everyone understands) this rule of thumb only applies to anchors of the same design and construction material.

 

[Neeves]

+1

Holding power is roughly proportional to anchor weight. As you correctly point out (and I think everyone understands) this rule of thumb only applies to anchors of the same design and construction material.

Proportional means, in your explanation, that if you double weight you double hold. this is not correct - not even roughly. Maybe you can quote some, even any, evidence to support the contention. Please provide any data that a 20kg anchor, of any design, holds 2 times (even roughly) more than a 10kg anchor, of the same design in the same seabed.

Jonathan