Calculating Scope Can Be Very Misleading

[Seaworthy Lass]

Originally Posted by BigBeakie

It seems to me deciding to go with heavier chain to achieve catenary would be wiped out by just a modest increase in wind since the force of wind difference would be way more than the weight of the chain difference, say between 10mm and 12mm as in Seaworthy Lass's example.

Is my thinking correct on that?

Yep .

Originally Posted by BigBeakie

Does it make sense to have quite heavy chain the last few meters, so that it remains on the bottom, and then lighter chain for the rest?

I'm thinking 100 meters of 8mm G70 with much heavier chain, say 15mm, the distance of the windlass to bow roller (approx. 4 meters).

It helps a little initially in keeping the pull parallel to the shank, but increasing the chain diameter from 8 to 15mm for the last 4 metres only gives you about 14kg extra (about 12kg under water) and when the wind picks up it will take little to pick that up off the seabed. You are much better off putting that weight into a bigger anchor to achieve a bigger fluke area if you want to improve holding.

The BIG problem with increasing the thickness of the final bit of chain is that it will decrease the ability of the anchor to dive. Some anchor manufacturers even recommend a SS wire strop leading to the shank, not chain of any thickness at all .

[BigBeakie]

SL,

Good point about the thicker chain inhibiting the anchor burying into the substrate. I was thinking it would give me more abrasion resistance near the anchor, like rubbing on rocks etc, compared to the 8mm.

As in most things, there's pro's and con's.

[Cottontop]

A link to a vey interesting site on ground tackle design. If the author's equations are correct, the forces necessary to "straighten" out the chain and eliminate the catenary effect are surprisingly light.

Also, according to the author, when the angle of the pull on the anchor shaft exceeds 10 degrees above horizontal, many anchors will begin to pull out, instead of digging deeper.

Combine the two above, and this thread by Seaworthy Lass takes on tremendous importance. Maybe now we can understand why so many boats drag.

Rode - Static Behavior

[Seaworthy Lass]

Originally Posted by Cottontop

A link to a vey interesting site on ground tackle design. If the author's equations are correct, the forces necessary to "straighten" out the chain and eliminate the catenary effect are surprisingly light.
......
Rode - Static Behavior

I can confirm this is the case from visual observation. I have snorkelled extensively looking at anchors for several years now (nothing much else of interest underwater here in the Med ). Moderately strong gusts typically lift most chain all the way to the shank for most boats I have observed. With strong wind I venture less far and can only report mainly our own chain, but typically above about 30 knots there is little catenary left (it was better, but not by a huge amount when we had 13 mm chain).

This article is very useful to read:
Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats

A few very relevant bits that underscore the importance of scope (and it is really "effective scope" that is being discussed):

"The practical upshot to this as it will interest most boaters is that the lore of heavy chain is demonstrably false as it applies to small boats and modern anchors. Chain is still necessary for a number of other reasons, but it need not be unnecessarily heavy......

Further, it should be recognized that adequate scope is very important in strong conditions. While catenary disappears, geometry cannot be argued with; for a constant depth, a longer rode means a lower maximum possible angle of pull on the anchor. A ratio of about 3:1 should be considered minimal, and adequate only for light to moderate conditions. Generally speaking about 5:1 is a good ratio to aim for. In worse conditions, the scope should be maximized, up to a ratio of around 8:1 – there is little benefit in higher ratios.

With the weight saved by replacing heavy chain with a lighter but stronger grade, a “weight surplus” is gained which may then be reinvested into the anchor system to increase performance elsewhere. The obvious item to benefit from this weight is the anchor itself – a large anchor upgrade could result in a massive performance improvement, including greater tolerance to short scope anchoring, and still represent a huge weight saving."


Knowing your "effective scope" is critical when assessing the likely holding of your system.
Remember to take the slope of the seabed into account when determining this .

[Jim Cate]

SWL, it is indisputable that catenary disappears as the wind loads increase, and that chain weight matters little in ultimate holding conditions. But, my observation is that in more normal conditions heavier chain is a good thing. First, the energy absorbed in straightening out (lifting) part or all of the rode is greater. This energy absorption slows acceleration of the boat in gusts and, like a snubber, reduces shock loads on teh anchor itself. Second, if your boat sails around at anchor (as ours does), dragging the part of the chain still lying on the bottom through the mud again slows down the boat's wanderings. This difference is really apparent if you compare the actions of boats on chain and those on mixed rodes with lots of rope content.

In both of these cases, greater scope adds to the benefit... no doubt about that! And being able to utilize a bigger anchor is always good IMO... but some weight in the chain is also good.

Say, I know!: Lets use hefty chain and a big anchor too! Oh, and always drop in the center of a depression in the sea bed so that it is uphill in all directions!

Cheers,

Jim

[Pelagic]

Originally Posted by Jim Cate

......Oh, and always drop in the center of a depression in the sea bed so that it is uphill in all directions!

Cheers......,Jim

Actually Jim, I am a big fan of doing that wherever practical.

I think that comes from 6 years running a steam yacht where, when the boilers were cold, it took up to 8 hours for the engine to be on standby.

Old habits just stuck.

[Seaworthy Lass]

Originally Posted by Jim Cate

SWL, it is indisputable that catenary disappears as the wind loads increase, and that chain weight matters little in ultimate holding conditions. But, my observation is that in more normal conditions heavier chain is a good thing. First, the energy absorbed in straightening out (lifting) part or all of the rode is greater. This energy absorption slows acceleration of the boat in gusts and, like a snubber, reduces shock loads on teh anchor itself. Second, if your boat sails around at anchor (as ours does), dragging the part of the chain still lying on the bottom through the mud again slows down the boat's wanderings. This difference is really apparent if you compare the actions of boats on chain and those on mixed rodes with lots of rope content.

In both of these cases, greater scope adds to the benefit... no doubt about that! And being able to utilize a bigger anchor is always good IMO... but some weight in the chain is also good.

Say, I know!: Lets use hefty chain and a big anchor too! Oh, and always drop in the center of a depression in the sea bed so that it is uphill in all directions!

Cheers,

Jim

Ah, if only we could .

Jim, I agree that thicker chain is preferable to thinner for all the reasons you stated. One other benefit you left out is that it helps the initial set as well.

BUT, the drawbacks with thicker chain are:
- Any extra weight at the bow is detrimental to the boat's performance.
- You can carry less length of chain (generally there is no room in the chain locker for an ideal amount of extra diameter).
- You will swing very differently to other boats in the anchorage. The separation between anchors dropped in crowded anchorages is not the sum of their respective rodes and you up your chance of a collision carrying relatively heavier chain than other boats.

Carrying thicker chain but less of it and using rope rode is a nice thought, but in practice it is a PITA and chafe can be a huge problem and swinging into other boats is even more of an issue.

Bigger vessels (freighters) use the hefty chain principle and the view initially was to do the same in yachts. This is simply not the best solution for little vessels . The improvement in holding is only a small fraction of what would be achieved adding the weight to the anchor. But, when faced with a choice of how to improve their holding, many people are still sticking to the big vessel technique and opting to increase the diameter of their chain rather than increasing their anchor size .

Jim, if you had a choice would you want to add any extra weight at the bow? And if you wanted to improve your holding, would you add 20kg extra to your anchor or your chain?

SWL

[TrentePieds]

This has been one of my best mornings in ages - thanks to SWL's cogent and irrefutable presentation of the fundamentals :-0)!! And to Jim Cate's probing for the soft spots. Is that your specialty, Jim ;-)?

Having long-held opinions vindicated is always pleasant, but even more useful to me, at this time, is Peter Smith's article. Some of you may recall that MyBeloved is an absolute greenhorn. She also needs teaching with a VERY gentle touch :-). We are a year and a half away from "the big one", and that's but a short time for a lubber to learn the basics. So the content of this thread, and the Smith article, is now "assigned reading" if for no other reason than to demonstrate that "what the geezer is banging on about" really does relate to the real world, a world so totally different from the one where MB spent her working life taking care of other people's infants and toddlers.

So, SWL and JC: Thanks for the back-up :-)!

TrentePieds

[SVNeko]

Originally Posted by Seaworthy Lass

A few very relevant bits that underscore the importance of scope (and it is really "effective scope" that is being discussed):

"The practical upshot to this as it will interest most boaters is that the lore of heavy chain is demonstrably false as it applies to small boats and modern anchors. Chain is still necessary for a number of other reasons, but it need not be unnecessarily heavy......

Further, it should be recognized that adequate scope is very important in strong conditions. While catenary disappears, geometry cannot be argued with; for a constant depth, a longer rode means a lower maximum possible angle of pull on the anchor. A ratio of about 3:1 should be considered minimal, and adequate only for light to moderate conditions. Generally speaking about 5:1 is a good ratio to aim for. In worse conditions, the scope should be maximized, up to a ratio of around 8:1 – there is little benefit in higher ratios.

With the weight saved by replacing heavy chain with a lighter but stronger grade, a “weight surplus” is gained which may then be reinvested into the anchor system to increase performance elsewhere. The obvious item to benefit from this weight is the anchor itself – a large anchor upgrade could result in a massive performance improvement, including greater tolerance to short scope anchoring, and still represent a huge weight saving."

We subscribe to this way of thinking and use 5/16 Maggi G7 on a 51' cat without any problems. However, you do face the issue of finding fittings to connect anchor or swivel to chain that has the requisite strength. The holes in the smaller chain are just too small!

[Paul L]

Originally Posted by SVNeko

We subscribe to this way of thinking and use 5/16 Maggi G7 on a 51' cat without any problems. However, you do face the issue of finding fittings to connect anchor or swivel to chain that has the requisite strength. The holes in the smaller chain are just too small!

Also some regalvanizers will not do G70 chain as they are concerned about the strength loss, making the lifetime cost of G70 pretty high if you are an active anchorer.

[noelex 77]

Originally Posted by SVNeko

The holes in the smaller chain are just too small!

Connections for G7 chain can be a problem. There is no point having a weak shackle with strong chain.
The easiest solution is to get the factory to weld a larger link on both ends, but this means the chain is a special order. If this is not done there are still solutions.

This is a new connector from Maggi. Galvanised with a titanium pin it is strong and seems like a good alternative, although we haven't personally used one. It is expensive:
https://www.jimmygreen.co.uk/item/55...librated-chain



Another possibility is this sort of G8 shackle. Maggi once offered these with a galvanised finish, but I am not sure if these are still produced. Alternatives from other companies are available in G8, but these are not typically galvanised:



Another alternative is G8 shackles in a conventional bow pattern. These are relatively cheap and reasonably readily available from Crosby, Green Pin, Titan etc. Unfortunately, even with the best of these shackles, there is usually a slight loss of strength compared to the chain. In some chain sizes it is possible to get an acceptable alternative, but you need to check the specifications. As the shackles can subject to some side loading it is helpful to have the shackle stronger than the chain if possible.

As a final option, some people slightly squash the final chain link to accept a larger pin. I think there is too much danger of weakening the link doing this. It would be good to have independent testing done.

[SVNeko]

Originally Posted by noelex 77

Connections for G7 chain can be a problem. There is no point having a weak shackle with strong chain.
The easiest solution is to get the factory to weld a larger link on both ends, but this means the chain is a special order. If this is not done there are still solutions.

This is a new connector from Maggi. Galvanised with a titanium pin it is strong and seems like a good alternative, although we haven't personally used one. It is expensive:
https://www.jimmygreen.co.uk/item/55...librated-chain



Another possibility is this sort of G8 shackle. Maggi once offered these with a galvanised finish, but I am not sure if these are still produced. Alternatives from other companies are available in G8, but these are not typically galvanised:



Another alternative is G8 shackles in a conventional bow pattern. These are relatively cheap and reasonably readily available from Crosby, Green Pin, Titan etc. Unfortunately, even with the best of these shackles, there is usually a slight loss of strength compared to the chain. In some chain sizes it is possible to get an acceptable alternative, but you need to check the specifications. As the shackles can subject to some side loading it is helpful to have the shackle stronger than the chain if possible.

As a final option, some people slightly squash the final chain link to accept a larger pin. I think there is too much danger of weakening the link doing this. It would be good to have independent testing done.

That Maggi fitting is interesting. I haven't seen it in the US. We considered those hammerlocks but we didn't find any galvanized ones and, even though I know they are used commercially, I can't get comfortable with the pressed pin holding it all together. You don't get enlarged links in the US. So we went with a Crosby g209A shackle one size larger. Its a little less strong, but not significantly.

[boatman61]

I prefer to splice a length of multi-plait anchor rope to my bitter end.. a shackle at the hook is unavoidable however.. so I peen well and then run superglue down the thread from the inside to add to the grip.

[noelex 77]

Originally Posted by SVNeko

Its a little less strong, but not significantly.

This is a reasonable solution. As you point out, with a conventional shaped hi test shackle (G8) like the Crosby 209A or Titan black pin shackle, the strength of the largest shackle that will fit through a normal link is slightly less than G7 chain, although make sure you check both the SWL and ultimate strength to get a proper perspective (the saftey factors and therefore the relationship between SWL and ultimate strength are not always the same) for your particular size chain.

On one hand, it can be argued that chain very rarely breaks so a shackle that is only slightly weaker than the chain should be fine. On the other hand, it seems a shame to have say 80m of strong expensive chain only to make the final couple of inches weaker. Of perhaps a more practical concern is that side loading weakens a shackle considerably. At 45° the SWL is reduced by 30% and at 90° by 50%.

Underwater I do occasionally see shackles that have jammed on the anchor shank in unusual positions. It is worth trying different directions of pull and seeing if this can happen with your anchor/shackle combination. If so, an alternative shackle can usually be found that will not do this. This is particularly important if the shackle is not stronger than the chain.

To put this in perspective, most anchors are held on with no name shackles of rather dubious quality, or even worse, no name swivels. I don't have much confidence in the quality control of these and a defect like an inclusion could dangerously weaken the structure. Despite this, there a very few instances of problems. If you are using rated shackles with a published SWL that is reasonable for your sized boat, you are doing much better than most people.

[SVNeko]

We have an oversized ultra swivel after the shackle, so side loading is an issue . However, it is a 1/2" size swivel and a massive oiece of kit. To deal with shackle issues we have a soft shackle in there for a backup. One made from 1/4" dyneema just fits.

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