What Size Anchor Chain ? (Again!)

[Ocean Girl]

Originally Posted by Camille1

I watched my chain (and rope being Bahamian moored) on board my 50' monohull yacht during hurricane Ivan. The rope (22mm(7/8") soon parted and I lay to chain only. What happened was that the yacht weaved. By that I mean that she would lie direct to wind which reduced thrust on the hull so she moved forward and catenation increased. It is neigh on impossible to keep dead to wind so she would start to veer. Because of the catenation the bow could veer off. The more she did so the greater the exposed bowside and the greater thrust so the more and the faster she veered until the catenation was eliminated. Wham! That is the strain the anchor and the chain and the stemhead and the cleat(s) must all bear. I am now a believer in a Vee shaped riding sail and to have the anchor and chain, say, 20 degrees off the wind so the thrust is consistent but acceptable, catenation is maintained, and she does not swing to the other bowside. I had 10mm (3/8") high tensile chain with a 25Kg (60lb) Spade anchor (bolt welded) which I remain well pleased with. No, I am not seeking another hurricane to try my theory out! My point is that technique is important as well as chain and anchor size.

Fantastic post!

[klem]

Barnakiel,
You are directly relating breaking strength and catenary which is not quite fair. The maximum force seen by the anchor rode has to do with the shock loads it experiences. Ways to lessen these shocks include catenary and using a shock absorber (snubber). As I argued above, I feel that catenary does not make a good shock absorber because of how little boat movement will greatly change the load applied. If we look at a standard snubber made of nylon line which is what I see most people using, they have good stretch characteristics with stretch of around 15% at 10% load and increasing all the way to somewhere around 50% stretch at 100% load. This means that you can actually tune your shock absorption by changing the length of your snubber (longer means more shock absorption but you will likely sail more on the anchor). With a properly sized snubber, the maximum force seen by the chain will be about the same as with much heavier chain. You are correct though that if you use smaller chain, you will need to use a higher grade to get that same working load limit. The required snubber diameter is not 6" as you suggest, it is probably 3/4" or 7/8" for the given vessel depending on how concerned they are of chafe. Remember, none of the components in the system will see forces greater than that required to pull the anchor out of the bottom. I did the calculations above guessing at 3000lbs of average force in 60 knots for the OP's boat but the peak loads might well be 1.5 times that even with proper shock absorption. It could well be that the boat in question puts on greater or smaller loads than those that I guessed but I think that should be in the ball park.

My own observations of being anchored in 70-80knots was that as the boat settled back, the snubber would slowly start to stretch, the catenary would come out of the chain and the snubber would keep stretching for a two or three feet. This meant that the shock absorption of the catenary had been exhausted and we were relying on the shock absorption of the snubber for the last few feet it took to stop the boat's backwards motion.

I hope that we haven't strayed too much in this debate and have provided the OP with some useful information. You will not hurt your anchoring abilities by using heavier chain, the only things that you are hurting are your wallet and sailing performance. Weight in the ends of boats is important so an excess weight should be eliminated. I argue that you should take the weight out of your chain which will not hurt your anchoring performance and if you have concerns, add it back into the anchor itself by buying a larger one which will definitely improve holding power.

[nitpik]

To all who "weighed in" on this topic (pun intended), a big thank you! So much useful information - even the fact that some of it was contradictory does not detract from it's usefulness in helping me make a decision. I just ordered my Lofrans Tigres with a 3/8 HT gypsy. (OK, wildcat, if you prefer!)
I have always used a snubber of smaller diameter than one would want for a primary rode. This gives even better shock-absorption, and is still of adequate strength, considering that, if it does part, the boat is still attached to the anchor. So, 3-strand nylon with a breaking strength of around 3,000 - 4,000 lbs. should do in my case. (I don't have to worry about chafe in my particular installation)
Again, thank you all for your input - glad I joined the forum. I shall have to get more active with other 'threads'.

[nitpik]

ps., ....... why is my avatar not showing?

[conachair]

Originally Posted by klem

Barnakiel,
. You will not hurt your anchoring abilities by using heavier chain, the only things that you are hurting are your wallet and sailing performance. Weight in the ends of boats is important so an excess weight should be eliminated. I argue that you should take the weight out of your chain which will not hurt your anchoring performance and if you have concerns, add it back into the anchor itself by buying a larger one which will definitely improve holding power.

I can see your argument but not sure if I agree. Playing with this calculater..Cable Sag Error (Catenary Curve Effect) Calculator , for, say, 100´anchor chain length shouldn´t the straight line distance be 200´ (there abouts) as the anchor is at the bottom of the catinary? May be wrong anyway ~chain weight will be less underwater? Anyway, the figures don´t matter hugely to your argument. I have 3/8 chain and am happy with it, along with a new lofrans kobra waiting to get fitted. Extra weight will mean little on my boat and ultimate conditions are just one aspect. With heavier chain you sail around the anchor less in normal conditions and one very important factor which the calcs don´t address ~ I sleep better at night an worry less when away from the boat. This may or may not have anything to do with the underlying science but has everything to do with day to day contentment. There´s a lot more to life than number crunching

[klem]

Originally Posted by conachair

With heavier chain you sail around the anchor less in normal conditions and one very important factor which the calcs don´t address ~ I sleep better at night an worry less when away from the boat.

You nailed the most important point.

[mesquaukee]

Originally Posted by barnakiel

This is an opinion. Others will claim to the contrary.


I have never anchored in true storm conditions (to me = exposed location, swell, and winds in excess of 45 knots) but judging from how my boat behaves in less extreme conditions (eg. a protected anchorage and gusty wind with 45 knots sustained) I must state that the more chain we paid out, the steadier the boats behaviour was and less snubbing we experienced.

b.

I think we can all agree that scope is the answer in high winds and dampening the movement of the boat. This is what minimizes the lifting of the anchor shank and reduces shock loads.

For a scope of say 10 to 1 using 200 ft of 3/4" nylon in winds speed of 60 knots on a 42 ft boat we have a strain of 1500 lbs. the angle on the shank is 4.8 degrees and the rode stretches 36 ft. (100 knts 60 feet)
We all know the wind is not steady so when it subsides the boat is pulled forward and in so doing veers.
Using 3/8" chain the shank is not lifted, about 10 ft of chain lies on the bottom. When the wind subsides the force pulling the boat forward reduces more rapidly than nylon (only 20 ft not 36 ft). The chain lying on the bottom dampens movement and as a consequence less veering.

There is much less veering with the chain since the boat has not been hauled as far forward and consequently the bow will not be pushed around as far by the wind before it is caught by the rode. We all know when watching boats in an anchorage who is on chain or nylon.

A reefed mizzen or a riding sail will reduce or even prevent shock loads due to the boat veering.

At a 10 to 1 scope the angle of the rode in worst case is 4.8 degrees. Any vertical movement of the boat has minimal effect on the anchor (10 ft vertical movement is a few inches of elongation of the rode).

For a 4 to 1 scope 10 feet ft vertical movement of the bow is 2.5 ft of "stretch" on the rode. The anchor has lost a lot of its holding power due to the angle of the shank, 12 degrees, less than 1/2 of ultimate holding power. 25% of the force on the anchor is uplift, this is 375 lbs of lift on the anchor.

In storm force winds scope of in excess of 10 to 1 will:
1) keep the shank from being lifted,
2)chaff resistance on the bottom,
3)minimize shock loads on the rode due to the vertical movement of the bow
4)chain will minimize elasticity of rode so boat is not hauled forward as far between gusts due to veering.
Use a riding sail or reefed mizzen angled to the wind to keep boat on one tack.

I enjoyed reading Peter Smiths Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats for what he emphasized;
All chain of heavy weight not required, nylon and light chain
And then mentioned briefly.
1. In deep water scope can be reduced only if rode is all chain due to catenary.
2. Shock loads or overloads on BBB chain stretches (links deform) and therefore absorbs the strain. High Strenght chain breaks.
3. Nylon chaffs.

The presentation and interpretation of data.....
So are we anchoring in:
deep or shallow water?
Heavy mud or light coral sands?
Smooth bottom or coral/rock covered?
Protected water or open to the weather?

Observing activity in various anchorages during +60 knot blows reveals much. Who stays and who moves.

Snubbers at 60 kts
10 ft, stretches 1.8 ft
20 ft ,.............3.6 ft.

[Bash]

3/8".

Plenty for your boat.

[jkleins]

Originally Posted by barnakiel

This is an opinion. Others will claim to the contrary.

While I am not taking sides on the controversy because I have no idea I was wondering if the above is really true. Is this an issue that does require an "opinion" or can it be mathematically figured out with a reasonable degree of assurance? We know all the loads, weights and issues involved can't a mathematician figure out the loads on the anchor and just tell us whether the heavier chain offers an advantage.

We may still like heavy chain for other reasons (reduced swinging, etc) but surely we don't really need to have a belief system built around something that we can figure out mathematically. That way we can save our opinions for important stuff like battery types, anchor shape and hull material

Jim

[conachair]

Originally Posted by jkleins

but surely we don't really need to have a belief system built around something that we can figure out mathematically.

Jim

Very interesting thread. Anyone can (and probably will) argue one way or the other. Another possible situation.. In a blow in the anchorage you are feeling smug about your calcs seeming to be proved correct and have a head full of figures. Just as a boat upwind drags down towards you and all the figures go out the window as you panic, dragging boats were not in the equation! DoH I find all the calcs very interesting but also find the real world to be a complicated and unpredictable place, even more so on a boat which somehow often seems to prefer the unexpected. At the end of the day we are each on our own boat and can blame no-one when it goes wrong. But keep the calcs coming! Any info is interesting, it´s up to each skipper to decide what´s best for his boat.

[mesquaukee]

Originally Posted by jkleins

...We know all the loads, weights and issues involved can't a mathematician figure out the loads on the anchor and just tell us whether the heavier chain offers an advantage.

We may still like heavy chain for other reasons (reduced swinging, etc) but surely we don't really need to have a belief system built around something that we can figure out mathematically. That way we can save our opinions for important stuff like battery types, anchor shape and hull material

Jim

Mathematical models or computer modeling all suffer from the same problem, assumptions.
There are a number of models that predicts the dynamic load on the rode for gusts of wind, sea state, etc. using all all chain, all nylon, a combination of, with or without kellets, etc. for an assumed average boat.

What the models predicts would be acceptable for one set of conditions can be unacceptable for another.
To be prudent we need a rode that works reasonably well in all conditions.
Shallow water. Nylon or nylon-chain combination is marginally better in reducing shock loads (ignoring chaff).
Deep Water. Nylon is useless, as the scope required to prevent uplift (tear out) on the anchor requires an excessively long rode. (75 ft depth=750 ft nylon rode, chain or combination would be 250-300 ft ). All predictions indicate all chain is superior and the heavier the better as the wind strength increases.

None of the models can mathematically predict Chaff. To prevent failure due to chaff some chain is required, the nylon should never be allowed to touch the bottom. If we use 6 to 1 scope in 20 ft we need 100 of chain, the rest can be nylon. If we expect storm force winds we can take a chance and let out enough nylon to give a 10 to 1 scope to maximize the anchors holding power.

If you are or plan to be a full time cruiser that plans to range far and wide the choice is simple (as any mathematical model will predict for the unpredictable).
Storm force, all chain, as heavy as possible.
Below storm force, minimum 100 ft of heavy chain and nylon.

[jkleins]

Originally Posted by mesquaukee

Mathematical models or computer modeling all suffer from the same problem, assumptions....

Storm force, all chain, as heavy as possible.
Below storm force, minimum 100 ft of heavy chain and nylon.

While I am not disagreeing with your conclusion, I have a hard time wrapping my head around the fact that we can calculate the load on a skyscraper in an earthquake and size the foundation anchors appropriately and calculate the thickness of the windows to withstand a 70 knot blow but not a 100 knot blow and cannot figure this one out. There seems to be many situations in life (airplanes, skyscrapers, bridges, etc) where we depend on engineering assumptions that are much more critical then this and have at least as many variations and assumptions and we have no problem staking our lives on those assumptions.

You argument about chafe is a good one but one that is a decision separate in my mind from the main argument of whether chain catenary benefits the holding of the anchor as there are many ways to reduce chafe besides increasing the size of the chain.

Just wondering.

Jim

[klem]

With some of the fancy fluid modeling software available today, I suspect that a model is possible but I do not know of anyone doing one. The issue is that there are so many variables which lead to so many different assumptions. Unfortunately, one of the variables is the hull and rig design. This means that a separate model would be necessary for each boat. Most of the numbers that are out there are based on a simplification of projected area and displacement but that is an extreme simplification. In my mind, one of the hardest things to simulate would be the boat's behavior as it falls off and fetches up. I have done a lot of simulation but not using the fancy fluids programs and don't know how they handle this sort of thing.

mesquakee nailed the biggest problem for creating a model which is creating the assumptions. You can tell your model that you need a fluid flow of x but you need to define that number x. You also need to choose things like wave size and period, the amount of spray in the air, how the wind is gusting, etc. If there were only one variable, you could build a model, do a sensitivity analysis and get a good idea of the behavior but with each variable, more uncertainty arrises.

Doing a simulation for the actual rode would not be too difficult. You could simulate a boat pulling back on it in different ways and seeing how it reacts. You essentially have a fixed point (anchor), a wire in a medium (chain in water), and a spring (snubber), that are attached to a moving object (boat). The only problem is, we don't have a numerical idea of exactly what the boat is doing because of the problems described above.

Someone could certainly build a model if they had the knowledge of modeling (the real skill is in the assumptions) and access to an expensive computer program. It would be interesting to see how accurate their model is to real world conditions. When something like a skyscraper is modeled, it is broken down into small bits. A window is analyzed by building a model of a window and subjecting it to different wind speeds from different wind directions and seeing the behavior. To model a building structure, they use a simplified model of a building with different loading. The key is that there are not as many factors at play here. The other important thing to realize is that engineering involves safety factors since models are not perfect.

[mesquaukee]

Originally Posted by jkleins

You argument about chafe is a good one but one that is a decision separate in my mind from the main argument of whether chain catenary benefits the holding of the anchor as there are many ways to reduce chafe besides increasing the size of the chain.

Just wondering.

Jim

In deep water chain catenary is of vital importance for the holding of the anchor as Peter Smiths briefly mentions in Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats . It is barely noticeable but he does acknowlege it.
Mathematics has proved the validity of this fact for some time. There is no need for a sophisticated model to determine the validity of this. The models available do show you need heavy chain unless you want an 800 ft rode in 80 ft of water if the wind picks up.
There isn't an anchor design that can change this fact (that would fit on a boat).
So if Deep Water is the constraint, and deep water needs heavy chain then you end up having heavy chain for shallow water. To minimize shock loads use a bridle.
Of coarse if you never plan to anchor in water deeper than 40-50 feet then heavy chain may not be required in most situations. Plans change.

Buildings are easy to model, they stay still and bend. The complexities of a boat at anchor while appearing to be easy is extremely complex. There are layers and layers of fluid dynamics that makes a buildings seem trivial.

[Andrew Troup]

Good point in the last post about deep water. Good enough that I thought it worth rescuscitating this dormant thread.

Consider this also: Heavy chain can do double duty. Doubly duty is great on a boat: it should be a specified minimum (triple and quadruple are even better. I try for octuple whenever possible)

Heavy chain makes GREAT internal ballast. Preferably short-link, stud-link is even better.

In a grounding, it can be jettisoned even more quickly than fresh water can (unless you've made special provision).

The chain should be buoyed for later recovery - you might shortly be needing it if your lightening efforts prove insufficient.

Speed is of the essence on a falling tide. If you can do it quickly, though, I recommend padding the edge of a saloon hatch and the toerail, (use a serrated carving knife to cut a deep vee across one face of a laminated rectangular foam fender, so you can 'fold' the fender to a right angle) and run it straight overboard.

If you've got padding, you'll let it run a lot quicker: it's human nature. One of the fenders might do double duty as a float if attached using a line as deep as the water (which is presumably shallow anyway, unless you're a superyacht)

Remember, double duty is double good!

- - - - - -

Catenary is not just in a vertical plane. There's also a horizontal curve in that portion of a heavy chain which is lying on the bottom in deep water. If the chain link nearest the anchor never moves sideways, then there is no 'walking out' tendency, despite the vessel wandering about at the other end of the catenary.

Anchor tests AFAIK never apply cyclical variations in load direction on an anchor over time. (They don't usually even apply variations in load intensity, although this probably wouldn't prove much, unless the direction was also changing)

It seems possible to me that this is partly why 'new generation' anchors score so well: they are 'designed to the test'. Like kids who are 'taught to the test', they may have limitations in real world situations.

And the recommendation to use light, hi tensile chain is consistent with the simplified representation of the test, rather than the messier and more complicated real world.

[It seems conceivable that the formerly revered (and now maligned) genuine CQR might have a certain capability not captured in such tests, perhaps in this connection]

It occurs to me it's even possible that heavy chain may help the latest wonder-anchors, in certain situations. Blasphemy, I know, in these newly religious times....

If you carry a shot of extra-heavy chain, it should be attached outboard of the usual chain cable when it's needed for deep water/ challenging conditions. This way the favourable influence on catenary (both vertical and horizontal) is in the ideal location relative to the anchor.

Think of it as a kellet, without any of the nasty characteristics of a kellet.