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Old 30-04-2010, 12:49   #1
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Anchor Loads

OK, a quick break at work and thought I'd ask something that has been bugging me.
I have a 30 ft, low freeboard, full keel vessel weighing approx 6 tons


OK hypothetically..

My 35 lb manson has been rated at over 1000-5,000 lb of holding power (depending what web site you read)
everyone says bigger chain is better so I get 200 ft of 5/16 HT with BL of over 11000 (I am rounding the numbers here)
Why have 5/16 when the 1/4 HT is rated at 7600 BLL?
When I get into a situation that has these kind of loads, wouldn't the anchor drag at anything over 5000 (generally)? well before the chain load is in danger of breakage?

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Old 30-04-2010, 13:16   #2
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Is this a friday afternoon pop quiz? I think that whether you drag or not is completely bottom dependant. I do not think your boat would ever experience those kind of loads. But if you must, try calculating wind load using surface area of boat exposed worst case in 200 hundred MPH winds.
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Old 30-04-2010, 14:07   #3
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It is very difficult to compute anchor loads because almost all data refer to steady loads. In reality, waves come from a variety of directions, the wind gusts, the boat sails at anchor, etc. When the anchor rode becomes taut, the tension is very high for a short moment. If the rode is only chain and has almost no intrinsic elasticity, a shock load can be transmitted to the anchor. You can accept that the anchor drags for a few inches at this time but the rode must not break.

If you want to run serious computations, see for example
Principia - Deeplines : Global analysis of risers, moorings and flowlines

Another point is: all figures for rode strength are given for new chain and rope. After some years of rusting and chafing and shock loading, how much remaining strength is there?

For this reason, the accepted wisdom is to estimate the safe working load at about one-third of breaking strength.

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Old 30-04-2010, 14:30   #4
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For the sake of the exercise see the attached file (below). See also http://www.cruisersforum.com/forums/...tml#post391297
Attached Files
File Type: xls anchor rode tension.xls (100.5 KB, 279 views)
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Old 30-04-2010, 14:59   #5
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The trouble with anchors is that there are so many variables, you end up needing to oversize stuff so that they will work in most situations. Your anchor has been tested many times up to 5000lbs and it can do that in the correct bottom but in some bottoms, it can't hold anywhere near that number and in others (the correct type of rock), it could hold until the steel yielded. For the most part though, you shouldn't see over 5000lbs from your anchor.

The numbers that you gave for chain are breaking load limit, not the working load limit which is what you should size your chain by. The working load limit is the amount of force that you can routinely go up to and still count on your chain. A few things are going on that make the breaking limit (it really is that, it will break somewhere near that point) a poor choice to size by. Fatigue is a function of the stress and the number of cycles. There is a stress level below which you can essentially have unlimited cycles without failure. In addition, you will reach the yield point before you reach the break point, the yield point being where the chain is permanently deformed but has not broken.

Then there are the forces that your boat will realistically put on the whole system. This has more to do with sizing the anchor than sizing the chain but you don't want to make the chain the weak link of the system. Therefore, you should have a chain that can hold 5000lbs even though on most days, you won't come anywhere close to this amount. Realistically, even in a cat I hurricane you will probably see average loads around 2000lbs with higher peak loads determined by how much shock absorption you have.

In my opinion, the notion that heavier chain is better is not correct. I feel that a chain should be sized by its WLL for what your anchor could reasonably hold. It is true that heavier chain will provide catenary in normal winds but when you really need it because there is a storm, all of the catenary will have disappeared. Catenary provides 2 benefits when present, keeping the pull on the anchor horizontal and shock absorption. Several of us have debated this at length before in this forum and the fact of the matter is, when it is really blowing, scope and a proper snubber do much more than having extra weight in the chain. You would be much better off in almost all situations to put all that weight savings in the chain back into the anchor if you were really worried about dragging. Your boat won't do well with a lot of weight in the bow though so keeping everything light but strong enough is a very good idea. For comparison, on the same boat I use a 33lb Rocna, 50' of 5/16" grade 40 chain and 200' of 5/8" line. Where I anchor, the bottom tends not to be foul so having a mixed anchor rode works well and from where it sounds like you are going, having a lot more chain than me seems to make a lot of sense.
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Old 30-04-2010, 15:24   #6
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If we put 5000 lbs of pull on the foredeck cleats of our 31 foot boat, they would probably pull straight out. Indeed didn't this happen 6 months ago on the last ARC? yacht with a problem was towed and then abandoned after they pulled the cleats out?

If you have some time and a quiet room this is worth a read, as is the Rocna website to give you some idea of anchor loads.

Forces

My synopsis of Alains work is that he recommends a mis rode as Klem has suggested.

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Old 30-04-2010, 15:37   #7
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Originally Posted by Pete7 View Post
If we put 5000 lbs of pull on the foredeck cleats of our 31 foot boat, they would probably pull straight out...
Design Loads for Deck Hardware
Design Loads for Deck Hardware - ABYC Section H-40, table 1 Cruisers & Sailing Photo Gallery
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Old 30-04-2010, 16:01   #8
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The trouble with anchors is that there are so many variables, you end up needing to oversize stuff so that they will work in most situations. Your anchor has been tested many times up to 5000lbs and it can do that in the correct bottom but in some bottoms, it can't hold anywhere near that number and in others (the correct type of rock), it could hold until the steel yielded. For the most part though, you shouldn't see over 5000lbs from your anchor.

The numbers that you gave for chain are breaking load limit, not the working load limit which is what you should size your chain by. The working load limit is the amount of force that you can routinely go up to and still count on your chain. A few things are going on that make the breaking limit (it really is that, it will break somewhere near that point) a poor choice to size by. Fatigue is a function of the stress and the number of cycles. There is a stress level below which you can essentially have unlimited cycles without failure. In addition, you will reach the yield point before you reach the break point, the yield point being where the chain is permanently deformed but has not broken.

Then there are the forces that your boat will realistically put on the whole system. This has more to do with sizing the anchor than sizing the chain but you don't want to make the chain the weak link of the system. Therefore, you should have a chain that can hold 5000lbs even though on most days, you won't come anywhere close to this amount. Realistically, even in a cat I hurricane you will probably see average loads around 2000lbs with higher peak loads determined by how much shock absorption you have.

In my opinion, the notion that heavier chain is better is not correct. I feel that a chain should be sized by its WLL for what your anchor could reasonably hold. It is true that heavier chain will provide catenary in normal winds but when you really need it because there is a storm, all of the catenary will have disappeared. Catenary provides 2 benefits when present, keeping the pull on the anchor horizontal and shock absorption. Several of us have debated this at length before in this forum and the fact of the matter is, when it is really blowing, scope and a proper snubber do much more than having extra weight in the chain. You would be much better off in almost all situations to put all that weight savings in the chain back into the anchor if you were really worried about dragging. Your boat won't do well with a lot of weight in the bow though so keeping everything light but strong enough is a very good idea. For comparison, on the same boat I use a 33lb Rocna, 50' of 5/16" grade 40 chain and 200' of 5/8" line. Where I anchor, the bottom tends not to be foul so having a mixed anchor rode works well and from where it sounds like you are going, having a lot more chain than me seems to make a lot of sense.
YES!

This is kinda the conclusion I was coming too (in a very agonizing meandering fashion- I can be a bit slow at times ), the added strength of 5/16 compared to 1/4 is not applicable when the rest of the tackle could never even handle or come close to those loads, so why wouldn't it be more prudent to have a bit bigger anchor and a bit more rode?

My boat is not a tank that you can dump tons of weight in, I need to be prudent on weight but at the same time do not want to skimp on my tackle, therefore I am looking at the tackle as a whole unit, my anchor can only hold so much (how much that is I am still working out), my chain WLL should be well within that range, along with my shackles being right in line with the rest of the system, and then a darn good snubber and attachment points.

I will be in the pacific so need lots of chain.

I figure my windage and size boat 1/4 can handle up to 98 mph...

Erika
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Old 30-04-2010, 16:12   #9
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Using AYBC guidelines is probably the best approach, unless you are and engineer and want to get into the details. Everytime I have gotten into the engineering of some boat system I have seen sense in their recommendations. I don't agree that anything is one-size-fits-all, but they are the best starting point.

Read this, if you want some more thoughts. Tuning an Anchor Rode and read this file: rodeanch.xls. It contains a lot of discussion of wind load, and I have found it compared well with the few measurements I have taken.

Curiously, if you go all-chain, the impact strains are greater and you may want 5/16". With a mixed rode, 1/4" will be fine.
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Old 30-04-2010, 16:16   #10
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Just for grins, there is another reason for chain weight... Catenary action. See here: Anchor Catenary
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Old 30-04-2010, 16:27   #11
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If your rode breaks, the most likely spot is where it passes through the deck chock or hawse pipe. For that reason, I use a bobstay fitting with two eyes: one for the bobstay itself, the other for a snubber line that I tie to the rode with a rolling hitch and then ride to the snubber rather than the rode itself. Result is no chafe, and a better ride when tethered at the waterline - which also increases the functioning scope. Of course, my boats are gaff rigged with bowsprits. Second most likely break is where the line attaches to the chain, and where the chain attaches to the anchor. Finally, carry a kellet you can use in an emergency.
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Old 30-04-2010, 16:30   #12
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OK hypothetically..

My 35 lb manson has been rated at over 1000-5,000 lb of holding power (depending what web site you read)
Erika
I'm curious as to what exactly this means. Does it mean the anchor will physically fail at the load or does it mean in "average" bottom conditions it will drag/pull through the bottom at that force?

I personally see little advantage to buying chain well beyond the strength you require. It seems to make more sense to me to buy an oversized anchor. It will hold better and be less weight than upsizing your chain.

I think dragging is more a function of the bottom, how well you set your anchor, scope and the anchor characteristics than it is of the gauge chain you use.
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Old 30-04-2010, 17:09   #13
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Just for grins, there is another reason for chain weight... Catenary action. See here: Anchor Catenary
I think you will find that the effect of catanery is well treated in the calculations.

The difficulty is that catanary is not very effective in strong gusty conditions, when peak loads occur; the chain become nearly straight. The Rocna site reaches the same conclusion. But read the web site, follow the math, and you will see.
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Old 30-04-2010, 17:14   #14
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The difficulty is that catanary is not very effective in strong gusty conditions, when peak loads occur; the chain become nearly straight.
Understood. I merely mentioned it because there is more to chain selection than just break strength and working load...
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Old 30-04-2010, 18:00   #15
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In my opinion, the notion that heavier chain is better is not correct

Klem a great post and thanks for Pete7 for that link which attempts to put some numbers where the myths are - I'll have a read later.

I'd like to extend Klems summary a bit to add that the most efficient boat anchoring system is widely? considered to be 50:50 chain/nylon combination (not trying to reopen inveitable debates on this). However chaffe and the larger swing room required (scope is larger but overall weight is less) are practical disadvantages.

Ocean Girls "YES" moment does this for me too. Use an all chain rode with reasonable chain length/weight for 'normal' anchoring conditions but with a heavier anchor to better allow for the uncertainties of bottom condition.

However beyond this I'd revert to a chain/nylon combination for more severe conditions. Even with all chain rode system most skippers would only put out enough scope and a bit to meet the expected conditions and not for the ultimate cyclone/hurricane. So essentially there is no real difference between an all chain and nylon combination approach for 'normal' conditions with my plan, except for less weight in the reserve rode. For severe conditions more than one anchor is probably required anyway and I don't see many yachts with full chain setups for their secondary anchors.

What are the normal conditions is the difficult bit and it really depends on how well you can sleep at night. Not so bad if you are on board and you can let a bit more go but not the same if you happen to be ashore. Sooo on it goes.

A little history: Van Hoon in his 1974 book Oceanography and seamanship was the first I know to advocate this nylon combination approach and he backed it up with many tables and graphs. He had a picture of a coast guard cutter with a taught straight chain rode which subsequently snapped. This was before the advent of PC's and the luxury of spreadsheets such as in Pete7's link. I can still recall consulting his book in the wee hours of the morning late 80's at a difficult deep anchorage with only 3:1 chain out. Never did resolve that night whether I should do something about it or go back to sleep - it but it served to keep me awake.
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