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Old 11-02-2010, 15:36   #1
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What Size Anchor Chain ? (Again!)

Hello all - sorry to jump in with a thread that has probably been dealt with ad infinitum, but I am about to pour a whole bunch more money into my personal "hole in the water", and I want to do it right!
41' steel schooner, 17.3 tons reg. full keel, wide, bluff bow.
- when anchored, I like to feel secure
- 60 pound Manson Supreme, all chain rode
What size G43 HT chain? 5/16, 3/8 or 7/16"?
Will Lofrans Tigres 1200 do the job, or do I need the Falkon?
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Old 11-02-2010, 15:44   #2
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For my 33 I went with 3/8" probably overkill but when you sleep every night on the hook its nice to know your not going anywhere when gales arrive, which seems to be a couple of times a week this winter.
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Old 11-02-2010, 16:47   #3
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The thinking used to be on all vessels that the catenary in the chain was very important to the anchor's holding power. This is true for large vessels who sit very differently to their anchors and have enormous but it is not true for smaller vessels. The purpose of your rode is to establish a reliable connection between your vessel and anchor and provide some shock absorbtion. Since catenary doesn't provide any shock absorption when you really need it, you need to get that through the snubber.

This means that the important things for your chain are to be strong enough and compatible with your deck gear. Using grade 40 chain, this means that you will probably need 3/8" to be strong enough. You could go to a smaller size chain and save some weight if you went to a higher grade chain but that tends to be quite expensive. Unfortunately, loads on your anchoring system are not well known but are probably on the order of 3000lbs at 60 knots.

Coming from large vessels, it took me a while to accept that you should use the smallest chain that is strong enough and then use a heavy anchor but try putting a few thousands pounds of load on a chain and see how little catenary there is.
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Old 11-02-2010, 17:58   #4
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3/8 G40 or 7/16 BBB

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Old 11-02-2010, 19:27   #5
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G'Day all,

While Klem's view of the insignifigance of chain weight in ultimate holding power may be correct, I reckon that the catenary's effect of damping surges is quite useful in less-than-ultimate conditions. This effect is improved by additional weight in the chain, and so I have always used the heaviest chain suitable for the windlass in question. In boats of moderate size this has always worked out for us to be 10 mm/ 3/8". A further useful effect of larger chain is increased friction when dragging along the sea floor. The bugaboo of the additional weight forward seems less important than a good nights' sleep.

Cheers,

Jim and Ann s/v Insatiable II lying Broken Bay, NSW, Oz
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Old 11-02-2010, 22:03   #6
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Quote:
Originally Posted by nitpik View Post
41' steel schooner, 17.3 tons reg. full keel, wide, bluff bow.
What size G43 HT chain? 5/16, 3/8 or 7/16"?
Will Lofrans Tigres 1200 do the job, or do I need the Falkon?
The size of the chain will depend on the size of the windlass as well.
The Tigres is sized for you boat.
The Falkon would be nice if you can afford it.

The Tigres is maxed out at 3/8" chain. Go for that.
The Falkon can go to 7/16 or 1/2"

The price difference between the falkon and the tigres, and the hight sized chain is considerable. But of course you get what you pay for.
I have the Tigres on a 25K lb boat. with 3/8 chain.
Bob
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Old 12-02-2010, 23:19   #7
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Quote:
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The thinking used to be on all vessels that the catenary in the chain was very important to the anchor's holding power.
........smallest chain that is strong enough and then use a heavy anchor but try putting a few thousands pounds of load on a chain and see how little catenary there is.
The anchor holds best when the shank of the anchor lies parallel to the bottom and not lifted up off the bottom. If the shank is raised up by the strain on the rode the holding power diminishes as the flukes now have less of an angle in the bottom. The smaller the angle = less area = less holding power.
That is the purpose of a heavy chain, the heavier the better, 3/8", 7/16". You could use 75-100 feet of heavy chain then 3/4 nylon.
For an average 42 ft boat, 150 ft of rode at 7.5 scope and everything else is equal
7/16" chain would require 950 lbs of strain (or 50 knts of wind) before the shank begins to be raised,
3/8", 700 lbs of strain or 40 knts of wind.
5/16", 500 lbs, 35 knts of wind.
If it was all nylon the shank would be lifted up in a light breeze and the typical anchor would loose about 1/3 to 1/2 of its holding power.
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Old 13-02-2010, 07:55   #8
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The anchor holds best when the shank of the anchor lies parallel to the bottom and not lifted up off the bottom. If the shank is raised up by the strain on the rode the holding power diminishes as the flukes now have less of an angle in the bottom. The smaller the angle = less area = less holding power.
That is the purpose of a heavy chain, the heavier the better, 3/8", 7/16". You could use 75-100 feet of heavy chain then 3/4 nylon.
For an average 42 ft boat, 150 ft of rode at 7.5 scope and everything else is equal
7/16" chain would require 950 lbs of strain (or 50 knts of wind) before the shank begins to be raised,
3/8", 700 lbs of strain or 40 knts of wind.
5/16", 500 lbs, 35 knts of wind.
If it was all nylon the shank would be lifted up in a light breeze and the typical anchor would loose about 1/3 to 1/2 of its holding power.
This is the old thinking that I was referring to and used to believe as well. What you say about the anchor's holding power is correct, the less vertical component to the pull, the higher the holding power. The problem with this is that when you really need the catenary in the chain is when it is blowing hard. The problem is that there is almost no catenary when it is blowing hard. This is why I suggested taking a length of chain and putting it under a few thousand pounds of strain and seeing how much catenary there is. Over a hundred feet, you might see a few inches and the chain is really bar-tight. Therefore, the lack of catenary does not provide any change in the angle of pull on the anchor and it does not provide an shock absorption. If you are looking to anchor on really short scope in light winds or have a very small swinging radius without using a kellet, then there is a benefit to heavy chain.

The reason that I distinguish from large and small vessels is that chain is very important on the larger vessels for exactly the reasons that you stated. I have worked on these larger vessels and seen how they maintain scope even in extremely high winds but that is because the boats lies very differently to the anchor and the chain is much heavier.

The best source of information on the subject is at the website I have listed below. Peter Smith (most people know him from Rocna) shows simulations of the effects of catenary and how it does not play a significant role in high winds.
Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats
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Old 13-02-2010, 09:43   #9
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The problem is that there is almost no catenary when it is blowing hard. This is why I suggested taking a length of chain and putting it under a few thousand pounds of strain and seeing how much catenary there is. Over a hundred feet, you might see a few inches and the chain is really bar-tight.

(...)

The best source of information on the subject is at the website I have listed below. Peter Smith (most people know him from Rocna)

If the rode is bar tight does not indicate there is no catenary. And I would not use a maker of any particular type of anchor as a bench mark.

BTW Each and every (of the two ;-) real life measurements made !!!on small boats!!! I have seen showed considerably less rode tension than in any pre-calc tables. If catenary were that insignificant we would have all been anchoring with a nylon string, which is simply not the case.

I think catenary is essential, and it 'co-operates' with the scope to make sure the pull is as horizontal as possible. It is not either/or, it is rather 'get all that you can = heavy chain and lots of it, big, efficient anchor, and extra scope', and add a duper kellet on top of it all.

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Old 13-02-2010, 11:50   #10
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My own experience is that I have seen all of the catenary taken out of the rode plenty of times on small boats. Part of the difference might be that we are talking about different conditions, I am referring to storm force conditions which I feel an anchoring system should work for. Many people size their anchoring system for conditions closer to 30 knots when the forces will often be 1/4 or less so there is still catenary but my argument is for storm conditions when there simply won't be catenary in any usefull amount. I am not arguing for an undersized anchoring system, I am arguing for a well designed one where the weight is in the correct place (the anchor).

Regarding citing Peter Smith's website, while he is an anchor manufacturer, his argument does hold up to an engineering analysis. Essentially the argument that he makes is the same argument that Hazelett marine uses with their rodes. They argue that during storm conditions, chain will loose it catenary effect (and that is mooring chain which is much heavier), and what becomes important is shock absorption.

I was looking for a picture where someone was testing a hazelett rode compared to a chain rode an comparing catenary and increase in force with moving a boat back but cannot find it. It pretty graphically illustrates the lack of catenary on a heavy mooring chain. Since I can't find that, I will result to math. Using a catenary calculator, I plugged in 100' of 3/8"BBB chain (1.7lbs/ft) with a force of 3000lbs which is roughly equivalent to the given boat in storm conditions. I calculated that the catenary sag would be 0.71ft and that the total stretch left is 0.013ft. As you can see, there is effectively no catenary in this. Even at 200', the values are only 4 times as large. I then plugged in 8lbs/ft for the chain weight which is equivalent to 3/4" chain and got a sag of just under 4' so there actually is a little catenary there. However, no one will use 3/4" chain on that size boat.

For storm conditions, catenary does not play a major role so the chain only needs to be large enough so that it is strong enough for the forces and it is the snubbers job to provide the shock absorption. Saving weight on chain will save you money, weight in the bow or will allow a heavier anchor.
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Old 13-02-2010, 12:07   #11
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For what it is worth Raya has a gear sizing chart on their site.

Off hand it looks like they are suggesting 1/2"

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Old 13-02-2010, 12:21   #12
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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.
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Old 13-02-2010, 13:54   #13
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For storm conditions, catenary does not play a major role so the chain only needs to be large enough so that it is strong enough for the forces and it is the snubbers job to provide the shock absorption. Saving weight on chain will save you money, weight in the bow or will allow a heavier anchor.
You donīt mention the dynamic effect at all, worst Iīve been in was 60kN in the gusts, the catinary definitely did take the worst of the snatch load out of the chain as the gusts pushed the boat around. There would be a lull and the chain (3/8) would pull the boat forward as the catinary deepened then next gust would have to straighten the catinary again substantially dampening the forces on the anchor. Your argument seems to suggest that the boat will have constant forces acting on it which is not the case in the real world. In my limited experience it is the the dynamic loads which do the damage, anything which can dampen these forces will be very benificial, like heavy chain and a 10m nylon snubber between the boat and the chain.
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Old 13-02-2010, 14:34   #14
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conachair,

You are correct that I ignored dynamic loading and let me give you my reasoning. We both agree that the boat will sail on the hook, ideally the wind is constant and the boat sits well and doesn't move around a lot but that is obviously not true. In addition, waves in the anchorage can really cause severe shock loading which needs to be dampened on the anchor. The two places that you could provide this shock loading are in the chain's catenary and the snubber.

The reason that I did not discuss this before is that catenary in reasonably sized chain does not have the ability to absorb the shock loading. If you take the example that I gave above, the boat will have moved ahead less than 2" and the load on the anchor rode (I am assuming no snubber here to take out that variable) will be down to 1000lbs or 1/3 of the original value. At 5" of boat movement(4.3' of catenary), the load will be down to 500lbs. Ideally, you want to apply a relatively constant high loading and allow the boat to move back and forth several feet. This would mean that when the boat came up on the anchor, it had significant resistance while it was falling back and taking the catenary out of the chain. By having a large resistance, the boat will be decelerated at a more constant lower rate. Unfortunately, the chain does not come close to approximating this behavior and goes from relatively little resistance to bar-tight in less than a foot of movement of the boat. In discussing dynamic loading, I have omitted one factor which is the resistance of the water to the movement of the chain. This is much harder to model and should be considered but is present for any rode under the water although obviously larger diameters and less hydrodynamic shapes have a larger damping effect from this.

As you can see from above, the chain is not an ideal shock absorber. Hence the reason that Hazelett is so succesfull with their moorings. Since the chain doesn't do a great job, it is up to the snubber to provide shock absorption which means that you need a properly sized and length snubber with adequate chafe protection.

I used to be a strong believer in catenary coming from large vessels but as an engineer, I observed that the chain tended to jerk the bow as it came taught and started looking into why. If you disagree with any of the assumptions made in my arguments, please let me know but I believe that they are reasonable assumptions and they match up with what I have observed. I will stick to a very large anchor and smaller diameter chain until someone can find the fault with the analysis. There are an increasing number of people doing this with very good luck such as the Dashews who use grade 70 3/8" chain on their 83' vessel(that is pretty extreme even to me).
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Old 13-02-2010, 17:48   #15
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Quote:
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For storm conditions, catenary does not play a major role (...)
This is an opinion. Others will claim to the contrary.

Quote:
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(...) so the chain only needs to be large enough so that it is strong enough for the forces (...)
I believe, looking thru the storm rode load tables, only G70 chain might be strong enough to accept the worst of the loads. So either the tables are off or the catenary does count. I bet it does.

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(...) and it is the snubbers job to provide the shock absorption. Saving weight on chain will save you money, weight in the bow or will allow a heavier anchor.
So, if the loads are what they are believed to be, what size snubber? 6 inches dia will do?

Seriously, I have been thru a number of publications and features on the subject (call me an anchoring freak) and I did find in them that chain beyond some amount/ratio adds very little to rode 'performance', but note the word BEYOND.

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.

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