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Old 18-08-2018, 17:46   #1
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Anchor chain diameter, weight and holding ?

I am about to reduce my chain from 70m of 10mm (L) to 100m of 8mm (G43), maybe it would be a mistake to go with lighter chain. Should I?

Lots of talk about strength but I'm sure the chain spec is over kill for strength and that weight/scope is more important for holding, I expect I just let out 20% more scope to maintain the weight and probably get better holding from length. Does anyone have experience down sizing chain diameter and the effect on holding.

About the boat...
My boat is a little over 7t as a weekender and I'd say maybe as much as 8t on a long run. We are a 15m cat with a Vulcan 33. We are much more wind effected than by current which reduces the drag. I looked up the Rockna chart and it said 25kg, then go up one for a cat so 33.
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Old 18-08-2018, 17:54   #2
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Re: Anchor chain diameter, weight and holding ?

I don’t think there will be a difference in holding. At worst, a bit more scope might be needed. As long as the strength rating is OK.
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Old 18-08-2018, 18:05   #3
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Re: Anchor chain diameter, weight and holding ?

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Originally Posted by Dave_S View Post
I am about to reduce my chain from 70m of 10mm (L) to 100m of 8mm (G43), maybe it would be a mistake to go with lighter chain. Should I?

Lots of talk about strength but I'm sure the chain spec is over kill for strength and that weight/scope is more important for holding, I expect I just let out 20% more scope to maintain the weight and probably get better holding from length. Does anyone have experience down sizing chain diameter and the effect on holding.

About the boat...
My boat is a little over 7t as a weekender and I'd say maybe as much as 8t on a long run. We are a 15m cat with a Vulcan 33. We are much more wind effected than by current which reduces the drag. I looked up the Rockna chart and it said 25kg, then go up one for a cat so 33.
On small boats the weight of the chain has been much over rated in its importance to anchor holding. Look at your chain when the wind is blowing a steady 25 knots. I'll bet it is bar tight with almost no sag left in it. Far more important to reducing shock loading is a proper snubber of suitable elasticity.

The anchor catenary is a nice concept, but it really doesn't apply in winds that I would categorize as "modest" at most.

Be sure you are strong enough, and then lose the weight.

Here is a good article by the designer of the Rocna anchor with lots of mathematical models to back up what you can observe just by looking...

https://www.petersmith.net.nz/boat-anchors/catenary.php
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Old 18-08-2018, 18:51   #4
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Re: Anchor chain diameter, weight and holding ?

The recommended High test chain for a 33 kg Rocna is 10mm, 8mm for G70
Chain (Rocna Knowledge Base)
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Old 19-08-2018, 08:28   #5
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Re: Anchor chain diameter, weight and holding ?

Modern thinking from people like Steve Dashew and Peter Smith (designer of the Rocna) is the weight should be in the anchor, not the rode. When it blows the rode, whether chain or a combination chain/rope, is almost straight. As long as strength is met light chain is better. Put the saved weight into the anchor where it will do more good.
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Old 19-08-2018, 10:40   #6
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Re: Anchor chain diameter, weight and holding ?

Thanks all, I'll go with the 8mm

I haven't been able to get G70 here, except for non marine grade but the 8mm G43 I can get rates better than the standard 10mm chain I have now and I can buy the right windlass gypsy for it.

If I use nylon for my bridal do I need a snubber?
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Old 19-08-2018, 11:10   #7
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Re: Anchor chain diameter, weight and holding ?

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Thanks all, I'll go with the 8mm
I haven't been able to get G70 here, except for non marine grade but the 8mm G43 I can get rates better than the standard 10mm chain I have now and I can buy the right windlass gypsy for it.
G43 chain is normally imperial not metric so I am not sure exactly which grade you are considering, but you do need to be careful when comparing the strength of different chain specifications. Different safety factors are used when calculating the SWL, so comparisons can be misleading. The best solution is to look at the UTS.

10mm G30 chain will be stronger than G40 8mm chain. The UTS of 10mm G30 is 5.0 tons and 8mm G40 is 4.0 tons. So the 8mm G40 is weaker, not stronger than the 10mm G30.
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Old 19-08-2018, 12:26   #8
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Re: Anchor chain diameter, weight and holding ?

Thanks, I better have another look.
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Old 19-08-2018, 12:35   #9
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Re: Anchor chain diameter, weight and holding ?

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...If I use nylon for my bridal do I need a snubber?
The bridle is the snubber. That said, the legs should be about 1.5 times the beam in order to get enough length. Some of us use even longer legs, often cleating them farther back on the boat, in order to get more stretch. But that is a whole nuther' topic. 1/2-inch should be large enough. Remember that bridles are disposable and should be replaced every 150-250 days at anchor, depending on the weather and wear. Watch out for chafe.
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Old 19-08-2018, 21:46   #10
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Re: Anchor chain diameter, weight and holding ?

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Originally Posted by noelex 77 View Post
G43 chain is normally imperial not metric so I am not sure exactly which grade you are considering, but you do need to be careful when comparing the strength of different chain specifications. Different safety factors are used when calculating the SWL, so comparisons can be misleading. The best solution is to look at the UTS.

10mm G30 chain will be stronger than G40 8mm chain. The UTS of 10mm G30 is 5.0 tons and 8mm G40 is 4.0 tons. So the 8mm G40 is weaker, not stronger than the 10mm G30.
Thanks for that, found a problem somewhere, turns out not much matches up.

It is called 5/16th on the brochure, the guy called it 8mm, it is listed as 8.4mm wire diameter and data shows 1.6kg/m V's 2.3kg/m but unless the material has a significantly different density the weights don't match up either. I've used a generic carbon steel for both calculations @ 7.87g/cm^3. I can't insert any variable of the data to get a weight match.

Using their numbers I get 1.89kg/m V's 1.91kg/m (they are close because of the shorter & wider link in the 5/16th chain)

Might have to go down a measure it.

CMP Global brochure lists

10mm, Grade G30, ID 30x12.5mm, SWL 1270kg, MBS 5080kg,
5/16, Grade G43, ID 26.2x12.95, SWL 1769kg, MBS 5310kg, NOTE: It is a shorter, wider link

Something is wrongera............. probably me
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Old 19-08-2018, 22:14   #11
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Re: Anchor chain diameter, weight and holding ?

I went from 100 meters G40 to 120 meters g70 and really like the change. The weight is the same but I can anchor in 100' of water now very safely. In that deep of water, even when the wind is high, the chain doesn't go "bar" tight which allows a much steadier ride on the chain. I would guess that most chain failures are when it gets stuck in a rock, not from the actual stress of a weather event.
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Old 20-08-2018, 02:03   #12
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Re: Anchor chain diameter, weight and holding ?

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Originally Posted by mitiempo View Post
Modern thinking from people like Steve Dashew and Peter Smith (designer of the Rocna) is the weight should be in the anchor, not the rode. When it blows the rode, whether chain or a combination chain/rope, is almost straight. As long as strength is met light chain is better. Put the saved weight into the anchor where it will do more good.

The difference in weight between 8 & 10mm chain is .5kg/m so on 100m you drop 50kg. Even if you only add half of that to the anchor it would be impossible to handle! The amount of extra wt you can add to the main anchor is insignificant in the total as the anchor weight is normally only 5-10% of the total. From experience I have anchored in 50kn on 12:1 scope ond not snubbed. If I did I would led out nor chain.
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Old 20-08-2018, 02:24   #13
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Re: Anchor chain diameter, weight and holding ?

Personally I would not be happy putting 7ton of boat with high windage on 8mm chain. If you goby the boats weight you are within spec (8mm up to about 10T) but if you go by length 15m should be 10mm. As so this you piont out the boat has a high windage for its weight so I wouls definatly look at lenght not weight as the important factor. In any case I would normally go for the highr number in there sort of scantlings for cruising. If you stick to 70m road I would aslo look at adding 50m 20mm 8 plait to extend the scope and give some stretch if you do ever need a storm anchor.
Consider, What are you reasons for going minial on ground tackle? presumably on a boat that size you have a windlass so weight is not an issue.
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Old 20-08-2018, 05:24   #14
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Re: Anchor chain diameter, weight and holding ?

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Personally I would not be happy putting 7ton of boat with high windage on 8mm chain. If you goby the boats weight you are within spec (8mm up to about 10T) but if you go by length 15m should be 10mm. As so this you piont out the boat has a high windage for its weight so I wouls definatly look at lenght not weight as the important factor. In any case I would normally go for the highr number in there sort of scantlings for cruising. If you stick to 70m road I would aslo look at adding 50m 20mm 8 plait to extend the scope and give some stretch if you do ever need a storm anchor.
Consider, What are you reasons for going minial on ground tackle? presumably on a boat that size you have a windlass so weight is not an issue.
I have joiners in my chain now which I'm not confident about and before I venture too far I want to replace it with one length of rated chain. If I can do so and have longer and stronger chain without a weight penalty all the better. The G43 rated 8mm chain should be stronger than the G30 10mm chain and lighter......... maybe.
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Old 20-08-2018, 19:37   #15
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Re: Anchor chain diameter, weight and holding ?

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Lots of talk about strength but I'm sure the chain spec is over kill for strength and that weight/scope is more important for holding, I expect I just let out 20% more scope to maintain the weight and probably get better holding from length. .
Could be a problem in some crowded anchorages.
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