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Old 09-11-2023, 23:19   #1
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5/16" or 3/8" Chain

Trying to figure out whether to go with 5/16" or 3/8" G4 to go with the 55lb Mantus M2 I'm about to buy. Looking at getting 200ft of chain.* Boat weighs 20,000 dry and I'm wanting this setup to be our better/stronger setup (already have a 35lb CQR, 40' of 1/2" chain + 3/4" nylon rode for normal day stuff) but eventually looking to replace our manual windlass with an electric one and there's a pretty big cost difference between 5/16 and 3/8 there too, so just trying to figure out if 5/16" will be good enough or if i should go with 3/8".* Thanks for any advice.
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Old 09-11-2023, 23:38   #2
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Re: 5/16" or 3/8" Chain

Monohull 38ft and say >10T , per anchor sizing chart for Rocna recommended anchor is 25Kg with 3/8 G40 . and their website states :


" Unlike other manufacturers, our anchor sizing recommendations are intended to provide an anchor adequate for use in most all conditions. We base our calculations on 50 knots of wind, associated surge, and poor holding bottoms."


https://rocna.com/anchor-sizing/

Obviously other manufacturers will have similar charts, the above is just an example..


The working load of G40 5/16 is 3900lbs 3/8 is 5400 lbs - depends if you are likely to encounter violent storms whilst at anchor
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Old 09-11-2023, 23:39   #3
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Re: 5/16" or 3/8" Chain

Are you going to anchor in a Hurricane? Assuming your boat is about 40', by manufacturer and ABYC recommendations, 1/4" is just strong enough for a "Storm" anchor, but not a "Violent Storm" 5/16 is substantially stronger, but still not quite enough for a "Violent Storm" 3/8" is strong enough for a "Violent Storm."

https://www.mantusmarine.com/mantus-...e/rode-sizing/
https://peerlesschain.com/pleasure/a...test-iso-chain
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Old 10-11-2023, 04:44   #4
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Re: 5/16" or 3/8" Chain

Our boat is 38 feet long and displaces 20,000+ in cruising trim. We have been using 5/16" G4 chain for about 18 years now with no problems, first with a 45lb Bulwagga and then with a 45lb Mantus. In that time we have weathered up to about 60 knots, again with no noticeable degradation. In fact, I am still using the chain purchased in 2005! In good holding ground of thick mud our Mantus has held us in 45 knots, and when we pulled it up I could tell it would hold a lot more! I doubt the anchor would drag in that bottom before something broke. The most annoying thing I have found is that the Crosby 209A alloy shackles (2T WLL) rust out quite quickly. Based on studies of actual anchor loads you will probably never see greater than 1,200-1,400 lbs. of load, assuming you use a decent length of snubber with the chain. The MWL of 5/16" G4 chain is around 3,900 lbs. so there is a good safety factor there.
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Old 10-11-2023, 06:12   #5
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Re: 5/16" or 3/8" Chain

5/16" G43 will be plenty strong enough for a 38 foot boat. You'd need to be in a decently strong hurricane to exceed the working load of the chain and require an up-size.
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Old 10-11-2023, 06:22   #6
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Re: 5/16" or 3/8" Chain

The 5/16" G4 has a SWL value half way between 40 knots and 60 knots. How often do you realistically expect to anchor in more than 40 knots, with no protection? And remember, this is the SWL, breaking load is 5 times that. I personally would not want to haul all that extra weight around, in the bows, for an extremely rare event. And I don't, my boat is 40', 11 tons, and I use 200' of 5/16" G4.

And lastly, the same source, Rocna, claims you can reduce loads by a third with a good snubber.
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Old 10-11-2023, 06:25   #7
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Re: 5/16" or 3/8" Chain

5/16 this be plenty strong enough BUT there is always a but, check that your existing anchor to chain connection will fit through the smaller links.


Also most 5/16 & 3/8 rope chain gipsies will only accept 5/8"(14mm) rode.
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Old 10-11-2023, 06:26   #8
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Re: 5/16" or 3/8" Chain

Quote:
Originally Posted by fourlyons View Post
The 5/16" G4 has a SWL value half way between 40 knots and 60 knots

Assuming that's based on the ABYC load values, the actual load won't be anywhere near that high. Those values are basically a worst case for sizing cleats and such.
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Old 10-11-2023, 07:45   #9
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Re: 5/16" or 3/8" Chain

Exactly. My understanding is these load values have been empirically tested, including by a forum member here, Thinwater, who wrote an excellent book on the subject, to be very conservative.
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Old 10-11-2023, 07:52   #10
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Re: 5/16" or 3/8" Chain

I chose 5/16 inch grade 43 chain [so I could squeeze in a barrel of 550 feet divided between two bowers] and a 99 lb Spade primary anchor for our 43’ 44,000lb [cruising weight] ketch.



[More details here.]

We anchor in some sporty conditions cruising above 56N, and typically anchor in deeper water [say 60 to 100 feet]. I always deploy a properly sized bridle, and have not seen any indications that the chain is undersized after 9 years full time use.

We have experienced 60 kts gusting 90 at anchor- for days- on more than one occasion, and often have williwaws [catabatic gusts; think mild to 60+ knots in a few seconds] that hit suddenly [and disappear almost as quickly] from various directions in some anchorages during storms. We have yet to have our primary bower drag once properly set.

In case this is useful.

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Old 10-11-2023, 08:27   #11
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Re: 5/16" or 3/8" Chain

On a 38 ft boat I would likely go 5/16 G4.
On a heavy 47 ft boat I went with 3/8 G4.
On a 42 ft Cat I went with 5/16 G4

The weak link is often the shackle not the chain.

I have snorkeled the anchor in 30-35 knots of wind (protected from sea action). Surprisingly there is still catenary in the chain which is like having a "spring" to absorb shocks.
The worst conditions for the rode in my experience have not been strong wind but rough water causing the chain to snub up tight when the bow pitches and the snubber pulls very tight, especially when trying to depart that situation so the snubber is off.
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Old 10-11-2023, 09:24   #12
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Re: 5/16" or 3/8" Chain

Bigger is relatively low cost long term insurance and provides better peace of mind and restful nights sleep. JMHO
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Old 10-11-2023, 10:21   #13
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Re: 5/16" or 3/8" Chain

That Rocna Chart puts you at either 5/16 or 3/8. The next column up (right also shows (5/16 or 3/8), then the next column shows 3/8.

I'd say that 5/16th is probably more accurate, though I'd probably go with 3/8 if weight weren't an issue AND I were going to be out for months at a time.
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Old 10-11-2023, 10:54   #14
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Re: 5/16" or 3/8" Chain

A question to ponder is: when was the last time you heard about a boat adrift because the anchor chain was undersized?


Weight in the bow is almost always bad.


I have 5/16" chain. 275 feet of it.
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Old 10-11-2023, 13:05   #15
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Re: 5/16" or 3/8" Chain

Quote:
A question to ponder is: when was the last time you heard about a boat adrift because the anchor chain was undersized?
Yep, the breaking strength of 5/16" G4 is 11,600 lbs. Something else will break before you'll reach that load.
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