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Old 05-05-2011, 21:34   #1
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Anchor Chain and Rode Sizing

I am building up a new anchor rode and chain for my boat, a Pearson 323, 32 feet long, 12800 lb displacement. So, I did a sizing calculation based on a couple of books recommendations, but at the end of it, I think I am getting an unreasonable sizing. I could use some advice.

First off, I decided to size for a tensile load of 7200 lb max. This load calculation includes normal horizontal load, and added load for windage (at 60 knots, Beaufort 11), and some allowance for wave action.

Second, for my cruising grounds, there is not any coral (mainly just mud bottoms), so it is okay to use line, and chain. My plan was to use roughly 200-250 feet of Samson Deep Six line, and 50 feet of high test chain (galvanized).

Okay, my questions center around what diameter to use. First consider the nylon rode. The 5/8" Deep Six has a breaking strength of about 13200 lbs, so at 60 knots, the 7200 lb load would be 55% of the nylon rode breaking strength. So, it seems the line should be bigger than 5/8", but almost everyone I talk to seems to think 5/8" is okay for my boat. What am I missing? I mean even if I size for 42 knots of wind (Beaufort 10), the net load drops to 5600 lb, which is 42% of breaking strength. I thought it was prudent to size the nylon rode to be at roughly 10 to 20% of breaking strength, not more. I would need a tremendous line diameter in that case. Help!

Now consider the chain. Almost everyone at the supply stores say 5/16" hi-test should be fine. However, this only has a breaking strength of 11700 lb, and at 7200 lb load, that is 62% of breaking load. From what I have read, shouldn't load the chain be less than or equal to the working load limit, which is about 35% of chain breaking load? Using that rule, and 7200 lb max load, I would need roughly 5/8" to 3/4" hi-test chain (grade 40). This can't be right!

Is it that the chain/rode is sized for normal load at 30 knots, then you take your chances, i.e. allow a higher design factor, at 60 knots?

Any comments appreciated. I am adrift.
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Old 05-05-2011, 23:28   #2
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Re: Anchor Chain & Rode Sizing

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Originally Posted by StringBimini View Post
I am building up a new anchor rode and chain for my boat, a Pearson 323, 32 feet long, 12800 lb displacement. So, I did a sizing calculation based on a couple of books recommendations, but at the end of it, I think I am getting an unreasonable sizing. I could use some advice.

First off, I decided to size for a tensile load of 7200 lb max. This load calculation includes normal horizontal load, and added load for windage (at 60 knots, Beaufort 11), and some allowance for wave action.

Second, for my cruising grounds, there is not any coral (mainly just mud bottoms), so it is okay to use line, and chain. My plan was to use roughly 200-250 feet of Samson Deep Six line, and 50 feet of high test chain (galvanized).

Okay, my questions center around what diameter to use. First consider the nylon rode. The 5/8" Deep Six has a breaking strength of about 13200 lbs, so at 60 knots, the 7200 lb load would be 55% of the nylon rode breaking strength. So, it seems the line should be bigger than 5/8", but almost everyone I talk to seems to think 5/8" is okay for my boat. What am I missing? I mean even if I size for 42 knots of wind (Beaufort 10), the net load drops to 5600 lb, which is 42% of breaking strength. I thought it was prudent to size the nylon rode to be at roughly 10 to 20% of breaking strength, not more. I would need a tremendous line diameter in that case. Help!

Now consider the chain. Almost everyone at the supply stores say 5/16" hi-test should be fine. However, this only has a breaking strength of 11700 lb, and at 7200 lb load, that is 62% of breaking load. From what I have read, shouldn't load the chain be less than or equal to the working load limit, which is about 35% of chain breaking load? Using that rule, and 7200 lb max load, I would need roughly 5/8" to 3/4" hi-test chain (grade 40). This can't be right!

Is it that the chain/rode is sized for normal load at 30 knots, then you take your chances, i.e. allow a higher design factor, at 60 knots?

Any comments appreciated. I am adrift.
I think your calculations for the load on the anchor might be highly overestimated. But even so we are talking about a "chain" where the strength is that of the weakest link. What load will drag your anchor? I suspect that the maximum hold for your anchor will be approx 500 lbs maybe 1000lbs in the best of circumstances. Use this as the SWL of your rode and you will find that 16mm (5/8") Nylon and 8mm (5/16") chain is overkill. Then remember that the 1000lb anchor loading is transferred to your bow anchor cleats, probably fastened with just two 1/4" bolts. They are going to be your overload fuse here. They will trip at a couple of hundred pound anchor load, hopefully leaving some of the deck intact.
Your sizing would be a correct if you want to suspend the boat in mid air hanging off the rode.
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Old 06-05-2011, 01:21   #3
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Re: Anchor Chain & Rode Sizing

I'd say 5/16'' would be sufficient.
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Old 06-05-2011, 06:30   #4
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Re: Anchor Chain & Rode Sizing

This subject has been discussed repeatedly. For an illustration of how to properly calculate wind loading on the yacht see: Windlass vs Size of Boat . The 5/16" is more than sufficient for what you have but what others think is irrelevant if you are uncomfortable with it. Unfortunately, however, you are not likely to find a rationale to support your "feelings". Different Ships. Different Long Splices. You make the call.

Also, look to the bottom of this page display for a list of other Threads with similar content.
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Old 06-05-2011, 07:03   #5
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Re: Anchor Chain & Rode Sizing

Quote:
Originally Posted by StringBimini View Post
... I think I am getting an unreasonable sizing. I could use some advice.
First off, I decided to size for a tensile load of 7200 lb max...
The ABYC "Design Loads for Sizing Deck Hardware" is often judged to be very conservative, leading to oversized gear.

They specify a 35 Ft boat requiring only about 2,700 Lbs for a permanent mooring, and 1,800 Lbs for a storm anchor.

See ➥ http://www.cruisersforum.com/gallery...r&imageuser=79

And ➥ http://www.cruisersforum.com/gallery...r&imageuser=79
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Old 06-05-2011, 07:14   #6
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Re: Anchor Chain & Rode Sizing

Thanks all. That's the feedback I needed. One offshore book I was referring to basically said, take the calculated load, then double it (service factor of two, to account for surge load). Sounds like I am doubling an already conservative number.

Plus, I don't want to hang my boat in the air, from the bow, with my weak-link anchor attached to a helicopter

Being basically conservative, I will go with 5/8" nylon rode, and and 50 feet of 5/16" galvanized hi-test.

Much appreciated.
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Old 06-05-2011, 10:07   #7
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Re: Anchor Chain & Rode Sizing

As stated in an above post, chain is no stronger than the weakest link. Make sure that the weakest link is not your bow cleat.

I use 5/16 HT chain. If I could have purchased a gypsy to accommodate both my windlass and 1/4" HT chain, I would have. I cannot believe that 1/4" HT chain is weaker than my deck/cleats.

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Old 06-05-2011, 10:24   #8
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Re: Anchor Chain & Rode Sizing

SB I have a Pearson 35 and am going with 50 feet of 5/16 G7, it's your boat you can go oversize if you want, I will be too, just so I won't have to replace it ever, if the 5/16 G7 wears to 1/4 I still have more than needed, now if it was 400 feet of chain then the extra wieght would change things a bit.
My current rode is unknown age, guessing 30+yo, 200' 5/8 3 strand and 10' 3/8 PC chain, hook is a 35# CQR so most likely the anchor will drag first, or a shackle will let go.
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Old 06-05-2011, 13:53   #9
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Re: Anchor Chain & Rode Sizing

Juniper--

You're right! I have too much weight now on my bow including well over 200# for chain alone.

There was a study in Massachusetts some years ago to understand why so many boats on anchor/moorings beached during Hurricane Bob. The reported results among others was nylon developed high internal heating when under heavy strain. The heating was even greater where the rode made sharp turns such as over a chock. Internal heating resulted in rode failure. This may not apply to you, just a small bit of nostalgic information.
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Old 06-05-2011, 14:55   #10
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Re: Anchor Chain & Rode Sizing

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Originally Posted by StringBimini View Post
Being basically conservative, I will go with 5/8" nylon rode, and and 50 feet of 5/16" galvanized hi-test.
I just went through this exercise and that's what I decided too (Tartan 3500). Figured the chain would break first. Also went through an interesting review of the nylon to chain splices. Either by accident or design, I ended up with what Brion Toss calls the "Traditional Irony Chain Splice", with 2 out of the 3 strands through the chain link. Claims is strongest splice and works well with the windlass.
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Old 06-05-2011, 16:54   #11
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Re: Anchor Chain & Rode Sizing

"Traditional Irony Chain Splice", with 2 out of the 3 strands through the chain link.

Not two going one way and the third going the other?
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Old 06-05-2011, 17:29   #12
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Re: Anchor Chain and Rode Sizing

Juniper, no, just 2 through the chain, with the 3rd being long laced/tied back about 2'.

The windlass manufacture and most examples show the 2 strands one way and the 3rd the other way, splicing back about 6". This is what I originally had and thought the rigger would do, but ended up with the above splice. I was somewhat doubtful, but after researching Brion Toss The Complete Rigger's Apprentice and Earl Hinz The Complete Book of Anchoring, both recommend this splice over the easier 3 strand splice. The claim is this is stronger as the loads are distributed over a block and tackle arrangement. It is also a very nice splice, runs through the windlass perfectly. This is not the best picture, with the 3rd strand buried:



To quote Brion Toss, "Many people wonder how this spice can be strong enough when only two strands pass through the chain. But the load is split four ways, like a line going through a two-sheave block. The link radius is small, but both strands bear fully on it. That's why this splice, done well, is the strongest of all chain splices."

I have to respect the claims in both books and the expertise of the rigger. Even if you don't believe the claims, the chain is still going to break before the 1/3 working strength of the anchor line. That's why by accident or design I ended up with this splice. I'm happy with the claims and would like to learn how to do this (Toss devotes 3 pages to this, 89-92). It is a difficult splice to do, which is probably why most recommend the easier 3 strand splice.
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