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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #1  
Old 03-11-2010
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Ground Tackle - Hummm...

Hello;

Back with another question, this time on ground tackle. I am planning on replacing the 20 lb Danforth anchor with a new Rocna 15 and keeping the Danforth as a backup. I have a bow roller so the Rocna will fit in nicely. I do not have a windless nor do I plan on getting one soon.

After a bunch of reading I thought I was set to purchase 30 feet of 1/4 inch Proof Coil chain, 200 feet of 1/2 inch New England Ropes Mega Plait anchor line and a 1/2" Galvanized Steel Screw Pin Anchor Shackle. The issue I have is that the Chain has a breaking load of 5000 lbs (swl of 1250 lbs), the rope has a breaking load of 7500 lbs (swl ?) and the shackle has a Breaking Load of 4000 lbs (swl ?).

Should I be trying to match the breaking loads of my ground tackle?

I think I should up my chain to 5/16 and my shackle to 5/8. That will move the breaking load to Chain - 7000 lbs / Rode - 7500 lbs / shackle - 6500 lbs. Is this over kill?

Boat - 33' Contest / 13000 lbs Displacement.
Sailing grounds - (next two months ) Cheasapeak, Delaware, New York Harbour, Hudson River. After that it will be Lake Ontario for the next few years.

Thoughts?

Thanks;

Robert
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Old 03-11-2010
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Umm, you really need to check your numbers... 5/8" megaplait has a breaking strength of 10400 lbs. A 5/8" shackle wouldn't fit into your anchor's stock, much less into a link of 5/16" chain, and would have a much higher breaking load than 4000 lbs. The SWL is NOT the breaking load... BY DEFINITION, a SAFE WORKING LOAD is far below a BREAKING LOAD.

First, I highly recommend the Rocna 15 anchor, as it is what is on the bow of my boat. It is a great anchor.

However, your rode choice leaves a lot to be desired. IMHO, you should go with 5/16" G43 high-test chain, since you don't have a windlass to match the chain to. I'd say 30' at a minimum, preferably 60'. This has a swl of 3900 lbs.

Next, add enough 5/8" megaplait or octoplait nylon to make the anchor rode 250' long overall. 5/8" octoplait has a breaking strength of 10400 lbs. Its got a working strength of about 3500 lbs at a 3:1 safety margin, which is a bit low IMHO, but doable.

Finally, add a 3/8" Crosby load-rated bow anchor shackle. This has a SWL rating of 4000 lbs.

Then splice the Octoplait/MegaBraid to the chain. Attach the chain to the anchor using the Crosby shackle. Mouse the pin on the shackle with stainless steel or monel seizing wire.
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Last edited by sailingdog; 03-11-2010 at 11:59 PM.
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Old 03-12-2010
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First I'd recommend the R as well, good choice, I have one and so far it works as advertised.

I would also suggest looking at Main Page (Rocna Knowledge Base) for more info. You bought the anchor may as well buy into the philosophy.

Which if I recall correctly would suggest about 100' 5/16th HT chain and 100' rode.
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Old 03-12-2010
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Quote:
Originally Posted by sailingdog View Post
Umm, you really need to check your numbers... 5/8" megaplait has a breaking strength of 10400 lbs. A 5/8" shackle wouldn't fit into your anchor's stock, much less into a link of 5/16" chain, and would have a much higher breaking load than 4000 lbs. The SWL is NOT the breaking load... BY DEFINITION, a SAFE WORKING LOAD is far below a BREAKING LOAD.

First, I highly recommend the Rocna 15 anchor, as it is what is on the bow of my boat. It is a great anchor.

However, your rode choice leaves a lot to be desired. IMHO, you should go with 5/16" G43 high-test chain, since you don't have a windlass to match the chain to. I'd say 30' at a minimum, preferably 60'. This has a swl of 3900 lbs.

Next, add enough 5/8" megaplait or octoplait nylon to make the anchor rode 250' long overall. 5/8" octoplait has a breaking strength of 10400 lbs. Its got a working strength of about 3500 lbs at a 3:1 safety margin, which is a bit low IMHO, but doable.

Finally, add a 3/8" Crosby load-rated bow anchor shackle. This has a SWL rating of 4000 lbs.

Then splice the Octoplait/MegaBraid to the chain. Attach the chain to the anchor using the Crosby shackle. Mouse the pin on the shackle with stainless steel or monel seizing wire.
Hey,

Thanks for the feedback, I know the difference between SWL and Breaking Strenght. I got my numbers from a mix of places, the manufactures websites and industry publications I received at the Toronto International Boat Show. The Mega Plait I said I was looking at was 1/2", not 5/8" and I have never held a shackle before. I got the numbers for the shackle from West Marine where I was sourcing the Anchor and the Shackle.

Obviously I have the wrong end of the stick on a couple things and I wanted to thank you for pointing me in the right direction. That's why I post here.

A question, if the the 5/8" octoplait has a low safety margin what would you recommend?
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Old 03-12-2010
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100' of chain is overkill IMHO, especially as he's not anchoring in any areas with coral heads...

Using too much chain can be an issue since it adds a lot of weight forward. Also, nylon rope is far more forgiving in terms of shock absorption.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 03-12-2010
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I'd recommend using one of the new anchor specific nylon double braids like Samson Super Strong Anchor Nylon Double Braid. It has a breaking strength of 15000 lbs. or almost 50% more than Megabraid.

Quote:
Originally Posted by krozet View Post
A question, if the the 5/8" octoplait has a low safety margin what would you recommend?
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 03-12-2010
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I would agree with Sailingdog's initial recommendation for 5/16" G40 chain and 5/8ths line. I figure that any part of the system needs to have a SWL that far exceeds any expected loads. Anchored in storm force conditions, you may well see loads of 2000lbs on the system which makes 1/4" proof cool too small. In selecting the line diameter, I would suggest 5/8" for a few reasons. First, chafe is your biggest enemy not peak load and the line will loose its strength over time. A thicker line can chafe through more in a storm and still have enough strength. Also, 1/2" line will have a tendency to act like too much a spring because you are reaching a higher percentage of its breaking strength. Stretch is really important for dampening shock loading but excessive stretch leads to the boat "sailing" on the hook. Lastly, it is just an easier size line to handle. Regarding shackle selection, 3/8" is the largest that will work with the Rocna 15 and it will be compatible with your chain.

I have the exact setup described by Sailingdog except that I use 5/8 3-strand since I already had it.
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I'd point out that the breaking strength of 1/2" megaplait is only 7500 lbs. His boat displaces 13000 lbs... and in a storm, the surge forces could easily top 7500 lbs.

I'd also recommend using WOVEN chafe protection for the anchor bridle or anchor rode. One of the most common modes of failure for anchor and mooring lines is from the rope heating internally from movement and then failing as the core is damaged by the high temperatures. Plastic or Rubber hose chafe protection makes this more likely as it prevents the heat from escaping. Using a woven chafe protection sleeve, like polyester or spectra tubular webbing allows water to get in to the rope and both cool and lubricate it—making it far less likely to fail from overheating.

2" tubular webbing should work nicely. Sections five feet or so long would work very well to protect the rode even under the worst conditions.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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The best reference I've seen on the subject is "The Complete Book of Anchoring and Mooring" by Earl R. Hinz. It's a methodical treatment of the subject; decide how much wind/surge you want to handle, calculate the loads, match them with the anchor and match the rode to that. I carry different anchors for different bottoms and capabilities. For a working anchor Bruce, plow and the newer anchors like your Ronca have better rollover and reset capability than the more traditional Danforth/Fortress. The latter however have far more holding power and over at higher wind speeds and it's hard to get that max holding power with other anchors. I carry a Bruce and Fortress on the bow. I divided the chain locker in half so each anchor has it's own bow roller, rode and hawspipe. If I'm in really deep water I can attach the rodes together. I also carry a smaller Danforth in the stern which in a realy big blow I can hook up to the Fortress in tandem. As for different bottoms I have a fisherman type anchor stowed below if I have to anchor in rock/weed. I also use a lead downrigger ball for a killet. This is a weight you slide down the anchor rode to lower the pull angle. It increases holding power especially if you're using a nylon/chain rode. I think it's best advantage is to give you the holding power at short scopes which inevidably occur in crowded anchorages. I found a nifty brass saddle to go around the rode to hang the killet from. When going thru this process I found that the bolts holding my bow cleats didn't match the loads they'd see in a blow. They were on the caprail with only two bolts. When I looked look at the shear loads that the threaded part of a stainless steel bolt can handle I found them to be it's surprisingly low. As there wasn't room on the caprail for a four bolt cleat I put one in on the foredeck with a large 1/8 in stainless backing plate.
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Old 03-12-2010
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Like most threads on anchoring, there are a number of unstated take aways...

...in these excellent recommendations:

* The ratio of breaking strength varies with the material. About 5:1 for ropes, about 4:1 for chains, and 3:1 for bolts that don't move or bend. This is also effected by quality of manufacture (better QC means less safety margin is needed - the part is predictable). Typically, you want to match SWL.
* Look over key areas every time you use your gear. Are tie-wires in place? Are the splices good? Any cuts? Chafing gear, as needed?
* An anchor is only as good as the bottom and the set. Some anchors are better in some places and some are better in most places, but none are good everywhere, eg. shells or soft silt over very hard clay. You need to judge the bottom and your set more than any other factor. This involves experience and diving on the anchor to have a look, until you learn to correlate the feel up the line with what is on the bottom.

And keep your existing rig for a second anchor; some of those areas have big tides. But be aware that setting a second anchor takes some time to learn; there are many pitfalls that have given two anchor rigs a mixed reputation. The angle between the anchors is critical, both anchor sets must be good, and you must make provisions to avoid fouling your keel. A big "dumb" anchor and loads of chain suits most folks best, but if you have no windlass you will need to be more clever with lighter tackle.

Good luck!
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