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Old 06-10-2019, 07:37   #1
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anchoring in 120kn gusts - which anchor will hold that??

Hi guys -

I am "the anchor" guy. I love the anchor manuever and I always trust my anchors to hold - which they always do perfectly.

well...sofar... in "normal" situations up to bf9.

But planing to go to the high latitudes and very remote and windy areas and trying to be prepared for the "unthinkable" - I see pulling forces of 24000 lbs for a boat of the size I want at windspeeds of 120kn.

The "wise" bobby schenk also runs a chain for 15000lbs pulling force for an 11m boat.


now... there are new researches showing that these "known" tables which state these forces are wrong and that forces are about 30% less.
I have read them, and I did understand their calcs.

anyway... chain or rope - all fine - but what about the anchor itself?

I do NOT FIND ANY numbers on the forces ankers can withstand.

a) I dont want to rip the anchor apart with my 24000lbs chain...(or 16000 or whatever)
b) the anchor needs to bring that pulling force into the ground.

there are some tests with 15kg anchors holding 2500-5000lbs. does that mean a 5 or 10 x15kg = 75kg-150kg anchor will do??

who tells me that my cleats will hold 12000-24000lbs??

somehow everybody talks about chains... but not about the rest
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Old 06-10-2019, 07:43   #2
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Re: anchoring in 120kn gusts - which anchor will hold that??

Nobody will give you figures for that. The weak point is the anchor holding in the substrate at the bottom of the sea. And there is so much variation in the bottom type that going further with figures is nonsense. Anyway figures for 120 knots are speculation.
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Old 06-10-2019, 07:50   #3
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Re: anchoring in 120kn gusts - which anchor will hold that??

well...there must be SOME... what ever... method to somehow reasonable prepar for such a situation.
If preparation is bad - all is lost. and... due to water and air temperatures - not only the boat.
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Old 06-10-2019, 07:53   #4
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Re: anchoring in 120kn gusts - which anchor will hold that??

In the south, shoreline with the boat tucking in quite close, are the preferred solution for potential winds like that.

This accomplishes three things (1) if you use the shore topology well the wind will shoot over the hull and not hit it directly, (2) you eliminate the uncertainty of the anchor/bottom/holding, (3) you can have multiple strong lines holding without tangle or kelp ball risks.

As to your direct question - Yea there are various high load tests of anchors, but they are not really practically useful. The holding Varys enormously based on bottom composition, and the raw anchor strength is quite high in a straight line but relatively low if the anchor gets trapped and the load comes on sideways.
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Old 06-10-2019, 08:08   #5
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Re: anchoring in 120kn gusts - which anchor will hold that??

Well, I have some figures. We were anchored in 120kts sustained winds during hurricane Ivan in a Grenada in 2004.

- Boat: 64’ loa, 16’ wide, 6’2” draft, 55,000lbs, 25 tons metric, 4-5’ freeboard
- Anchor: original Belgium Bruce, 176lbs, 80kg
- Chain: Acco 3/8” G7 hot dip galvanized with enlarged end links
- Shackle: galvanized 1/2” pin
- Swivel: galvanized, commercial fishery style 1/2” pin
- Chain stopper: Maxwell 5/8” pin

Damage: another boat dragged into our anchor chain which must have been the biggest load, as it bent and twisted our heavy duty anchor-sprit like a pretzel.

- chain: 3 links cold welded to the stainless steel anchor sprit. I had to use a hammer to break them loose after which part of the links was left on the sprit.
- shackle: elongated, had to cut it off
- swivel: surprisingly undamaged. The swivel stronger than the shackle.

The anchor was buried very deep into the mud-clay seabed, we estimate 10-15’. It took 5 hours to force it up using the engine, taking out slack.

Changes:
- replaced chain with new of the same type
- replaced shackle with new of the same type but with 5/8” pin
- replaced swivel with new of the same type but with 5/8” pin
- repaired anchor sprit, enlarged roller diameter to maximum possible and improved locking of the 1” pin

We have complete faith in our anchoring system and regularly anchor at 1:3 scope
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Old 06-10-2019, 08:08   #6
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Re: anchoring in 120kn gusts - which anchor will hold that??

5 minutes ago I came by chance across the website of the "lindemann-ag" in germany, which at least includes size in addition to weight for chosing their rocna anchors - made in deed from high tensile steel...and underlines that if in question the next bigger size should be chosen

the 75kg I mentioned before is indeed the right size in that case.
I think I go for the 110kg .. LOL
wow... I am indeed surprised about those figures. never thought about it before.

I know that 5 shorelines are better than one anchor in this situation - but you know.. this is about NOT running out of options in heavy weather!



edit... read your post after writing mine...
5h to get an anchor out... I bet you were glad every minute of these hours that your WERE STILL ABLE to do so.
ok.. very impressive. I gues I know what to do now.
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Old 06-10-2019, 08:16   #7
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Re: anchoring in 120kn gusts - which anchor will hold that??

About Rocna, Mantus anchors: I am convinced they will provide equal holding at one size down from my Bruce, but I would still get one the same weight because that weight is not a problem with the right windlass, roller etc. and that leaves one with more safety margin
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Old 06-10-2019, 08:24   #8
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Re: anchoring in 120kn gusts - which anchor will hold that??

used a bruce this summer for the first time.
only 7,5kg. at bf 9 with 25ft boat it was draged - nill. I could not identify 10cm of drag swimming 3m above the anchor next morning.

great anchor! better than all I used before in the med on sand
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Old 06-10-2019, 09:02   #9
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Re: anchoring in 120kn gusts - which anchor will hold that??

Whatever combination of chain and anchor you settle on, a properly designed and deployed snub line will reduce the force of the wind and wave to a fraction of the load without the snubber. Drive a car into a wall at 20 mph and you could easily be killed. Drive one into a row of water filled collision barrels at 70 mph and you will likely walk away. The longer the distance over which the force can be dissipated, the lower the final force on the ground tackle.

FWIW, I size my snubbers so that when stretched 20%, the line is near the SWL strength of the chain. The breaking strength of the snubber at that degree of stretch is available from the cordage company. I use 30' of snub line, so I have 6' of stretch before the (now dissipated) load is transferred to the chain. When deployed I will overlay the snub line with 6' of additional chain so at the point the snubber is nearing 20% stretch, the chain takes up the load. Whether the wind expected is 5 kn or 50 I almost always do the same thing so it is second nature.

I'd disagree that any hoop style anchor in a really serious blow is going to be as reliable as an anchor designed to bury itself without the impediment of the hoop. Bruce, Spade, Ultra, Fortress - all will bury themselves as the force increases subject to seabed conditions. Put a load on the hoop of a Rocna, and it exerts a lifting force on the tip of the anchor, which ain't good, IMHO.
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Old 06-10-2019, 09:19   #10
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Re: anchoring in 120kn gusts - which anchor will hold that??

walls and buckets?

Well... Anyway... I know what you Are taking about.
The peak momentum forces on the anchor posed by chain only Are about 2-3 Times higher than by a rope.
BUT That is ONLY when the Wind is so strong That the chain is a straight line. Otherwise the chain hanging in the water is the equivalent to the rope elasticity.

The Momentum forces of non-elasticity Are allready included in the high loads mentioned above.
For explanation: nobody ever calculatet that. calculations were made by windforce only and than a saftymargin of about 4 was added for the unknown.
tests with different boats have shown, that the forces are reliably lower and that the numbers from tables are about 50-60% to high. if you stick to the old numbers they still hold true for the extreme situations when the chain is completely pulled tight.

Thus... An additional elastic Module Would improve the System Not to a large extent.
(i doubt there are elastic modules that can stand 10000-25000 dna?) are there?
well...20000-50000 dna to still be elastic...

It would help If the complete System is somewhat underpowered or... To save weight.


actually i think this might be well a reason why in the example above the jedi yacht could stop the other boat ... because it hit the chain first... giving elasticity to the crash... and did not hit the bow of the boat directly ... with no elasticity left to the mooring system.
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Old 06-10-2019, 09:37   #11
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Re: anchoring in 120kn gusts - which anchor will hold that??

You do know that most of us do anything to avoid the conditions you seek. I live in sandals and shorts. I own one pair of long pants, somewhere. This is supposed to be fun, not survival. We may not be the best source of advice.
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Old 06-10-2019, 09:58   #12
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Re: anchoring in 120kn gusts - which anchor will hold that??

Quote:
Originally Posted by Ibetitsthisway View Post
walls and buckets?

Well... Anyway... if you stick to the old numbers they still hold true for the extreme situations when the chain is completely pulled tight.

Thus... An additional elastic Module Would improve the System Not to a large extent.
.
Ignoring the snark, from the above you don't seem to have quite the grasp of the physics involved you seem to think you do. Perhaps this will explain why your statement that adding elasticity to an anchor setup won't help much to reduce force at the vessel is, well, a bit childish?

https://www.engineeringtoolbox.com/impact-force-d_1780.html

In any case good luck with your hurricane force anchoring thought experiment...
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Old 06-10-2019, 10:10   #13
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Re: anchoring in 120kn gusts - which anchor will hold that??

Quote:
Originally Posted by Delfin View Post
Ignoring the snark, from the above you don't seem to have quite the grasp of the physics involved you seem to think you do. Perhaps this will explain why your statement that adding elasticity to an anchor setup won't help much to reduce force at the vessel is, well, a bit childish?

https://www.engineeringtoolbox.com/i...ce-d_1780.html

In any case good luck with your hurricane force anchoring thought experiment...

... did not read the link... but you got me totally wrong.
I did not say adding elasticity does not improve.
I said it does not improve if the system is allready very heavy incl. the peak forces for very high windspeeds with straight chains. (well...to be correct it would still improve - but there might not be the neccessecity to improve a system that can withstand 120kn wind)

- and I said it might be helpful if the anchor setup is ...relatively speaking... to weak - which might be a smart solution if you want to save weigth and cost.
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Old 06-10-2019, 10:17   #14
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Re: anchoring in 120kn gusts - which anchor will hold that??

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Originally Posted by Nicholson58 View Post
You do know that most of us do anything to avoid the conditions you seek. I live in sandals and shorts. I own one pair of long pants, somewhere. This is supposed to be fun, not survival. We may not be the best source of advice.
well... i know

And I am not seeking these conditions neither. but there are places on earth that I am interested in to visit, which are unfortunately very windy. And I think Id better be prepared.

in fact - most cases of losses - yachts and lives - are the result of crews not beeing preped enough... speaking about weather, navcharts, material, maintanence and human needs.
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Old 06-10-2019, 10:28   #15
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Re: anchoring in 120kn gusts - which anchor will hold that??

When we were in the far south, Beagle Channel/ Cape Horn area, we mostly did as per above comments, with an anchor out off the bow and backing up to the windward (westerly) shore and tieing off to trees or rocks with 1/2" diameter polypropelene floating line. The anchorage bottoms in this area are mostly very deep and the bottom slopes steeply to the shore. If you anchor to swing into the wind you will be blown out of the anchorage with the anchor sliding down the sloped bottom. We sometimes had the deck covered in leaves and small branches after a gale.
Boat: 39ft. catamaran; Anchor: 20kg Bruce; rode: All 10mm chain, 180ft, plus a 1/2" diameter nylon bridle about 25ft. long.
An exception to the above was when anchored in Caleta Martial, about 20nm north of Cape Horn, after rounding Cape Horn in about 25knots of wind, anchored during a gale when the Chilean Navy weather station was reporting 90knots of wind. We dropped the bruce in about 12ft of water in the hard packed sand bottom and sat there for nearly three days until those strong winds blew themselves out. We never dragged.
Compared with that was our experience with the same boat and anchors during Hurricane Sandy, October, 2012, anchored in the Navasink River of New Jersey.
A nearby airport was reporting gusts of up to 100knots. The bottom there is soft mud. I had three anchors set out, anchored in about 8ft (low tide) with the tidal surge adding another 14ft. Anchors out: the 20kg bruce on the all chain rode to the east, where the storm winds would first come from, large Viking aluminum anchor (like a Fortress), and a 20kg Fisherman/Paul Like style anchor, each on rope and chain, all of them tied to the bridle. I knew the winds would rotate as the eye passed us.

At the height of the storm, during the extreme gusts, the boat would surge back on the anchors and I could feel them slip a few feet at a time.
I went into the semi-protected cockpit and started the engines. When a gust hit - you could hear them coming like a freight train - I engaged the engines and motored at a fast idle into the wind. After about three hours the wind began to diminish and I could retreat to the cabin to change into dry clothes.
The difference between the Cape Horn experience (no dragging) and Hurricane Sandy (dragging) was the holding ability of the bottom.
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