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Old 19-11-2013, 09:06   #1
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Displacement to Anchor Weight Ratio

When I was considering upgrading to a Rocna from a Delta 70 lb for my 39 ft Amel Sharki (21,300 lbs.) I decided to look at what manufacturers of a few high quality boats provided with various models.
To make some sense of the information I looked at the ratio of the boat's displacement and the anchor's weight. This ratio may be meaningless scientifically. Anyway, here is what I found.

Amel 55 48,000lbs 66lb Brugel 727 to 1
Amel 64 75,000lbs 92.4lb Brugel 812 to 1
Morris 45 22,300lbs 45lb CQR 496 to 1
Morris 48 32,000lbs 45lb CQR 711 to 1
Morris 52 40,300lbs 55lb Delta 733 to 1
Oyster 475 35,273lbs 44lb CQR 802 to 1
Oyster 545 46,893lbs 75lb CQR 625 to 1
Oyster 635 73, 854lbs 75lb CQR 985 to 1
Halberg Rassy 48 40,700lbs 70lb Delta 581 to 1
Halberg Rassy 55 58,000lbs 70lb Delta 829 to 1
S/V Hanoah Amel 39 21,300lbs 73lb Rocna 292 to 1
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Old 19-11-2013, 09:17   #2
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Re: Displacement to anchor weight ratio

I would expect final decision should also take windage into account, not just displacement.

Example: we can't reef our flying bridge so we present much more windage than a sailboat of equal displacement... and have had to adjust our anchor selections accordingly.

Although perhaps manufacturer recommendations have already taken that into account, if offered as boat-specific suggestions...

-Chris
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Old 19-11-2013, 11:52   #3
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Re: Displacement to anchor weight ratio

Just to add a little spice to the discussion:

Quoting Don Dodds in Modern Seamanship page 145-146
"One of the fundamental truths of anchoring is that a 40-pound digging anchor holds more than a 16-pound digging anchor, not because it weighs more, but because the fluke area is larger. The additional weight is only a by-product of the larger fluke area and is of little value once the anchor is buried."

Dodds then provides a mechanical engineering analysis (trig and force vectors) to support his thesis.

Maybe you need to develop a fluke area to displacement analysis?
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Old 19-11-2013, 12:01   #4
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Re: Displacement to anchor weight ratio

And a little more spice:

Dodds again on page 184:
"In order to calculate wind loads, it is necessary to deal with Cd, the drag coefficient, and Ae, the equivalent area exposed to wind. "

Again thru trig and force vectors Dodds goes on toe show the wind load of comparable displacement boats will vary significantly due to superstructure and rigging. The ratios for wind loads are approximately

2.5 x Beam x Height for a sailboat and 1.5 x Beam x Height for a powerboat.

He goes on to show that in winds of less than 10 knots the wind loading is insignificant compared to the hydraulic loads placed on the submerged hull by water forces (currents and waves).

However as the wind speed increases the wind loading quickly dominates due to the wind load being a factor of the square of the wind velocity. Going from 10 knots to 20 knots increases the wind load by a factor of 4.

On page 189 he uses the magic of trig and force vectors to determine required anchor size based on total load (wind and hydraulic, static, and dynamic).

If you like good engineering - you'll love Dodds!
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Old 19-11-2013, 12:03   #5
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Re: Displacement to anchor weight ratio

The problem with the information is that boats that anchor a lot will generally upgrade the anchor supplied by manufacturer. Usually the supplied anchor is retained as spare.

Amels tend to be an exception.
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Old 19-11-2013, 12:36   #6
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Re: Displacement to anchor weight ratio

As others have stated, overall anchor performance is related to weight, fluke area and ease of setting and resetting.

A Danforth style anchor, particularly a Fortress has a very high fluke area and low weight, particularly for the Fortress. It will hold like a beast once set. But they are often difficult to set (due to low weight presumably) and very difficult to reset in a reversing current.

A Bruce is a breeze to set and reset, but it has less fluke area than others and doesn't have a sharp point to penetrate easily. It will set easily but not set deeply.

The "best" anchor is one that combines all three attributes. I like the Rocna or Manson Supreme.

I had an 18,000 lb sailing catamaran and I wasn't happy until I installed a 55# Rocna.

David
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Old 19-11-2013, 12:37   #7
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Re: Displacement to anchor weight ratio

Quote:
Originally Posted by TacomaSailor View Post

Quoting Don Dodds in Modern Seamanship page 145-146
"One of the fundamental truths of anchoring is that a 40-pound digging anchor holds more than a 16-pound digging anchor, not because it weighs more, but because the fluke area is larger. The additional weight is only a by-product of the larger fluke area and is of little value once the anchor is buried."

I suspect that's one reason the alloy Fortress anchors are sometimes said to hold better than standard steel Danforth-style anchors of the same weight. The Fortress flukes are simply bigger...

Not a recommendation, just an observation...

-Chris
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Old 19-11-2013, 13:33   #8
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Re: Displacement to anchor weight ratio

Quote:
Originally Posted by ranger42c View Post
I suspect that's one reason the alloy Fortress anchors are sometimes said to hold better than standard steel Danforth-style anchors of the same weight. The Fortress flukes are simply bigger...

Not a recommendation, just an observation...

-Chris
The Fortress anchor does much better than a tradition Danforth even matching the equivalent fluke area.
The big 2006 multi magazine test (sail yachting monthly etc) selected a Fortress anchor that was unfortunatly larger than it should have been (FX-37). The next size down (FX-23) would have been a better match for the steel anchors. There is a substantial increase in fluke area, but the dramatic increase in holding power of the Fortress cannot be explained by a increase in fluke area alone. The anchor is fundamentally working much better.

This was the approximate holding power at the 4 locations. The Danforth was a good quality west marine performance version. ( all results in pounds holding. The test was stopped at 5,000lbs so the holding may have been greater when 5000 lbs is recorded.

Danforth. (Performance 26.3 lbs)
100 lbs
50 lbs
400 lbs
700 lbs

Fortress (22 lbs)
5,000 lbs
5,000 lbs
5,000 lbs
1,500 lbs
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Old 20-11-2013, 03:01   #9
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Re: Displacement to anchor weight ratio

Quote:
Originally Posted by ranger42c View Post
I suspect that's one reason the alloy Fortress anchors are sometimes said to hold better than standard steel Danforth-style anchors of the same weight. The Fortress flukes are simply bigger...

Not a recommendation, just an observation...

-Chris
Chris,

The US Navy conducted a test with Danforth and Fortress anchors of comparable sizes. This test included a 90 lb Danforth H-3600 model along with the 47 lb Fortress FX-85, and these anchors are almost identical in physical size.

The sea bottom was sand/clay. The Danforth held to 9,000 lbs and under "Reason for Test Termination," the US Navy noted: "Anchor Pulled Out - Structural Damage"

The FX-85 held to 10,200+ lbs, and under "Reason for Test Termination," they noted, "Test Stop, Anchor Held - Shank Bent on Retrieval"

Two other Danforth models were tested, the T-6000 and T-7000 of the "Deepset II" model series, and they also pulled out during their tests due to structural failure.

The Fortress models FX-55 and FX-125 were tested as well. In the same sand/clay bottom, the smaller 32 lb FX-55 pulled out at 8,800 lbs in the first test, and in a second test, it held to 10,600 lbs when they stopped the test.

The 69 lb FX-125 held to 14,600 lbs when they stopped the test due to the engines overheating aboard the testing vessel.


Prior to this 1989 test, an aluminum alloy was probably never considered to be an appropriate material for a boat anchor, and afterwards the US Navy wrote this in their summary report:

"The fact that the Fortress anchors incurred no significant structural damage at such high holding ratios suggest that the anchors have been extensively engineered from both the hydrodynamic and structural standpoints."

And also:

"Under anticipated loading conditions, NAV-X (corporate name) has compensated for aluminum's susceptibility to deformations through careful structural design of their anchors."

Regards,
Brian
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Old 20-11-2013, 03:46   #10
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Talking Re: Displacement to anchor weight ratio

Quote:
Originally Posted by noelex 77 View Post
The Fortress anchor does much better than a tradition Danforth even matching the equivalent fluke area.
The big 2006 multi magazine test (sail yachting monthly etc) selected a Fortress anchor that was unfortunatly larger than it should have been (FX-37).

The next size down (FX-23) would have been a better match for the steel anchors.
Noelex,

The 7.1 kg FX-23 was pitted against steel anchors (including a 15 kg Rocna and 16 kg Manson) which weighed 2x more in a holding power test that was reported in the May 2012 issue of Voile, a French sailing magazine.

Afterwards, the FX-23 was called "The Champion of All Categories" as it held to over 2,500 kg (5,500 lbs) which was 350x it's weight.

Here's a short video from aboard the test boat during a pull of the FX-23, and if you know French, then you might hear a couple of profanities mixed in with the superlatives.

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Old 20-11-2013, 09:55   #11
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Re: Displacement to anchor weight ratio

Displacement/Anchor = Price/Earnings

Investors use the Price/Earnings Ratio (P/E) as an indicator in evaluating a company's financials. While the P/E is important it is not the only indicator in the invester considers. D/A = P/E because financial security is as important as anchor security for the long distance, long term cruiser.

For the primary anchor (this does not include Danforth type fluke anchors) the Displacement / Anchor Weight Ratio of around 300 may be a good number for a sailing monohull. This is an unlicensed ratio, use any of the popular primary anchors; Delta, Rocna, Brugel, Mantus, CQR, whatever.

The D/A Ratio does not take into account length of scope, nor does it consider the difference between all chain and a hybrid of chain and nylon. The sea bottom configuration is unknown. Power yachts and catamarans with greater wind exposure will have a different optimum D/A than monohull sailboats.

The Displacement/Anchor Weight Ratio gives us a place to start talking. So, what's your D/A?
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Old 20-11-2013, 10:12   #12
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Re: Displacement to anchor weight ratio

Thanks Brian
Good job my limited French learned at school was not broad or enough to cover some of those words

The only results I have seen from that test are on the Rocna website. Sometimes when manufacturers publish results from anchor test they are a little "selective" in the data they present. The published table does not even include Fortress although Rocna do mention in the text that Fortress did well.

Do you have more results (or a link)?
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Old 20-11-2013, 12:46   #13
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Re: Displacement to anchor weight ratio

Quote:
Originally Posted by Sharki127 View Post
When I was considering upgrading to a Rocna from a Delta 70 lb for my 39 ft Amel Sharki (21,300 lbs.) I decided to look at what manufacturers of a few high quality boats provided with various models.
To make some sense of the information I looked at the ratio of the boat's displacement and the anchor's weight. This ratio may be meaningless scientifically. Anyway, here is what I found.

Amel 55 48,000lbs 66lb Brugel 727 to 1
Amel 64 75,000lbs 92.4lb Brugel 812 to 1
Morris 45 22,300lbs 45lb CQR 496 to 1
Morris 48 32,000lbs 45lb CQR 711 to 1
Morris 52 40,300lbs 55lb Delta 733 to 1
Oyster 475 35,273lbs 44lb CQR 802 to 1
Oyster 545 46,893lbs 75lb CQR 625 to 1
Oyster 635 73, 854lbs 75lb CQR 985 to 1
Halberg Rassy 48 40,700lbs 70lb Delta 581 to 1
Halberg Rassy 55 58,000lbs 70lb Delta 829 to 1
S/V Hanoah Amel 39 21,300lbs 73lb Rocna 292 to 1

I should have specified that the Displacement/Anchor Weight Ratio applies to the primary anchor. The thread took a turn towards fluke type anchors which are frowned upon as primaries.

Also, the Displacement to Anchor weight ratio should not be confused with the Debt to Assets Ratio which is also abbreviated D/A.

So, what's your D/A?
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Old 20-11-2013, 13:44   #14
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Re: Displacement to anchor weight ratio

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Thanks Brian
Good job my limited French learned at school was not broad or enough to cover some of those words

The only results I have seen from that test are on the Rocna website. Sometimes when manufacturers publish results from anchor test they are a little "selective" in the data they present. The published table does not even include Fortress although Rocna do mention in the text that Fortress did well.

Do you have more results (or a link)?
Noelex,

Any test results that you might find on the web site of Peter Smith, the Rocna designer, are typically going to be inaccurately skewed in their favor, just like the chart that you posted. Please send me a PM with your e-mail address and I will send you a PDF file with the Voile magazine article.

The Smiths also whined about the Fortress FX-37 anchor being oversized in the West Marine test. Really? This anchor was outweighed 40-50% by the steel anchors in this test, and yet was still a top performer. In the Voile test, the FX-23 was outweighed over 100% by the Rocna and the other steel anchors….and still out-performed them all, so naturally they omitted this information from their homemade chart.

I do not believe that the new manufacturing license holder for Rocna anchors, CMP Global of Canada, would ever publish bogus test results on the Rocna web site.

Sharki127, may I humbly submit that the new 154-ft USCG Fast Response Cutter (FRC), which displaces 353 long tons (about 800k lbs) has approved the 70 lb / 32 kg Fortress model FX-125 for it's primary anchor, a ratio of over 11,000 to 1.

As I recall, the holding power requirement of this vessel was determined to be about 13,000 lbs, and we were able to show that the FX-125 had exceeded this holding power on two tests, one of which was conducted the US Navy.
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Old 20-11-2013, 14:05   #15
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Re: Displacement to anchor weight ratio

Quote:
Originally Posted by Sharki127 View Post
To make some sense of the information I looked at the ratio of the boat's displacement and the anchor's weight. This ratio may be meaningless scientifically. Anyway, here is what I found.


Oyster 545 46,893lbs 75lb CQR 625 to 1

Halberg Rassy 48 40,700lbs 70lb Delta 581 to 1
These ratios are close to each other and the boat sizes are close to each. But do people really feel that a CQR and Delta are that close to each other?


Meanwhile my 21,000 displacement boat has a 60# Manson Supreme, which is a ratio of 350. If I had a CQR would a 35 pound one be good enough as that would be a 600 ratio?

I think this whole displacement/anchor weight thing is useless. The only thing the weight has meaning to is the bending strength and maybe the surface area (if comparing the same materials).
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