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Old 09-08-2015, 10:19 AM   #1
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Large discrepancy in anchor sizing

My ground tackle calculations (based on Hinz Complete Book of Anchoring) produce chain and rope sizing that is consistent with the recommendations of the mainstream anchor manufacturers.

It's a different story however when it comes anchor sizing.

I've taken careful measurements of the boat windage with a 30 degree skew, displacement, wave surge, aerodynamic load, etc., for a working anchor (30 knot wind) and the calculations come out to anchor sizing of half the weight of the mainstream companies.

They only compare when I calculate for force 11 storms.

So why the discrepancy, are the mfg.'s just being conservative, trying to sell anchors?
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Old 09-08-2015, 11:02 AM   #2
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Old 09-08-2015, 11:06 AM   #3
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Not familiar with your reference material, but load calculations shouldn't vary much as it all just math when figuring out load force and chain size. Now anchors would be a totally different animal based on bottom substrate holding power. Far to many real world variables for an anchor manufacturer not to be extremely conservative in their recommendations.

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Old 09-08-2015, 11:08 AM   #4
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Not trying to pick another fight here .

But I get the feeling that a lot of boaters pick an anchor size because everyone says so. Certainly mfg.'s are not the most objective bunch around.

But we engineers tend to like to rely on numbers and calculations and silly things like that.

Anyway, just hoping to get some feedback.

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Old 09-08-2015, 11:16 AM   #5
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The anchor manufacturers are selling anchors and want you to buy the biggest anchor. Notice how much more expensive the next size up is. Also he wants you to have success using your anchor and bigger will come a lot closer to accomplishing that and you will tell your friends what a great success your XLM anchor is.

But perhaps the biggest variable is the expected performance of the anchor .. per pound. For me an 8lb Fortress may be fine but the same lever of performance w a Claw may require a 33lb unit. But if you're comparing sizes of very similar anchors like the Rocna and the Supreme or the Delta and the Davis then it should be more like lb for lb.

Windage seems to be a difficult thing to calculate. I've seen many attempts that don't compare w other calculations. For example most are based on area and fail to consider many many other wind drag sources ... like how many and how big are the hand railings, roof overhangs, rigging, bow pulpits, radar arches and if you considered them all you'd be look'in at door knobs. There is so many and they all add up. Windage numbers seem all over the place and if all the other variables are considered like waves and boat displacement the job of calculation becomes impossible.

I think I can come up w appropriate weights for many anchors but it's only a guess based on all I've read, heard and at times experienced. But it helps if you've used a certian size anchor in a big storm and it's performance was perfect. If someone says their anchor is great it would be nice to be able to look in his log book and see under what conditions the anchor was great in and how many times that performance was repeated.

As you say calculating rode size is more or less easy. But calculating anchor size is more guessing than science and numbers .. and good guessing. Guessing is probably a bad word to use w an engineer. How-a-bout estimating w lots of experience. And of course all the elements mentioned in this thread (so far) haven't mentioned the most important variable of all ........ the bottom. Important because we have little control over it and that variable has such a wide range ......
Look at the Rocna. In a good holding power bottom it usually is top dog in holding power if only by a small amount. But but it in really gooey soft mud and it's practically useless. Practically any other anchor out performs it but many of us have never found mud that slimy and soft.

So Makobuilders it looks like there are too many variables to use numbers to determine anchor size. The simplest is to observe others and follow suit or go up a size in weight. So just use the best tools you have including common sense.

Looks like RT chickened out. But he's here.
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Old 09-08-2015, 11:36 AM   #6
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Mako

What is your vessel length, color and weight. Given that data the experts here can provide you all the advice you'll never need.

For starters, why not go with a 25 kg Manson, 5/16 chain and 1500 Maxwell. 😀
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Old 09-08-2015, 11:49 AM   #7
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Quote:
Originally Posted by makobuilders View Post
Not trying to pick another fight here .

But I get the feeling that a lot of boaters pick an anchor size because everyone says so. Certainly mfg.'s are not the most objective bunch around.

But we engineers tend to like to rely on numbers and calculations and silly things like that.

Anyway, just hoping to get some feedback.

Cheers
Hey if you feel that you know better than the anchor manufacturer what your vessels needs are, then by all means choose based on your calculations. What's the worst that could happen if you guess wrong?

Ted
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Old 09-08-2015, 01:00 PM   #8
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MakoBuilders:

Not everything in life can be calculated by an engineer with precision. How much force a boat places on an anchor is one of these. How much force a given anchor can hold is another. It takes experience.

I go the opposite way. I look at the manufacturer's recommendation and I buy the next size up, based on experience.

David
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Old 09-08-2015, 02:27 PM   #9
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I always liked Steve Dashew's method of anchor spec'ing: "You know your anchor is about the right size when people on the dock start laughing at you for how (overly) large it is"

Quote:
But if you're comparing sizes of very similar anchors like the Rocna and the Supreme or the Delta and the Davis then it should be more like lb for lb.
I like to size anchors simply by choosing one where my boat size is at the very bottom (or even a notch below that) of the boat-length range the anchor manufacturer gives for a given anchor weight (and in the case of the very large profile Hatt, that made a lot of sense). For Delta, that ended up being an 88 pounder, for Rocna, 110, just as a factoid.

Mako, this is probably why the Hinz numbers start matching the manufacturer's as wind speed increases. Getting blasted by an unforecasted microcell doesn't allow you the luxury of breaking out your hurricane anchor.
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Old 09-08-2015, 02:40 PM   #10
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NO you understand it perfectly
sometimes the answer is as simple as it seems.

Manyboats' first paragraph pretty much sums it up.

I may add that even here on TF, virtually everyone who goes on and on about their new anchor, glosses over the fact the the new anchor is heavier than the old one.

Basically it becomes an issue of needing to justify spending $1000 versus $300.
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Old 09-08-2015, 02:45 PM   #11
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Anchor size is just one factor in anchoring. But too many boats drag for the anchors to be universally oversized.
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Old 09-08-2015, 03:36 PM   #12
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I agree with the others who advise conservatism. Don't try to out engineer the professionals who have determined sizing. You may be a better engineer, but not likely you have their experience with anchors. Conditions vary a lot. We've certainly anchored in conditions where our anchor was far more than needed. However, in different situations we liked it's size. People do stare at anchors. I'd far rather people think mine is too big than too small.
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Old 09-08-2015, 05:33 PM   #13
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Quote:
Originally Posted by Bay Pelican View Post
Anchor size is just one factor in anchoring. But too many boats drag for the anchors to be universally oversized.
To paraphrase Yogi Berra "Anchoring is 90% technique and the other half is physical"
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Old 09-08-2015, 07:06 PM   #14
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And be absolutely sure you have the correct shackle between the anchor and the chain. The anchor is useless if the shackle breaks.
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Old 09-08-2015, 10:21 PM   #15
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And be absolutely sure you have the correct shackle between the anchor and the chain. The anchor is useless if the shackle breaks.
Now you've done it!
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Old 09-08-2015, 10:50 PM   #16
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Quote:
Originally Posted by djmarchand View Post
MakoBuilders:

Not everything in life can be calculated by an engineer with precision. How much force a boat places on an anchor is one of these. How much force a given anchor can hold is another. It takes experience.

I go the opposite way. I look at the manufacturer's recommendation and I buy the next size up, based on experience.

David
Exactly!!
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Old 09-08-2015, 11:11 PM   #17
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Hey if you feel that you know better than the anchor manufacturer what your vessels needs are, then by all means choose based on your calculations.
Heil Manufacturer!

I would agree that the mfg should be the most knowledgeable on the holding strength of their anchors under various conditions and averaged over many tests. But that doesn't mean that they give the correct guidance.

So now I'm able to answer my own question from the opening statement - the reason that the numbers vary so much is because the person answering inquiries from technical support department is some part time school kid or grandma who solely replies based on the same simplified chart that is posted on the internet.

Since my boat's design does not share the same overall profile of the average production trawler (which the charts are based on), I had sent technical information, calculations and load estimates to various mfg.'s. Chose three similar designs so that the comparison would be "apples to apples" as Manyboats was stating.

One came back with what I believe is an accurate recommendation. The other two were so incredibly, blatantly wrong that I laughed out loud and scared my cat.

Anchor discussions are always hot and my intention is not to start a pissing contest. But the wonderful purpose of forums is to share information from a variety of sources. Perhaps a newcomer to the sport will appreciate some objective feedback. Perhaps someone with decades of experience will appreciate opening up a closed mind.

Personally I really appreciate this forum above the others out there on the 'net. It's educational, interesting, funny and insulting all at the same time.

Cheers
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Old 09-09-2015, 12:05 AM   #18
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Anchor size is just one factor in anchoring. But too many boats drag for the anchors to be universally oversized.
Marty,
You really think anchor dragging is common? Don't think it's common in the PNW. But you're right if it is .. bigger is certianly better. Think I'll try my 15lb Manson Supreme w/o the roll bar this weekend. Only setting will be tested as wind is to be less than 10 knots. I could try and turn it up side down though.
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Old 09-09-2015, 06:12 AM   #19
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Marty,
You really think anchor dragging is common? Don't think it's common in the PNW.
Agree with you, during my time in the PNW (years ago) dragging didn't seem to be an issue. Experiences in the Bahamas (Georgetown) and the Eastern Caribbean were different with frequent draggings. Could be just the large number of boats which were anchored or the higher winds or perhaps the number of charters. Don't know. But when I do find out details of the type and weight of the anchor it is more likely than not that the anchor weight is lighter than I would have expected.
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Old 09-09-2015, 06:32 AM   #20
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Anchors have changed very little over the last 6 or 7 . decades

Sure the new guy is touting his miracle at $50 a pound as the cure for anything ,
BUT the standards are still being made and still work just fine.

Happily the standards can be found used in fine condition for a buck or two a pound.

If the windlass is full sized pulling up a 45 or 60 lb anchor is no more effort than hauling a 20lb watch fob..

If you go cruising , you will loose an anchor some time , as you will go aground sometime.

A second, third or fourth set of ground tackle is nice insurance.
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