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Old 17-05-2009, 08:46   #1
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Dense Sand - Mushroom Anchors Just Float

I'm about to buy a 1,000 mushroom anchor for our Beneteau 50. Then I remembered the mushroom anchors I saw "floating" on the sand where we keep our boat. I even saw one whose shank eye had worn through and it was only 1/2 way into the sand. I don't have confidence that a mushroom will "set" and provide good holding power.

I think the same problem would exist with a pyramid anchor, since they are also made of steel and would have the same density. Here's an email from dor-mor about their pyramid anchor, from earlier this month. They were very responsive to my email questions.
Quote:
Based on the tables of American Boat and Yacht Council, the wind load on your vessel in winds of 64 knots would be 7250 lbs. and in wind speeds of 100 knots would be 16,000 lbs.

Our Anchors when fully imbeded using 3:1 scope achieve approx. 10 times their dry weight in holding power.

When you asked about the 1000 lb. anchor - if you are using 3:1 scope and have a penetrable bottom, then it should achieve approx. 10,000 lbs. in holding power...
There's the "when fully imbeded" condition again.

So now what? I'm thinking that I'm back to a 3-anchor system, like I was considering last summer.

With the philosophy that each of the 3 anchors should be able to hold our boat individually, we could use 33kg Rocna anchors. Our boat is 50 feet, 12.7 metric tons. The Rocna 33kg work for 52 foot 15 metric ton boats. See the sizing chart[/url]. From their website: "We base our calculations on 50 knots wind, associated surge, and soft moderate holding bottoms into which it is assumed the anchor has set. Adequate scope is assumed." The lake is 3 miles long, 8 feet deep, with few other boats anchored nearby. Adding some extra scope is not a problem. The Rocna will surely set in the hard sand.

Finally, I found no local rules about anchor size when I looked last summer. And I intend to call my insurance company to see if they have any requirements that I need to meet.

Any thoughts on all this? Am I missing something here?
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Old 17-05-2009, 09:59   #2
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I'm no expert on mushrooms vs. other options for mooring. Which is why I always... ALWAYS ..... rely on those individuals with the best local knowledge. Usually someone like a harbormaster or yacht club manager, or someone with a boat like mine who has moored their boat in the subject loaction for a long time.
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Old 17-05-2009, 09:59   #3
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Given you are setting up a permanent mooring system, I would go with the 1000 pound mushroom. Eventually it will settle into the sand and provide excellent holding, just not in the first week. The problem with the Rocna is that it may reset each time you have a load on the mooring and the wind or current changes direction. When the wind or current changes, it may drag for a while until it does reset. Or it could get dragged into deeper water and never reset. Yes Rocna's are excellent boat anchors, but your mooring buoy application I think is better suited to a very heavy permanent anchor. People have also used less expensive alternatives like railroad car wheels or large truck engine blocks as mooring buoy anchors.
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Old 17-05-2009, 10:22   #4
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The best moorings I've seen are the railroad wheels or axles. The idea is a huge mass down there requiring minimal maintenance. If it's a permanent mooring for your 50'er I'd put down 2 - 3 thousand lbs.
The issue with the 3 anchor system is that there are a lot of parts that will wear, shackles that may work loose. That's the mooring system I had when moored off La Haina Maui & it worked well - I stayed on the mooring in storms with 10' seas. I dove on it twice a year because it did need to be looked after.
That's the important thing to provide security - MAINTENANCE.
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Old 17-05-2009, 11:09   #5
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Thanks for the posts.

DavidM - With 3 anchors there would not be any real swing. I suppose I could use deep-set danforths instead, for that reason, but I like the thought that the Rocnas would reset quickly if needed.

Randy - Yes to diving on it. The water is shallow enough to be able to do that easily. The seaon for us in only 3-4 months long - mostly due to school sports. I could retrieve the Rocnas after Labor Day and give them a good inspection over the winter. I actually like this choice because I can look after it. Anything else that's "heavy" won't ever be brought up for inspection and I won't really know when it is reaching end-of-life.

speedo - Good advice. There aren't any other big sailboats [Edit: aren't any other big boats at all] moored in the lake there.
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Old 18-05-2009, 01:34   #6
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Bene,

I would always go for the 3 anchor option. Use 1/2" or bigger chains to the center of the circle and shackle them together with 1" or bigger shackles. From there on you can just use your primary chain anchor rode with 6:1 scope or so.

But I wouldn't use three Rocna's for that. I would use my primary anchor and two back-ups. Fortress back up anchors would work great. Don't just "set" them; dig them in by hand, completely burrowed in the seabed with a short piece of polypropylene rope tied to their back/stock/whatever that floats up a couple of feet from the seabed to find them or pull them out with later. Only then pull on each anchor with the main engine, punish them!

You can leave the chains and shackles in place when you go cruising; they stay good as long as they are submerged. For retrieving, connect your secondary anchor rode to the center shackles first after which you can move the primary rode to the primary anchor and retrieve what you want to take with you.

A good anchor buried deep is many times better than a weight.

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Old 18-05-2009, 05:27   #7
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You could try a screw in mooring which might be ideal for this bottom. Find someone local with experience setting them.

The dor mor sets by its weight, while the mushroom must be set by going in a circle to get it to bury. When buried the shank will be point in one direction.

What are the other mooring over there on that bottom? What size are boats on them? I don't believe that sand has a higher specific gravity than steel so the steel should not "float" atop the sand.

The eye of the dor mor is far stronger than a mushroom and you can see that many have their shanks bent.
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Old 18-05-2009, 06:40   #8
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Quote:
Originally Posted by defjef View Post
You could try a screw in mooring which might be ideal for this bottom...
I don't believe that sand has a higher specific gravity than steel so the steel should not "float" atop the sand...
I'm a fan of helical screw-in anchors.

Even a "heavy" embedement mooring may have difficulty penetrating a compacted sand substrate.
Wet compacted sand can have a density of around 130 Lbs/Cubic Ft; whereas steel is about 490 Lbs/Cu Ft.

The following paper might be interesting.
The static equilibrium of drag anchors in sand:

http://pubs.nrc-cnrc.gc.ca/rp/rppdf/t96-083.pdf
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