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  #1  
Old 05-17-2009
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Dense sand - mushroom anchors float on it

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. 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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  #2  
Old 05-17-2009
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Getting a properly installed helix mooring would probably make a lot more sense than getting and installing three 33 kg Rocnas...
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