Mailport July 2013 Issue

Mailport: July 2013

Anchor Bending

I would greatly appreciate your thoughts regarding the snatch load a 27-foot sailboat weighing 6,000 pounds with a fin keel is likely to encounter under extreme storm conditions on the Northern Chesapeake Bay (mud bottom). Letís assume these storm conditions are caused by 6-foot waves and continuous but varying wind gusts of 30 to 55 miles per hour hitting the boat as Practical Sailor tested (PS, April and May 2013); well set but not completely buried with a 5:1 scope and snatched at 90 degrees from the set direction.

Photo courtesy of Walt Grifel

Reader Walt Grifel is seeking a pump that will drain the bilge, but not the batteries aboard his Sabre 28, Impromptu (at right).

By any chance, have you developed an algorithm for predicting safe snatch loads to avoid bending the anchorís shank?

Do you have a guideline for estimating the load at which an anchor is likely to be pulled out of a good setting? I am familiar with the holding power guidelines my anchorís manufacturer provides, but I am interested in your estimates as I have a feeling they will be different.

I have been caught in squalls when the anchor held well for the first half-hour, but after encountering repeated blows from aggressive waves for an extended period of time, the anchor drags. Are you aware of any anchor-holding test conducted over time to assess an anchorís holding power?

Richard Paden
Petite Cherie, 1974 Dufour 27 (#193)
Chesapeake Bay, Severna Park, Md.

We delved into the topic of rode loads in the November 2012 issue. We arenít aware of any tests that could answer your questions about your anchorís holding power over time or under snatch loads. Often, a good anchor will set deeper in the conditions you describe, but because you are in shallow water, you lose most of the advantage of chain catenary in absorbing shock loads. A long nylon snubber with chafe protection, where needed, is essential in your situation. † As we stated in our November 2012 report on rode loads, there are many published charts of theoretical loads. Probably the most commonly cited table of anchor loads in the U.S. is the table of theoretical loads used by the American Boat and Yacht Council in its Standards and Technical Information Reports for Small Craft, revised most recently in 2008. Indicating minimum design loads for deck hardware used for anchoring or mooring (cleats, bitts, samson posts, etc.), the table lists calculated loads according to boat size, ranging from 10 to 60 feet. According to that table, permanent mooring hardware on a 35-foot sailboat (allowing a margin of safety in your case) should be able to withstand 2,700-pound working loads.

Assuming this as your baseline, and including a margin for error, an appropriately sized (see previous letter), brand name anchor constructed from declared, high-tensile raw materials, such as ASTM 514, should be reliable.

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Comments (1)

I fully agree with PS's response here and use of more logical metric system. The relative numbers are far more important then the units, and psi would be wholly inappropriate. We may have a sense for a few hundred psi, but what is 50,000 psi? And since the US Imperial system does not scale easily, we would be force to use something convoluted like tons per square inch. In American engineering schools, material science is taught with megapascals because it is widely used and of appropriate scale.

Posted by: DAMON L | July 14, 2013 6:59 PM    Report this comment

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