March 27, 2008


Stupid Question ™
July 13, 2000
By John Ruch
© 2000

Q: Why are ship anchors shaped like double fishhooks? If they’re supposed to hook onto something underwater, how are they unhooked? And if they’re not, why have hooks?
—Curious Landlubber

A: A couple special cases aside, anchors are indeed supposed to hook onto stuff—in fact, they’re called “hooks” in sailing jargon.

They either dig into the muck at the sea bottom, or hook into crevices in rock or coral.

Anchors have to hook into the sea floor because even medium-sized boats are too big to be held still by only a big weight (or at least by a weight that could be practically carried around on board).

Anchors are certainly heavy—up to 30 tons each on the biggest ships—but the most significant factors in their holding power are how deeply they dig in, the surface area of their flukes (or points), and the way the rode (rope or chain) connecting them to the ship is laid.

Anchors actually end up lying horizontal on the bottom. If the rode ran straight up to the ship, the vertical pull would work the anchor loose. Instead, the rode is played out in a curve, so that at the sea floor it’s lying flat and providing continuous horizontal pull on the anchor to keep it dug in.

The type of anchor you’re talking about is the “stock” or “Admiralty pattern” anchor. As two-dimensional representations of a three-dimensional object, drawings of stock anchors are misleading. The stock, or top crosspiece, isn’t parallel with the hooks. It’s perpendicular to them, which ensures that no matter how the anchor lands on the bottom, it will pivot so one of the flukes digs in.

Hauling an anchor out of seabed muck is fairly easy. Winches are almost always used, and the bobbing of the ship itself helps work it loose.

Unhooking an anchor from rocks or coral can be a different story—one involving scuba divers—or even an impossibility. That is, unless you prepare a trip line first. That is a line running from the bottom of the anchor to a buoy.

If the anchor won’t come loose the normal way, you go to the buoy and tug on the trip line, pulling the anchor out backward until it comes loose.
The stock anchor used to be the industry standard, but is today used almost exclusively for rocky-bottom areas.

Far more common is the “stockless” anchor, which looks something like a shovel with the blade split in half and the handle bent forward until it’s between the split. It’s used exclusively in sandy or muddy seabeds (the most common type for anchoring in anyway), into which it digs so deeply that it actually becomes completely buried.

This avoids another problem with the stock anchor—it always left one fluke sticking up from the seabed, where it could puncture the ship (in shallow waters) or snag the rode.

In fact, those jaunty drawings of anchors that show rope wrapped prettily around the shank are a reference to the stock anchor’s rope-snagging tendencies—a pain in the neck to sailors of yore.

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