Anchors aweigh?
September 14, 2011 12:44 PM   Subscribe

If a ship in the current can't pull an anchor -- that's the purpose of the anchor, right? -- how do sailors pull the anchor up when they need to?
posted by LonnieK to Travel & Transportation (11 answers total) 1 user marked this as a favorite
The current isn't strong enough to pull the anchor...unless it is strong enough in which case the anchor will drag. Also, when you pull the anchor you are lifting it up as well rather than dragging it horizontally.
posted by dabug at 12:48 PM on September 14, 2011

Most larger sea anchors will provide a mechanism to collapse the anchor for retrieval. This is called a trip line, and attaches to the rear of the anchor, allowing it to be pulled in back first, shedding water rather than filling. This trip line can be rigged a number of ways, depending on the preference of the user.
posted by KokuRyu at 12:49 PM on September 14, 2011

Anchors are designed to dig into the sea floor at an angle. They prevent the ship from drifting in a horizontal direction while in use. The force on them is coming at an angle to the sea floor, not directly perpendicular to it. When pulled straight up, the force is less.

(Note, this answer is valid only for small anchors, like those on small sailboats and large canoes, generally described as fluke anchors, as those are the anchors I'm familiar with. They tend to have a very different design than those that are either of classical design or probably those that are used by larger vessels these days)

An illustration of this can be seen on the wikipedia page.
posted by Hactar at 12:50 PM on September 14, 2011 [2 favorites]

Sailors pull up anchors by winching them up with a windlass, in most cases.

Anchors in general are designed to hold primarily when pulled in one direction. When pulled from above or behind, they break free from the ground and are relatively easy to retrieve. Most anchors are also designed so that if pulled from a new direction (aside from up), they will re-bury themselves and hold against the new pull.
posted by gyusan at 12:55 PM on September 14, 2011

If a ship in the current can't pull an anchor

Ever heard of dragging an anchor? Totally happens, particularly in strong winds and/or tides.

how do sailors pull the anchor up when they need to?

With a capstan. The thing with the current is that it's just pushing on the ship, but the length of the cable doesn't actually change. The capstan actually shortens the cable between the ship and the anchor, so though this can result in moving the boat a bit--clever captains can actually use anchors to maneuver in tights spaces--either the anchor will rise, the cable will break, or the ship will founder. Generally the first one, as there's very little that an anchor could get snagged on which is stronger than the cables employed, the additional downward pressure exerted by pulling on the cable isn't going to represent any significant percentage of what it's already carrying, and cables are usually carefully positioned to avoid exerting pressure along the ship's short axis.
posted by valkyryn at 12:56 PM on September 14, 2011 [1 favorite]

The wikipedia anchor page has the basics of what dabug is explaining. To "use" an anchor, you pay out hte line so that it is much longer than the water is deep. To raise an anchor, you get right up over it and pull up. Some anchors must be circled around in order to get them free of the bottom or the rocks they're lodged into, but I've never had to deal with one of those myself.

But, in addition, remember that very heavy things are raised using pulley systems, which makes the weight much easier to deal with.

Finally, some anchors are designed to stay under water! Which is pretty cool or terrible depending on your point of view.
posted by bilabial at 1:02 PM on September 14, 2011 [1 favorite]

Others have covered the mechanics of anchors well enough so I'll skip that portion but I wanted to comment on KokuRyu's link which is a slightly different thing and may be confusing...

A 'sea anchor' is not intended to keep a boat stationary. It is a specialized device that adds extra drag through the water to slow or stabilize a boat in adverse conditions. Sea anchors do not rest on the bottom.
posted by Babblesort at 1:02 PM on September 14, 2011

My husband is a Naval Officer and he refers you to this: Weighing Anchor

Also worth noting: on large ships, it is not the weight of the anchor that holds the vessel, it's the weight of the chain.
posted by illenion at 2:02 PM on September 14, 2011 [1 favorite]

So I think everyone else has covered that anchors do sometimes drag. Basically, anchors are designed so that they hold on the seafloor when they are pulled horizontally. Anchors aren't all that heavy, so it isn't too hard to pull them up vertically (depending on the size of the ship, of course). When you anchor, you let out many times the depth of the water in chain or rope (from 3:1 to 12:1 are common). This ensures that the pull on the anchor from the boat is mostly horizontal (especially when you have heavy chain that lays on the seafloor). When you want to pull up the anchor, you pull in the chain/rope. At first, this pulls the whole boat towards the anchor until the angle and tautness of the chain/rope is such that the anchor is pulled upwards enough to break free of the seafloor. Then the anchor is simply pulled up to the boat. Alternately, one can move the boat towards the anchor as one pulls it in. In other words, the anchor has two modes: long chain/rope, it generally stays put as the pull is mostly horizontal, and short chain/rope, it can be pulled up easily.
posted by ssg at 3:57 PM on September 14, 2011 [1 favorite]

If you have an ordinary cruising sailboat (with an inboard motor), and an anchor like this, you huck the anchor in the water while motoring very slowly until it catches on the seabed. (If it doesn't catch, you pull it up and try again.) When you're ready to leave, you motor very slowly in the opposite direction, while pulling on the anchor line (with a winch) until you feel the anchor release, then you stop the engine and haul it up.

It is very much like putting on tire chains -- a bit of a pain, more so in heavy weather, you usually don't have as much room as you would like, sometimes you need a couple tries, and even when you do it right, sometimes they will come loose in a scary fashion.
posted by pH Indicating Socks at 4:17 PM on September 14, 2011

Anchors are designed to be easy to move up and down but hard to move sideways along the seabed.

Imagine swinging an axe down at a tree stump and sticking it into the wood? Can you pull it upwards and get it out? Yeah, probably. Can you pull it sideways and get it out? No, because of the way it's dug into the wood.

When you set an anchor, you let out 5-10x as much line as is required to get the anchor to the bottom, which is to say that if you're in 20 feet of water, you're letting out 100-200 feet of line. Your boat will then drift down current for 100-200 feet before the line is pulled taut. You can imagine then that the angle that the boat is pulling on the anchor is actually pretty close to parallel to the bottom. It is not straight up and down.

When you want to pull the anchor up, you drive the boat up over the anchor (pulling in line as you go), and then pull the anchor straight up, which it does easily.

Also, anchors drag *all the time*. You have to carefully check that they're actually set before you go off and do something else, especially at night. I have a friend who has a good story about dragging anchor almost onto the rocks at Angel Island in San Francisco Bay, and then panicking, trying to motor away, and driving over the anchor rode and fouling his prop with it.

For fun: my charter boat at anchor in Greece last summer. It's really easy to check if you're anchored well when the water is warm and super-clear. You just swim over with a snorkel and look at it.
posted by tylerkaraszewski at 11:30 PM on September 14, 2011

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