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I've long been wondering about this, but couldn't read myself to the answer.

I'm talking about the classic, large iron anchors on large ships "back in the day". It seems to me that there must've been numerous situations where even a long chain could not make the anchor reach the bottom of the ocean when they were too far away from land and had to stop for whatever reason.

Or even that, in spite of being close to land, the water was simply too deep for the anchor to reach the bottom.

In such cases, did the anchor still provide sufficient/some "stability" by simply hanging like that in the water under the ship? Or does an anchor only work properly/meaningfully when it actually is resting on the bottom?

The whole concept of an anchor has always fascinated me, because it seems useless out on the sea, yet basically pointless when you are already so close to land and the water is relatively "calm". But clearly I'm missing something and I bet the truth is that it was crucial to have an anchor for any ship.

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  • For stability at sea during storms, there are 'sea anchors' ( en.wikipedia.org/wiki/Sea_anchor ) which are drogues or buoys with long ropes that they would deploy so the ship would be able to control which direction the weather and waves hit the ship when it had minimal sails up.
    – Dave X
    Aug 25 at 17:35
  • 1
    An iron anchor that does not reach bottom will have some effect, because of the additional drag caused in the water, but it won't stop the ship from moving. Aug 25 at 17:51
  • When you say "had to stop for whatever reason" - can you explain why you think there would be a need to do this?
    – Rory Alsop
    Aug 29 at 17:05
  • To be clear- the anchor doesn't actually do much in shallower water- it's just the end of the chain. It's the weight of the anchor chain that actually keeps ships in place.
    – adeadhead
    Aug 31 at 14:33
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Anchors are important when the ship can't control its own motion and would be in danger if if drifted uncontrollably.

In dead calm, in deep water far from danger of running around, there is no danger in drifting, and you wouldn't use an anchor.

In shallower water where there is danger of running aground, a sailing ship may use an anchor if it doesn't have enough wind to maneuver with.

In a storm that would limit control, visibility, and navigation, you might dangle an anchor in the hopes that you could catch the bottom and keep your ship from going aground.

In fair weather, I used an anchor in Woods Hole, Massachusetts when the tidal current was stronger than our engines and sails could keep up with when we stupidly misjudged the currents.

The length of the anchor chain(+rope) system should be at least 3x, 7x, or 10x the depth of the water, depending on conditions. The anchor chain is protection against damage from dragging on the bottom, additional weight backing up the anchor, and a bit of shock-absorber as the curve of it lifts off the bottom. Often, you extend the chain with longer rope to get a good sideways pull across the bottom without any vertical lift.

In deep water during storms where you can't control your ship, you use sea anchors to manage the angle between the wind and the waves and your ship. A drogue, whether special-purpose, the iron anchor and chain, or just long ropes, dragging in the water off of the bow would keep the strongest part of your ship pointed into the weather and slow you from being blown out of control.

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  • 1
    Depending on conditions and amount of safety, the length of the chain/rope connecting the anchor to the boat (the "anchor rode") should be 3 times the depth, 7 times the depth or 10x the depth. The jargon for the ratio of anchor rode to depth is called "scope" or "anchor scope" en.wikipedia.org/wiki/Anchor#Scope
    – Dave X
    Aug 25 at 21:30
  • The outdoors.stackexchange.com/a/11804/16418 answer considers "scope"
    – Dave X
    Aug 25 at 21:59
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Dave's answer is good, and I'd like to provide some additional examples:

  • When reaching a shore and wanting to leave the boat and use a dingy to reach the ground, you need that the boat will not drift away or into the ground, so you use an anchor.
  • When you want to sleep in the boat without someone on the helm, you can anchor when you're in shallow enough waters.
  • In some places (in most of the Mediterranean) you use the anchor when attaching the boat to a dock, the anchor holds the front of the boat, and the ropes attach to the side and/or the back of the boat.

Basically an anchor isn't the parallel to normal breaks on a car, but mostly like a parking break.

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While a sea anchor slows drifting, and can help to keep the bow pointed in the direction that reduces the risk from wind or waves, it does not hold a boat or ship stationary.

An anchor that connects to the ground will stop a vessel moving, but as you noted, will require more chain/rope than the depth of the water, so is only useful in coastal waters.

But for the use case you are querying, there is no reason to stop. If you want to turn off engines, or take down sails, you can drift, and it doesn't present any greater risks than continuing powered. The only things out there are other vessels, and you'd need a way to alert to them anyway.

There are specific exceptions, such as certain types of deep water exploration, and they use motors and gps to position themselves accurately, no matter the current or wind.

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