30

When a merchant vessel drops an anchor, the anchor chain is initially stretched. The captain is waiting until the chain unstretches and after that he is assured that the anchor keeps the vessel in position.

The opposite seems reasonable to me: that only a stretched chain can keep anything in position. Does anybody know something more on this?

Glorfindel
  • 1,424
veronika
  • 2,706
  • 1
    Are you asking about why you want the line to be slack, or are you asking why that means the anchor is "ready". The current answers only seem to focus on the former – BioPhysicist Mar 16 '20 at 10:07
  • FYI: The captain doesn't "wait until the chain unstretches." The captain (and crew) let out an appropriate amount of chain for the depth of the water (maybe between 5x and 10x as much chain as depth), and then they "back down" (i.e., pull) on the chain in order to "set" the anchor. After that, there's nothing left to do except monitor the vessel's position, and be prepared to take action in case the anchor "drags" on the bottom. The chain will only go slack in the absence of any strong wind or current that wants to move the boat. – Solomon Slow Mar 16 '20 at 15:25
  • @SolomonSlow No, the captain does not pull on the chain and the chain is really "slack" He is informed from the crew on the ship's bow that the "anchor is dead" ( on greek or russian marine slang that I understand) After that the anchoring procedure is over and the bridge watch, as you say, monitors the position – veronika Mar 16 '20 at 21:02
  • 1
    The search term you can find more info on anchoring - "anchor scope" (ratio between chain length and depth - 5:1 to 7:1 for metal chain, more for other materials). https://www.boatus.com/boattech/articles/anchoring.asp – Alexei Levenkov Mar 16 '20 at 21:45
  • @veronika, Fair enough. I only have the experience of anchoring small vessels. I had not considered the possibility that a large vessel could reliably set anchor without backing down. – Solomon Slow Mar 17 '20 at 11:11

1 Answers1

47

There are many different kind of anchors with different working principles (see wiki). Most of them, however, rely on the chain lying on the ground for the anchor to have its effect.

To understand this, let us look at a stockless anchor and how it holds onto the ground (picture from the wikipedia article):

enter image description here

From the picture it is clear that the anchor has its main strength when the chain pulls parallel to the ground. In fact, the way the anchor is liftet is by pulling it upwards (pciture also from the wikipedia article):

enter image description here

So what about the chain stretching/slackness? Let's look at the following picture (from this quora question):

enter image description here

So essentially what happens is that most of the chain lies on the ground. The rest is fairly loose, which gives the ship some swaying room, but when pulled by wind in one direction it will stretch sufficiently to balance the wind force on the boat. The weight of the anchor chain therefore also plays a crucial role, since the chain should not be stretched fully by the wind force.

In summary, if we stretch the chain too much, the anchor cannot operate. Instead, a large part of the chain has to lie on the ground, such that the force on the anchor is rougly parallel to the ground. The weight of the anchor chain ensures that forces on the boat such as waves and wind do not fully stretch the anchor chain.

Aaron Stevens pointed out that the question also asks about the equilibration phase after anchoring, which I missed initially in my answer. Specifically, why one waits until the anchor relaxes again.

Let us look at what happens after throwing the anchor.

  • Anchor hits ground.
  • Ship still moves. Anchor is dragged along.
  • The anchor gets a grip. Remaining movement of the ship stretches the chain, drags the anchor further.
  • Eventually, the anchor gets stuck in its final position, with the chain most likely stretched due to the ship movement.
  • The ship is stopped and dragged into an equilibrium position by the stretched chains weight.
Wolpertinger
  • 11,535
  • 1
    I'd suggest mentioning in your 4th bullet point that the ship is pulled back in the opposite direction to which it was pulling the anchor by the chain moving into equilibrium after the anchor gets a grip. – Makyen Mar 16 '20 at 18:17
  • 3
    This also accommodates tides, which can reach +18m in some parts of the world. If the anchor chain is tight at low tide you rip the chain box clean off the ship on the way to high tide. – Stian Mar 16 '20 at 21:07
  • 4
    @StianYttervik Why would it damage the ship rather than just lifting the anchor as if some of the chain was drawn in? – JPhi1618 Mar 17 '20 at 15:14
  • 5
    Because you can lift the anchor (more or less straight up, you navigate the ship to get a clean lift) - you can absolutely not drag the anchor in, which would be the case with a taut chain and wind/current on the ship. I am not saying that each and every time, the chainworks will be catastrophically damaged, I am saying it is a fairly likely event - and should be avoided. – Stian Mar 17 '20 at 16:25