[Physics] Anchor of a boat


hope you can help

I've been teaching my kids about pirates and when we were talking about pirate ships this morning I confused myself greatly.

When carrying an anchor a pirate ship (or any vessel with an anchor) is free to float about where ever the crew pleases, however once the anchor is in the water the ship is held in place.

This seems unusual as (according to Archimedes principal) the weight of the anchor when it's in the water is less than when it's on the ship (immersed weight = weight – weight of displaced fluid) so how come the boat can carry the anchor but as soon as the anchor hits the water the boat cannot move?

As you can see below the ship on the left can move freely, the one in the right cannot.

I thought of a few possible solutions but maybe I'm shooting in the dark here.

Initially I thought maybe the part of the anchor that curls out at the bottom is holding a large volume of water (much more than that which the anchor displaced) and so holds the ship in place, like a scoop.

The other idea I had was that maybe the anchor is on the bottom of the ocean but this seems absurd, I've read that the average depth of the ocean is around 14,000ft (obviously parts are much deeper) and a 30mm chain of that depth would weight almost 100,000kg, which not only seems like a lot for the boat to carry but also essentially adds to the weight of the achor itself, perplexing the question further still

Thanks for your time

Best Answer

Most anchors have to "hook" into the seabed, so you will need a long chain (rode). In some cases, the rode can have several thousand feet and weigh 100 tonnes. But in most cases, ships won't anchor in deep water.

Note that the anchor's rode must be several times longer than the sea depth:

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To remove the anchor, you just have to shorten the chain until it's vertical. Then you pull "up".

In deep water, see the: sea anchor, which is a kind of sea "brake".

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