An oil tanker drops anchor and the anchor imbeds itself sufficiently to keep the tanker in a stable position. How does the tanker pull up the anchor if the movement of the tanker itself wasn’t enough force to pull it up?
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There are two things to note here.
The first is that typically the anchor itself isn’t doing the job of holding the boat in place – that is being done by all of the heavy chain tied to the anchor. When a boat let’s out its anchor it doesn’t just unwind it until the anchor his the bottom and stop, but instead they keep unwinding the chain so that it starts to pile up and drag on the sea floor. When the boat then starts to drift it is actually the weight of the chain dragging on the sea floor that holds the boat in place.
Separately though, you could still ask how the anchor itself doesn’t get stuck when it digs into the ground, and the answer to this is in the shape of the anchor.
If you look at the shape of an anchor, the chain fixes to the very top, and then at the other end the spades that dig into the ground are attached to the bottom pointing upwards. This means if the anchor drags along, these spades will dig in and hold it steady (until there is enough chain piling up to do that job). When it comes time to wind in the anchor, the boat isn’t pulling it sideways along the ground any more, but pulling directly upwards towards the boat. Here the shape of the anchor acts as a lever as it gets pulled up – it pivots around the bottom tip of the anchor, with a big advantage in leverage on the spades compared to the top of the anchor where the chain fixes, this will cause the spades to basically rip up out of the ground.
If the anchor is stuck under a big rock rather than just buried in squishy mud this does make it harder to lift as the rock will need to be pulled up, but remember that the size of the anchor will be relative to the size of the boat. So a big anchor stuck under a big rock will be being pulled up by a very big boat and winch – a big cargo ship will have no issue with lifting quite a few tonnes of rock (after all, it already carries loads of hundreds or thousands of tonnes worth of material, a few extra won’t make any difference). A smaller boat will have a smaller anchor, which will be harder to get stuck under something big.
And occasionally an anchor may have to be cut free if it gets truly stuck…
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