and ship anchors


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?

In: 2

On large modern ships its actually not the anchor itself that holds the ship from moving, its the weight of the anchor chain that also lies on the bottom. The cleats (they have a name, can’t remember) or spade like bits on the anchor are to keep the anchor from dragging along the seabed, but only in a lateral direction. When the chain lifts straight up, they disengage and the anchor is lifted out of the seabed.

So, to lift the anchor, the windlass only has to lift the length of chain that is off the seafloor, not the whole thing. And due to the magic of gearing, a moderate size motor can lift quite a bit of weight.

An anchor can keep a ship in place because it digs into the seabed and holds against being pulled *sideways*. When the ship wants to leave, it reels in the anchor chain until it’s above the anchor, and then pulls the anchor *upward.*

As long as the winch (or crew) is strong enough to lift the anchor, and it hasn’t gotten wedged into a crack in bedrock, it’ll come up.

When dropping anchor you also drop a lot of rode (heavy chain). If the water is 100′ deep then you want to drop 700′ of rode. Most of it is lying on the bottom of the ocean. When the wind or current pushes the ship, the rode provides most of the stopping power, with friction along the bottom and with its weight. As the ship gets pushed, it tries to lift the heavy road off off of the bottom.

When want to lift the anchor, the boat slowly motors towards the anchor, lifting the rode. When it is directly above the anchor, it rotates and breaks free. (Rarely it doesn’t and you need to cut it free).

The anchor falls to its side when it hits the seafloor and its flukes dig into the sand or whatever. This is enough to resist lateral forces if the boat is pushed around by currents or wind. When they want to leave it’s pulled up vertically which lifts its flukes out of the seafloor:

Ships and boats don’t intentionally move while anchored. Any movement while anchored is due to environmental factors.

A ship isn’t just going to drag an anchor behind them.

An anchor is not nearly the same weight as a ship, it’s just heavy enough to keep a boat in place.

Like if I am in a canoe, I don’t need something 250 lbs, I only actually need like 10-20 lbs

When the anchor chain is straight up and down the anchor doesn’t have much holding power so the tanker can easily pull it up. Anchors work because of the angle of pull, the more horizontal the better it holds.

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…

These are great answers! Thanks to everybody.