if an anchor is heavy enough to hold a ship in place, how does the ship not sink when hauling it?
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Well, the ship doesn’t sink when it drops the anchor either, does it? The anchor only needs enough weight to keep the ship from floating away… it doesn’t need to support the full weight of the ship.
But also, the anchor doesn’t rely on weight only… it has spikes that grab onto the ocean floor and in fact most anchors are not heavy enough to hold the boat without digging into the mud.
It’s heavy enough to hold the ship in place but not heavy enough to sink it. The weight threshold of holding the ship in place and sinking the ship are very different. It takes more weight to sink the ship than to anchor it.
Because an anchor isn’t just a weight that sits on the bottom. It has [teeth that dig it to the bottom](https://media.istockphoto.com/id/154246219/photo/anchor-in-sand-under-the-water.jpg?s=1024×1024&w=is&k=20&c=sM6HVjldklTj0T4E6iP65kpPnsu9cxvWODvMFmld_wc=) and resist the pull of the ship tugging on it. If it was able to hold the ship by weight alone, they would just make them in a simple block shape since that would be easier to manufacture and smaller (and therefore cheaper). The fact that anchors are… [anchor-shaped](https://media.istockphoto.com/id/531062471/photo/anchor-in-desert.jpg?s=612×612&w=0&k=20&c=tsSrulrLGDP5jIBPzj0sptEnqfgF-hadxUpDNIOA5xE=) tells us that the shape (with big long shovel-bladed “teeth”) must be necessary for how the anchor works, not just the weight of it.
Assuming you know the basics of buoyancy. Weight in the ship displaces more water in the ship size. Adding the anchor weight to the ship means it displaces more water to stay afloat.
When the anchor is dropped, it sinks because it can only displace its own volume of water.
The other answers seem to cover your implied question of how the anchor works.
The anchor is more of a hook than a weight. It digs into the sea floor or grabs onto rocks. It’s very possible to drag the anchor if not properly set, and some locations are not suitable for anchoring because of this.
How do your tires keep your car from moving when parked? Same for the anchor resting on the sea floor.
The anchor is like a leash not a weight, it’s only heavy enough to drop through the water also there are lots of types of anchors
The same reason why you can brake on a bicycle by putting your feet down on the ground (not recommended) while riding. You don’t stop just because your feet are heavy
Here’s a way to think about it:
**A boat on the water simultaneously weighs 10,000 lbs (or whatever the boats dry weight is) and 0lbs**, which is what I tell everyone when I describe what it’s like to pilot a big boat. At rest, a single person can easily push even a pretty large yacht around when it’s floating on the water. This is why you see really tiny tugboats capable of towing really huge ships. This is important to understand why the anchor works.
If the water is calm enough, a moderately strong swimmer could move a large-ish boat. A large *breeze* can move the boat around.
So when a boat is *at rest*, the boat really only moves because of the current / wind. The boat itself, being buoyant, doesn’t really weigh much, while at the same time the weight of the boat is also resisting moving at all.
The anchor and chain weight only need to overcome ***the current***. It doesn’t actually need to hold the **weight of the boat** – it just needs to resist the forces that would normally just float the boat away.
This is why a 30lb anchor can hold a 10K lb boat in place. The weight of the anchor (and digging into the sea floor), plus the weight of the chain resting on the sea floor, is providing enough resistance to keep the boat from floating away.
Note also – a boat doesn’t stay *stationary* at Anchor. The anchor stays stationary, and the anchor chain has enough slack that the boat pivots around the anchor as the current and wind move the boat around. So when you anchor, you need to account for a the circular movement of the boat around the anchor pivot to make sure you don’t hit other boats.
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