An anchor is an object — usually made from metal — that is heavy and used to stop a ship moving while at water. A mushroom anchor is a specific type that is usually used in areas of water that have a fine sand seabed. As the name suggests, a mushroom anchor is formed in the shape of a mushroom — a design that allows the head to become buried in the sand or silt and hence anchor the ship. A mushroom anchor is a permanent anchor — it is rarely moved and provides a mooring for the boat.
The mushroom anchor was first used in the 1800s on a boat named Pharos. Pharos was an 82-ton boat and the anchor weighed 1.5 tons. During this time the boat was used as a temporary lighthouse while the real one was being built. Since then the anchors have been used on a number of different types of boats and range from something a person could lift to several tons.
In order for a mushroom anchor to become buried within the sand a counter weight must be used. This is placed to force the shank to lie down before it is buried. One of the benefits of a mushroom anchor is that it will usually continue to a sink into the sand until it has moved the same mass as itself. This means that it can displace a large amount of material and hence provide a high level of holding power for the ship.
One of the reasons why a mushroom anchor can only be used as a permanent anchor is that it takes a relatively long length of time for it to become fully embedded. For this reason it is usually first deployed during a calm part of the year. A disadvantage of mushroom anchors is that they can only be used where there is sand or silt covering the seabed — they provide little holding power on rocky surfaces.
A mushroom anchor is just one type of permanent anchor — there are several others which can be used in different situations. For example, the dead weight anchor is much simpler in design and relies on being very heavy. The advantage of this is that it can be used on any type of seabed. A screw anchor screws into the seabed and provides a large amount of holding power. The downside is that they require access to the seabed for installation and hence can only be used in areas that become exposed during lower tides unless a diver is available.