Permanent moorings use large masses (commonly a block or slab of concrete) resting on the seabed. Semi-permanent mooring anchors (such as mushroom anchors) and large ship's anchors derive a significant portion of their holding power from their mass, while also hooking or embedding in the bottom. Modern anchors for smaller vessels have metal flukes which hook on to rocks on the bottom or bury themselves in soft seabed. The vessel is attached to the anchor by the rode (also called a cable or a warp). It can be made of rope, chain or a combination of rope and chain. The ratio of the length of rode to the water depth is known as the scope (see below). Holding ground is the area of sea floor which holds an anchor, and thus the attached ship or boat. Different types of anchor are designed to hold in different types of holding ground. Some bottom materials hold better than others for instance, hard sand holds well, shell very poorly. Holding ground may be fouled with obstacles. An anchorage location may be chosen for its holding ground. History Evolution of the anchor Īn Admiralty Pattern anchor when deployed on the seafloor the stock forces one of its flukes into the bottom In poor holding ground, only the weight of an anchor matters in good holding ground, it is able to dig in, and the holding power can be significantly higher. The Admiralty Pattern anchor, or simply "Admiralty", also known as a "Fisherman", consists of a central shank with a ring or shackle for attaching the rode (the rope, chain, or cable connecting the ship and the anchor). At the other end of the shank there are two arms, carrying the flukes, while the stock is mounted to the shackle end, at ninety degrees to the arms. When the anchor lands on the bottom, it will generally fall over with the arms parallel to the seabed. As a strain comes onto the rope, the stock will dig into the bottom, canting the anchor until one of the flukes catches and digs into the bottom.
The Admiralty Anchor is an entirely independent reinvention of a classical design, as seen in one of the Nemi ship anchors. This basic design remained unchanged for centuries, with the most significant changes being to the overall proportions, and a move from stocks made of wood to iron stocks in the late 1830s and early 1840s.
Since one fluke always protrudes up from the set anchor, there is a great tendency of the rode to foul the anchor as the vessel swings due to wind or current shifts.
When this happens, the anchor may be pulled out of the bottom, and in some cases may need to be hauled up to be re-set. In the mid-19th century, numerous modifications were attempted to alleviate these problems, as well as improve holding power, including one-armed mooring anchors. The most successful of these patent anchors, the Trotman Anchor, introduced a pivot at the centre of the crown where the arms join the shank, allowing the "idle" upper arm to fold against the shank.
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