Anchor

An anchor is a device, normally made of metal, used to secure a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current.

The ship seeks a location that is sufficiently protected; has suitable holding ground, enough depth at low tide and enough room for the boat to swing.

[8] If the weight is suspended off the seabed it acts as a spring or shock absorber to dampen the sudden actions that are normally transmitted to the anchor and can cause it to dislodge and drag.

With the foremost anchor reducing the load on the aft-most, this technique can develop great holding power and may be appropriate in "ultimate storm" circumstances.

[clarification needed] It is used occasionally when it is necessary to limit the turning circle as the yacht swings when it is anchored, such as in a narrow river or a deep pool in an otherwise shallow area.

Historically, it was of particular relevance to sailing warships that used them to outmaneuver opponents when the wind had dropped but might be used by any vessel in confined, shoal water to place it in a more desirable position, provided she had enough manpower.

[10][11] When it is necessary to moor a ship or floating platform with precise positioning and alignment, such as when drilling the seabed, for some types of salvage work, and for some types of diving operation, several anchors are set in a pattern which allows the vessel to be positioned by shortening and lengthening the scope of the anchors, and adjusting the tension on the rodes.

However, using pure weight to resist the forces of a storm works well only as a permanent mooring; a large enough rock would be nearly impossible to move to a new location.

In the mid-19th century, numerous modifications were attempted to alleviate these problems, as well as improve holding power, including one-armed mooring anchors.

Although their holding-power-to-weight ratio is significantly lower than admiralty pattern anchors, their ease of handling and stowage aboard large ships led to almost universal adoption.

While there are numerous variations, stockless anchors consist of a set of heavy flukes connected by a pivot or ball and socket joint to a shank.

Cast into the crown of the anchor is a set of tripping palms, projections that drag on the bottom, forcing the main flukes to dig in.

Its weight also makes it relatively easy to move and carry, however its shape is generally not compact and it may be awkward to stow unless a collapsing model is used.

If there is much current, or if the vessel is moving while dropping the anchor, it may "kite" or "skate" over the bottom due to the large fluke area acting as a sail or wing.

[25] The Fortress is an American aluminum alloy Danforth variant that can be disassembled for storage and it features an adjustable 32° and 45° shank/fluke angle to improve holding capability in common sea bottoms such as hard sand and soft mud.

[29] Bruce gained his early reputation from the production of large-scale commercial anchors for ships and fixed installations such as oil rigs.

The Bruce and its copies, known generically as "claw type anchors", have been adopted on smaller boats (partly because they stow easily on a bow roller) but they are most effective in larger sizes.

A slab of rock with an iron staple in it to attach a chain to would serve the purpose, as would any dense object of appropriate weight (for instance, an engine block).

Modern moorings may be anchored by augers, which look and act like oversized screws drilled into the seabed, or by barbed metal beams pounded in (or even driven in with explosives) like pilings, or by a variety of other non-mass means of getting a grip on the bottom.

It was invented by Robert Stevenson, for use by an 82-ton converted fishing boat, Pharos, which was used as a lightvessel between 1807 and 1810 near to Bell Rock whilst the lighthouse was being constructed.

A mushroom anchor normally sinks in the silt to the point where it has displaced its own weight in bottom material, thus greatly increasing its holding power.

Its holding power is defined by its weight underwater (i.e., taking its buoyancy into account) regardless of the type of seabed, although suction can increase this if it becomes buried.

These anchors, which have one or more slightly pitched self-drilling threads, must be screwed into the seabed with the use of a tool, so require access to the bottom, either at low tide or by use of a diver.

[44] Knowing the location of potential dangers, as well as being useful in estimating the effects of weather and tide in the anchorage, is essential in choosing a good place to drop the hook.

The weight of the chain also helps keep the direction of pull on the anchor closer to horizontal, which improves holding, and absorbs part of snubbing loads.

Where weight is not an issue, a heavier chain provides better holding by forming a catenary curve through the water and resting as much of its length on the bottom as would not be lifted by tension of the mooring load.

[clarification needed] Some skippers prefer an all chain warp for greater security on coral or sharp edged rock bottoms.

Generally, the rode should be between 5 and 10 times the depth to the seabed, giving a scope of 5:1 or 10:1; the larger the number, the shallower the angle is between the cable and the seafloor, and the less upwards force is acting on the anchor.

[clarification needed] A major disadvantage of the concept of scope is that it does not take into account the fact that a chain is forming a catenary when hanging between two points (i.e., bow roller and the point where the chain hits the seabed), and thus is a non-linear curve (in fact, a cosh() function), whereas scope is a linear function.

[52][53][54] An anchor frequently appears on the flags and coats of arms of institutions involved with the sea, as well as of port cities and seacoast regions and provinces in various countries.