An anchor is an object that is used to attach a ship or boat to a specific point at the bottom of a body of water. The anchor prevents the vessel from drifting away by the action of winds and waves.
There are two primary classes of anchors: temporary and permanent. A permanent anchor, often called a mooring, is one that is rarely moved. It consists of a large mass, such as a rock, resting on the seabed, and it is usually not possible to hoist the permanent anchor aboard a vessel. By contrast, a temporary anchor is stowed aboard the vessel until it needs to be used. Almost all temporary anchors have metal flukes that hook onto rocks at the bottom or bury themselves in a soft seabed. A vessel may carry one or more temporary anchors, which may be of different designs and weights.
The earliest anchors were probably rocks, as many rock anchors have been found dating from at least the Bronze Age. A sea anchor (drift anchor, drift sock), often made of canvas and shaped like a cone or parachute, is not attached to the seafloor but floats just beneath the water’s surface and acts as a source of drag in the water. It pulls large amounts of water along as the boat moves, and it is used to counter the effects of high winds.
The anchor is attached to the vessel by means of what is called a rode, which may be a chain, cable, rope, or a combination of these. The hole in the hull through which the anchor rode passes is called a hawsepipe, because thick mooring lines are called hawsers.
An interesting element of anchor jargon is the term aweigh, which describes the anchor when it is hanging on the rope or cable, not resting on the bottom. This term is linked to the phrase to weigh anchor, meaning to lift the anchor from the seabed, allowing the ship or boat to move. An anchor is described as aweigh when it has been detached from the bottom and is being hauled up to be stowed.
The term aweigh should not be confused with under way, which describes a vessel that is not moored to a dock or anchored, whether or not it is moving through the water. Thus, a vessel can be described as under way (or underway) even when it is not moving.
Effects of wind and water
Wind and current generate steady forces on the hull, but the waves and motion of the vessel add the peak loads that the anchor and rode need to withstand. Reports from boats that have faced whole gale to cyclone winds at anchor say that it is not the winds that break anchor gear and upset anchors, but the accompanying wave action, which causes boats to pitch, surge, heave, and yaw. Surge is the worst of these motions, as the boat rides over the waves, alternately stretching and relaxing the anchor warp like a horizontal yo-yo. Surge is the motion forward and backward in the direction of boat travel, along the longitudinal axis.
Designs of temporary anchors
A modern temporary anchor usually consists of a central bar called the shank, and an armature with some form of flat surface (fluke or palm) to grip the bottom and a point to assist penetration of the bottom. The position at which the armature is attached to the shank is called the crown, and the shank is usually fitted with a ring or shackle to attach it to the cable. There are many variations and additions to these basic elements—for example, a whole class of anchors include a stock, such as the fisherman and fluke anchors.
The range of designs is wide, but there are actually trends in designs for modern anchors that allow them to be grouped as hook, plow (or plough), and fluke types, depending on the method by which they set.
- Hook designs use a relatively small fluke surface on a heavy, narrow arm to penetrate deeply into problematic bottoms such as rocky, heavy kelp or eel grass, coral, or hard sand. Two of the more common versions of this design are the fisherman and the grapnel.
- Plow designs are reminiscent of the antique farm plow, and are designed to bury themselves in the bottom as force is applied to them. They are considered good in most bottom conditions from soft mud to rock. North sea designs are actually a variation of a plow in how they work; they bury into the bottom using their shape.
- Fluke designs use large fluke surfaces to develop very large resistance to loads once they dig into the seabed. Although they have less ability to penetrate and are designed to reset rather than turn, their light weight makes them very popular.
In the case of sea anchors and drogues, they simply use the water itself as a weight, allowing the “anchor” to consist of merely a canvas and a rope that holds the textile in place.
In the past 20 years or so, many new anchor designs have appeared. Driven by the popularity of private pleasure boats, these anchors are usually designed for small- to medium-sized vessels, and are usually not appropriate for large ships.
A traditional design, the fisherman, also known as a kedge (not be confused with a modern-day light kedge anchor), is the most familiar among non-sailors. The design is a non-burying type, with one arm penetrating the seabed and the other standing proud. The anchor is ancient in design and has not changed substantially over time. It has a good reputation for use in rock, kelp, and grass, but is unlikely to be any more effective than a good modern design and its holding power-to-weight ratio is among the worst of all anchor types. Three-piece versions can be stowed quite compactly, and most versions include a folding stock so the anchor may be stowed flat on deck.
The primary weakness of the design is its ability to foul the cable over changing tides. Once fouled, the anchor is likely to drag. In comparison tests, the fisherman design developed much less resistance than other anchors of similar weight. It is difficult to bring aboard without scarring the topsides, and does not stow in a hawse pipe or over an anchor roller.
The most common commercial brand is the Danforth, which is sometimes used as a generic name for the class. The fluke style uses a stock at the crown to which two large flat surfaces are attached. The stock is hinged so the flukes can orient toward the bottom (and on some designs may be adjusted for an optimal angle depending on the bottom type.) The design is a burying variety, and once well set can develop an amazing amount of resistance. Its light weight and compact flat design make it easy to retrieve and relatively easy to store; some anchor rollers and hawse pipes can accommodate a fluke-style anchor. A few high-performance designs are available, such as the Fortress, which are lighter in weight for a given area and in tests have shown better than average results.
The fluke anchor has difficulty penetrating kelp and weed-covered bottoms, as well as rocky and particularly hard sand or clay bottoms. If there is much current or 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. Once set, the anchor tends to break out and reset when the direction of force changes dramatically, such as with the changing tide, and on some occasions it might not reset but instead drag.
A traditional design, the grapnel style is simple to design and build. It has a benefit in that no matter how it reaches the bottom one or more tines will be aimed to set. The design is a non-burying variety, with one or more tines digging in and the remainder above the seabed. In coral it is often able to set quickly by hooking into the structure, but may be more difficult to retrieve. A grapnel is often quite light, and may have additional uses as a tool to recover gear lost overboard; its weight also makes it relatively easy to bring aboard.
Grapnels rarely have enough fluke area to develop much hold in sand, clay, or mud. It is not unknown for the anchor to foul on its own rode, or to foul the tines with refuse from the bottom, preventing it from digging in. On the other hand, it is quite possible for this anchor to find such a good hook that, without a trip line, it is impossible to retrieve. The shape is generally not very compact, and is difficult to stow, although there are a few collapsing designs available.
Many manufacturers produce a plow-style design, named after its resemblance to a traditional agricultural plow (or more specifically two plowshares). These anchors are based on or direct copies of the original Coastal Quick Release (CQR), designed in 1933 by mathematician Geoffrey Ingram Taylor.
Based on a well-established history, plows are particularly popular with cruising sailors and other private boaters. They are generally good in all bottoms, but not exceptional in any. The CQR design has a hinged shank, allowing the anchor to turn with direction changes rather than breaking out, and it is arranged to force the point of the plow into the bottom if the anchor landed on its side. A more recent commercial design, the Delta, uses an unhinged shank and a plow with specific angles to develop slightly superior performance. Both can be stored in most regular anchor roller systems.
Owing to the use of lead or other dedicated tip-weight, the plough is heavier than average for the amount of resistance developed, and may take a slightly longer pull to set thoroughly. It cannot be stored in a hawse pipe.
The genuine CQR and Delta brands are now owned by Lewmar, although they have both been owned by several other companies during their lifetimes.
This claw-shaped anchor was designed by Peter Bruce from the Isle of Man in the 1970s . Bruce claims the invention to be based on a design used for anchoring floating oil derricks in the North Sea. The Bruce and its copies, known generically as “claws,” have become a popular option for smaller boaters. It was intended to address some of the problems of the only general-purpose option then available, the plow. Claw-types set quickly in most seabeds, and, although not an articulated design, they have the reputation of not breaking out with tide or wind changes, instead slowly turning in the bottom to align with the forces.
Claw types have difficulty penetrating weedy bottoms and grass. They offer a fairly low holding power to weight ratio and generally have to be over-sized to compete with other types. On the other hand they perform relatively well with low rode scopes and set fairly reliably. They cannot be used with hawse pipes.
Bruce Anchor Group no longer produces the genuine Bruce anchor, but other companies make reproductions.
In recent years, there has been something of a spurt in anchor design. Primarily designed to set very quickly, then generate high holding power, these anchors (mostly proprietary inventions still under patent) are finding homes with users of small- to medium-sized vessels.
- The German designed Bügel (or Wasi) has a sharp tip for penetrating weed, and features a roll-bar that allows the correct setting attitude to be achieved without the need for extra weight to be inserted into the tip.
- The Bulwagga is a unique design featuring three flukes instead of the usual two. It has performed well in tests by independent sources such as the American boating magazine Practical Sailor.
- The Spade is a French design which has proved successful since 1996. It features a demountable shank and the choice of galvanized steel, stainless steel, or aluminum construction, which means a lighter and more easily stowable anchor.
- The Rocna, designed in New Zealand, has been produced since 2004. It too features a sharp toe like the Bügel for penetrating weed and grass, sets quickly, and has a particularly large fluke area. Its roll-bar is also similar to that of the Bügel. The Rocna obtained the highest averaged holding power, according to SAIL magazine’s comparison testing in 2006.
Designs of permanent anchors
These are used where the vessel is permanently sited, for example in the case of lightvessels or channel marker buoys. The anchor needs to hold the vessel in all weathers, including the most severe storm, but only occasionally, or never, needs to be lifted, only for example if the vessel is to be towed into port for maintenance. An alternative to using an anchor under these circumstances may be to use a pile driven into the seabed.
Permanent anchors come in a wide range of types and have no standard form. 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 (e.g., an engine block). Modern moorings may be anchored by sand screws which look and act very much like over-sized screws drilled into the seabed, or by barbed metal beams pounded in (or even driven in with explosives) like pilings, or a variety of other non-mass means of getting a grip on the bottom. One method of building a mooring is to use three or more temporary anchors laid out with short lengths of chain attached to a swivel, so no matter which direction the vessel moves one or more anchors will be aligned to resist the force.
The mushroom anchor is suitable where the seabed is composed of silt or fine sand. 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. It was equipped with a 1.5 ton example.
It is shaped like an inverted mushroom, the head becoming buried in the silt. A counterweight is often provided at the other end of the shank to lay it down before it becomes buried.
A mushroom anchor will normally sink in the silt to the point where it has displaced its own weight in bottom material. These anchors are only suitable for a silt or mud bottom, since they rely upon suction and cohesion of the bottom material, which rocky or coarse sand bottoms lack. The holding power of this anchor is at best about twice its weight unless it becomes buried, when it can be as much as ten times its weight. They are available in sizes from about ten pounds up to several tons.
This is an anchor that relies solely on being a heavy weight. It is usually just a large block of concrete or stone at the end of the chain. 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. Consequently deadweight anchors are used where mushroom anchors are unsuitable, for example in rock, gravel or coarse sand. An advantage of a deadweight anchor over a mushroom is that if it does become dragged, then it continues to provide its original holding force. The disadvantage of using deadweight anchors in conditions where a mushroom anchor could be used is that it needs to be around ten times the weight of the equivalent mushroom anchor.
Screw anchors can be used to anchor permanent moorings, floating docks, fish farms, and so forth.
These anchors 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.
Weight for weight, screw anchors have a higher holding than other permanent designs, and so can be cheap and relatively easily installed, although may not be ideal in extremely soft mud.
The elements of anchoring gear include the anchor, the cable (also called a rode), the method of attaching the two together, the method of attaching the cable to the ship, charts, and a method of learning the depth of the water.
Charts are vital to good anchoring. 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. One can get by without referring to charts, but they are an important tool and a part of good anchoring gear, and a skilled mariner would not choose to anchor without them.
The depth of water is necessary for determining scope, which is the ratio of length of cable to the depth measured from the highest point (usually the anchor roller or bow chock) to the seabed. For example, if the water is 25 ft (8 m) deep, and the anchor roller is 3 ft (1 m) above the water, the scope is the ratio between the amount of cable let out and 28 ft (9 m). For this reason it is important to have a reliable and accurate method of measuring the depth of water.
The basic anchoring consists of determining the location, dropping the anchor, laying out the scope, setting the hook, and assessing where the vessel ends up. After using the chart to determine a desirable location, the crew needs to actually see what the situation is like; there may be other boats whose crew thought that would be a good spot, or weather conditions may be different from those expected, or even additional hazards not noted on the chart may make a planned location undesirable.
If the location is good, the location to drop the anchor should be approached from down wind or down current, whichever is stronger. As the chosen spot is approached, the vessel should be stopped or even beginning to drift back. The anchor should be lowered quickly but under control until it is on the bottom. The vessel should continue to drift back, and the cable should be veered out under control so it will be relatively straight.
Once the desired scope is laid out (a minimum of 8:1 for setting the anchor, and 5:1 for holding, though the preferred ratio is 10:1 for both setting, and holding power), the vessel should be gently forced astern, usually using the auxiliary motor but possibly by backing a sail. A hand on the anchor line may telegraph a series of jerks and jolts, indicating the anchor is dragging, or a smooth tension indicative of digging in. As the anchor begins to dig in and resist backward force, the engine may be throttled up to get a thorough set. If the anchor continues to drag, or sets after having dragged too far, it should be retrieved and moved back to the desired position (or another location chosen.)
With the anchor set in the correct location, everything should be reconsidered. Is the location protected, now and for the forecast weather? Is the bottom a suitable holding ground, and is the anchor the right one for this type of bottom? Is there enough depth, both now and at low tide? Especially at low tide but also at all tide states, is there enough room for the boat to swing? Will another vessel swing into us, or will we swing into another vessel, when the tide or wind changes?
Some other techniques have been developed to reduce swing, or to deal with heavy weather.
- Using an anchor weight, kellet or sentinel
- Forked moor
- Bow and Stern
- Bahamian moor
- Backing an anchor
A good anchorage offers protection from the current weather conditions, and will also offer protection from the expected weather. The anchorage should also be suitable for other purposes; for example, proximity to shore is beneficial if the crew plans to land.
Charts should indicate the type of bottom, and a sounding lead may be used to collect a sample from the bottom for analysis. Generally speaking, most anchors will hold well in sandy mud, mud and clay, or firm sand. Loose sand and soft mud are not desirable bottoms, especially soft mud which should be avoided if at all possible. Rock, coral, and shale prevent anchors from digging in, although some anchors are designed to hook into such a bottom. Grassy bottoms may be good holding, but only if the anchor can penetrate the foliage.
Depth and tides
If the anchorage is affected by tide, tide ranges, as well as the times of high and low water, should be known. Enough depth is needed so that low tide does not present obstacles to where the vessel might swing. This is also important when determining scope, which should be figured for high tide and not the current tide state.
If the anchorage is affected by tide, one should keep in mind that the swing range will be larger at low tide than at high tide. However, no matter where the vessel is anchored, the largest possible swing range should be considered, as well as what obstacles and hazards might be within that range. Other vessels’ swing ranges may overlap, presenting a further variable. Boats on permanent moorings, or shorter scope, may not swing as far as expected, or may swing either more rapidly or more slowly (all-chain cables tend to swing more slowly than all-rope or chain-and-rope cables.)
There are techniques of anchoring to limit the swing of a vessel if the anchorage has limited room.
Using an anchor weight, kellet or sentinel
Lowering a concentrated, heavy weight down the anchor line – rope or chain – directly in front of the bow to the seabed, behaves like a heavy chain rode and lowers the angle of pull on the anchor. 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. In light conditions, a kellet will reduce the swing of the vessel considerably. In heavier conditions these effects disappear as the rode becomes straightened and the weight ineffective.
Using two anchors set approximately 45° apart, or wider angles up to 90°, from the bow is a strong mooring for facing into strong winds. To set anchors in this way, first one anchor is set in the normal fashion. Then, taking in on the first cable as the boat is motored into the wind and letting slack while drifting back, a second anchor is set approximately a half-scope away from the first on a line perpendicular to the wind. After this second anchor is set, the scope on the first is taken up until the vessel is lying between the two anchors and the load is taken equally on each cable.
This moor also to some degree limits the range of a vessel’s swing to a narrower oval. Care should be taken that other vessels will not swing down on the boat due to the limited swing range.
Bow and stern
In the Bow and Stern technique, an anchor is set off each the bow and the stern, which can severely limit a vessel’s swing range and also align it to steady wind, current or wave conditions. One method of accomplishing this moor is to set a bow anchor normally, then drop back to the limit of the bow cable (or to double the desired scope, e.g. 8:1 if the eventual scope should be 4:1, 10:1 if the eventual scope should be 5:1, etc.) to lower a stern anchor. By taking up on the bow cable the stern anchor can be set. After both anchors are set, tension is taken up on both cables to limit the swing or to align the vessel.
Similar to the above, a Bahamian moor is used to sharply limit the swing range of a vessel, but allows it to swing to a current. One of the primary characteristics of this technique is the use of a swivel as follows: the first anchor is set normally, and the vessel drops back to the limit of anchor cable. A second anchor is attached to the end of the anchor cable, and is dropped and set. A swivel is attached to the middle of the anchor cable, and the vessel connected to that.
The vessel will now swing in the middle of two anchors, which is acceptable in strong reversing currents but a wind perpendicular to the current may break out the anchors as they are not aligned for this load.
Backing an anchor
Also known as Tandem anchoring, in this technique two anchors are deployed in line with each other, on the same rode. 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. It does not limit swinging range, and might not be suitable in some circumstances. There are complications and the technique requires careful preparation and a level of skill and experience above that required for a single anchor.
Kedging is a technique for moving or turning a ship by using a relatively light anchor known as a kedge.
In yachts, a kedge anchor is an anchor carried in addition to the main, or bower anchors, and usually stowed aft. Every yacht should carry at least two anchors – the main or bower anchor and a second lighter kedge anchor. 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 very narrow river or a deep pool in an otherwise shallow area.
For ships, a kedge may be dropped while a ship is underway, or carried out in a suitable direction by a tender or ship’s boat to enable the ship to be winched off if aground or swung into a particular heading, or even to be held steady against a tidal or other stream.
Historically, it was of particular relevance to sailing warships which used them to out-maneuver 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.
An anchor frequently appears on the flags and coats-of-arms of institutions involved with the sea, both naval and commercial, as well as of port cities and sea-coast regions and provinces in various countries.
There also exists in heraldry the “Anchored Cross,” or Mariner’s Cross, a stylized cross in the shape of an anchor. The symbol can be used to signify ‘fresh start’ or ‘hope’.
The Mariner’s Cross is also referred to as St. Clement’s Cross in reference to the way this saint was martyred (being tied to an anchor and thrown from a boat into the Black Sea in the year 102).
Anchored crosses are occasionally a feature of coats of arms in which context they are referred to by the heraldic terms anchry or ancre.