Anchor
From Free net encyclopedia
- For alternate meanings see anchor (disambiguation)
An anchor is a heavy object, often made out of metal, that is used to attach ships to the bottom at a specific point. There are two primary classes of anchors—temporary and permanent. A permanent anchor is often called a mooring, and is rarely moved; it is quite possible the vessel cannot hoist it aboard but must hire a service to move or maintain it. A temporary anchor is usually carried by the vessel, and hoisted aboard whenever the vessel is under way.
An anchor works by resisting the movement force of the vessel which is attached to it. There are two primary ways to do this — via sheer mass, and by "hooking" into the seabed. It may seem logical to think wind and currents are the largest forces an anchor must overcome, but actually the vertical movement of waves develop the largest loads, and modern anchors are designed to use a combination of technique and shape to resist all these forces.
Image:Anchor1.png The kind of anchor you probably envision is a temporary anchor, the kind which might be carried aboard a ship or a boat. 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, the whole class of anchors which include a stock such as the fisherman and fluke anchors.
A permanent anchor, on the other hand, may come in a wide range of types and has no standard form. A slab of rock with an iron staple in it to attach a chain to serves very well, as does a Chevy long-block motor. 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.
An interesting element of anchor jargon is the term under weigh, which describes the anchor when it is hanging on the rope, not on the bottom; this is linked to the term to weigh anchor, meaning to lift the anchor from the sea bed, allowing the ship or boat to move. Usually an anchor is described as under weigh when it has been broken out of the bottom and is being hauled up to the boat because all the weight of the anchor and rode (a term for the chain linking the anchor and the ship) are lifted, and when lifted from the water it becomes stowed. Although the terms may be linked, under weigh should not be confused with under way, which describes a vessel which is moving through the water,
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Development
The earliest anchors were probably rocks and many rock anchors have been found dating from at least the Bronze Age. Many modern moorings still rely on a large rock as the primary element of their design. It simply works. However, using pure mass to resist the forces of a storm only works well as a permanent mooring; trying to move a large enough rock to another bay is nearly impossible.
A simple anchor using a pair of wood arms under a rock mass is a primitive anchor which is still in use today. The wood arms are pointed to penetrate the bottom, and the mass will overcome normal movement forces. Together they comprise what may have been the first successful attempts to hook into the seabed and use the strength there to prevent a vessel from moving. Almost all future anchor developments combine these two elements—a penetrating point and a reasonable mass.
In the western world the vast majority of anchors worked on the concept of the grappling hook—multiple points on arms such that at least one will be aimed toward the bottom. Suddenly the concept of the stock, a bar placed perpendicular to the hooking arm at the other end of the shank which would roll the anchor over so the point would penetrate the bottom, was developed and within a single century became the standard anchor type.
In the East, however, another model of anchor had been known for some time which also used a stock, but with the stock located at the crown along with the arm. This successful model is still built today in virtually unchanged form. It also informed such modern designs as the US Navy's stockless Mark IV and the fluke-style anchor.
Designs of temporary anchors
The range of designs is wide, but there are actually trends in designs for modern anchors which allow them to be classed as hook, plow, 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, and 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 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. See modern designs.
Fisherman
Image:Fisherman2-sm.jpg
A traditional design, the fisherman, also known as a kedge, is familiar to people who have never used an anchor. The design is a non-burying type, with one arm penetrating the seabed and the other standing proud. The anchor is popular as the ultimate storm anchor, and has a good reputation for use in rock, hard bottoms, and kelp or eel grass covered bottoms. The three piece versions can be stowed quite compactly, and most versions include a folding stock so the anchor may be stowed flat on deck.
Image:Walraversijde28.jpg
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.
Fluke
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 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.
Grapnel
Image:Grapnel-sm.jpg 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. 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.
North sea
Image:Anchor Bruce.jpg Designed originally for anchoring floating oil derricks in the North Sea, this versatile design has become a popular option for smaller boaters as well. The burying design acts similarly to a large scoop, and is known for the speed with which it digs in. Although not an articulated design, it has the reputation of not breaking out with tide or wind changes, instead slowly turning in the bottom to align with the force. Some versions of the design, such as the Bruce, are reputed to be easy to retrieve once broken out of the bottom, and some anchor rollers can accommodate their shank.
North sea designs may have difficulty penetrating weedy bottoms, rock, and coral. They can be particularly difficult to break out. Although they can be got aboard without scarring the topsides, they take up an inordinate amount of locker space. They cannot be used with hawse pipes.
Plow
Many manufacturers produce a plow-style design, all based on or direct copies of the original CQR (a Scottish design dating 1933). Owing to a now 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 also arranged to force the point of the plow into the bottom if the anchor lands on its side. Another more recent commercial design, the Delta uses an unhinged shank and a plow with specific angles to develop very similar performance. Both can be stored in most regular anchor roller systems.
Owing to the use of lead or other dedicated tip-weight, the plow is heavier than average for the amount of resistance developed, and may take slightly longer pull to set thoroughly. It cannot be stored in a hawse pipe.
Modern designs
Image:Anchor Rocna.jpg In recent years there has been something of a spurt in anchor design. Primarily designed to set very quickly, then generate superior 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 has a sharp tip for penetrating weed, and features a roll-bar which orients the anchor to the correct attitude on the seabed
- The Bulwagga is a unique design featuring three flukes instead of the regular two. It has performed well in tests by independent sources such as American boating magazine Practical Sailor. Manufacturer's website
- The Spade is a French design particularly popular with sailors. Although relatively expensive, it performs well, and features a demountable shank and optional aluminium construction, which means a lighter and more easily stowable anchor. Manufacturer's website
- The New Zealand designed Rocna is a new anchor gaining popularity amongst cruisers. It too features a sharp toe for penetrating weed and grass, and has a particularly large fluke area. Its roll-bar is similar to that of the Bügel, and means the correct setting attitude is achieved without the need for extra weight to be inserted into the tip (an inefficiency common in other anchor types). Manufacturer's website
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.
Mushroom
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[1]. They are available in sizes from about 10 lb up to several tons.
Deadweight
This is an anchor which 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 equal to 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
Screw anchors can be used to anchor permanent moorings, floating docks, fish farms, etc.
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.
Anchoring techniques
Heaving an anchor over the side is not good enough. There are several elements to anchor gear to be considered, and there are techniques to ensure a good set. This article can discuss some of this information, but it is by no means a treatise for safe anchoring.
Anchoring gear
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 25ft (8m) deep, and the anchor roller is 3ft (1m) above the water, the scope is the ratio between the amount of cable let out and 28ft (9m). For this reason it is important to have a reliable and accurate method of measuring the depth of water.
A cable or rode is the rope, chain, or combination thereof used to connect the anchor to the vessel. Neither rope nor chain is fundamentally superior as a cable or there would not be continued argument over the issue; each has its strengths and its weaknesses and it is not the purpose of this article to address these.
Anchoring
The four primary questions to be considered before actually anchoring:
- Is the anchorage protected?
- Is the seabed good holding ground?
- What is the depth, tidal range, and the current tide state?
- Is there enough room?
Is the anchorage protected?
A good anchorage offers protection from the current weather conditions, and will also offer protection from the expected weather. You should also consider if the anchorage will be suitable for other purposes, for example can you get safely to shore in your dinghy if that is one of your goals. And keep in mind comfort; a rolly harbor is no fun.
Is the seabed good holding ground?
You should have charts to indicate the kind of bottom, as well as a tool such as a sounding lead to collect a sample from the bottom. 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, and 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 bottom.
What is the depth, tidal range, and the current tide state?
If your anchorage is affected by tide, you need to know the tide range and the times of high and low water. You need enough depth for your vessel throughout the range it might swing, at low tide, not just where you drop the anchor. This is also important when determining scope, which should be figured for high tide and not the current tide state.
Is there enough room?
If your anchorage is affected by tide, you should keep in mind that the swing range will be larger at low tide than at high tide. However, no matter where you anchor you need to consider what the larges possible swing range will be, and what obstacles and hazards might be within that range. Keep in mind that other vessels in the anchorage may have a swing range which can overlap yours. Boats on permanent moorings, or shorter scope, may not swing as far as you expect them to, or may swing either more rapidly or more slowly than your vessel (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.
Methods
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 figuring out on the chart where a desirable location would be, the vessel need to actually see what the situation is like; there may be other boats who thought that would be a good spot, or weather conditions are different than expected, or even additional hazards not noted on the chart which 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 to 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.
Forked moor
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
Not to be mistaken with the Bahamian moor, below.
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.
Bahamian moor
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 anchors, in this technique two anchors are shackled to a single cable running crown-to-shank. With the leading anchor holding the cable down and the tension between the anchors taking load off, this technique can develop great holding power and has been used in "ultimate storm" circumstances. It does not limit swinging range, and might not be appropriate for crowded anchorages.
Kedging
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 one or more anchors 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-manoeuvre 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.
References
- Edwards, Fred; Sailing as a Second Language: An illustrated dictionary, 1988 Highmark Publishing; ISBN 0-87742-965-0
- Hinz, Earl R.; The Complete Book of Anchoring and Mooring, Rev. 2d ed., 1986, 1994, 2001 Cornell Maritime Press; ISBN 0-87033-539-1
- Hiscock, Eric C.; Cruising Under Sail, second edition, 1965 Oxford University Press; ISBN 0-19-217522-X
- Pardey, Lin and Larry; The Capable Cruiser,; 1995 Pardey Boooks/Paradise Cay Publications; ISBN 0-9646036-2-4
- Rousmaniere, John; The Annapolis Book of Seamanship, 1983, 1989 Simon and Schuster; ISBN 0-671-67447-1
- Smith, Everrett; Cruising World's Guide to Seamanship: Hold me tight, 1992 New York Times Sports/Leisure Magazines
External links
- North Star Marine Supplies- Popular online anchoring store.
- Inamar recommendations for safe moorings
- Lightship anchors
- A Process of Evolution — An essay on boat anchors by New Zealand boatbuilder, offshore cruiser, & consultant Peter Smith
- the titanic task that put the town on the map
- titanics centre anchor.
- all about the anchor some good photos
- history of nethertonda:Anker (søfart)
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