Editing Anchor (section)

== Anchoring gear ==
[[File:Thomas Brunton.jpg|thumb|upright|Thomas Brunton invented and patented in 1813 studded-link marine chain cable, which replaced hempen cables and is still in use.]]
[[File:AS HMAS Canberra 1.jpg|thumb|upright|Naval anchor incorporated into {{HMAS|Canberra|1927|6}} memorial, [[Canberra]], [[Australia]]]]
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.

Vessels may carry a number of anchors: ''bower anchors''  are the main anchors used by a vessel and normally carried at the bow of the vessel.  A ''kedge anchor'' is a light anchor used for [[Warping (sailing)|warping an anchor]], also known as ''kedging'', or more commonly on yachts for [[Mooring (watercraft)|mooring]] quickly or in benign conditions.  A ''stream anchor'', which is usually heavier than a ''kedge anchor'', can be used for kedging or warping in addition to temporary mooring and restraining stern movement in tidal conditions or in waters where vessel movement needs to be restricted, such as rivers and channels.<ref>[http://www.wordnik.com/words/stream-anchor Stream Anchor], wordnik.com</ref>

Charts are vital to good anchoring.<ref>{{Cite book |last1=Jollands |first1=Simon |url=https://books.google.com/books?id=ypqeBgAAQBAJ&pg=PA74 |title=Safe Skipper: A practical guide to managing risk at sea |last2=Holmes |first2=Rupert |date=2015-03-12 |publisher=Bloomsbury Publishing |isbn=978-1-4729-1548-1 |pages=74 |language=en}}</ref> 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.

=== Anchor rode ===
The anchor rode (or "cable" or "warp") that connects the anchor to the vessel is usually made up of chain, rope, or a combination of those.<ref>{{Cite web|title=The Anchor Rode – Making the Connection|url=http://cruising.coastalboating.net/Seamanship/Anchoring/Rode/Anchor_Rode.html|access-date=2020-12-26|website=cruising.coastalboating.net}}</ref> Large ships use only chain rode. Smaller craft might use a rope/chain combination or an all chain rode. All rodes should have some chain; chain is heavy but it resists abrasion from coral, sharp rocks, or shellfish beds, whereas a rope warp is susceptible to abrasion and can fail in a short time when stretched against an abrasive surface. 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. Any changes to the tension are accommodated by additional chain being lifted or settling on the bottom, and this absorbs shock loads until the chain is straight, at which point the full load is taken by the anchor. Additional dissipation of shock loads can be achieved by fitting a snubber between the chain and a bollard or cleat on deck. This also reduces shock loads on the deck fittings, and the vessel usually lies more comfortably and quietly.

Being strong and elastic, nylon rope is the most suitable as an anchor rode.<ref>{{Cite book |url=https://books.google.com/books?id=szXN5UvNSwkC&dq=nylon+rope+is+the+most+suitable+as+an+anchor+rode&pg=RA9-PA65 |title=Cruising World |date= January 2001|language=en}}</ref> [[Polyester]] (terylene) is stronger but less elastic than nylon. Both materials sink, so they avoid fouling other craft in crowded anchorages and do not absorb much water. Neither breaks down quickly in sunlight. Elasticity helps absorb shock loading, but causes faster abrasive wear when the rope stretches over an abrasive surface, like a coral bottom or a poorly designed chock.  [[Polypropylene]] ("polyprop") is not suited to rodes because it floats and is much weaker than nylon, being barely stronger than natural fibres.<ref>{{Cite book |url=https://books.google.com/books?id=3mPt9q9vH0IC&dq=Polypropylene+is+not+suited+to+rodes&pg=PA135 |title=MotorBoating |date= March 2005|language=en}}</ref> Some grades of polypropylene break down in sunlight and become hard, weak, and unpleasant to handle. Natural fibres such as [[manila hemp|manila]] or hemp are still used in developing nations but absorb a lot of water, are relatively weak, and rot, although they do give good handling grip and are often relatively cheap. Ropes that have little or no elasticity are not suitable as anchor rodes. Elasticity is partly a function of the fibre material and partly of the rope structure.

All anchors should have chain at least equal to the boat's length.{{clarify|explain why|date=August 2021}} Some skippers prefer an all chain warp for greater security on coral or sharp edged rock bottoms. The chain should be shackled to the warp through a steel eye or spliced to the chain using a chain splice. The shackle pin should be securely wired or moused. Either galvanized or stainless steel is suitable for eyes and shackles, galvanized steel being the stronger of the two.{{cn|date=August 2021}} Some skippers prefer to add a [[swivel]]<ref>{{Cite web|title=To Swivel or to Twist, That is The Question|url=http://features.coastalboating.net/Editorials/swivels/index.html|access-date=2020-12-26|website=features.coastalboating.net}}</ref> to the rode. There is a school of thought that says these should not be connected to the anchor itself,{{clarify|Why should the swivel be in the chain?|date=August 2021}} but should be somewhere in the chain. However, most skippers connect the swivel directly to the anchor.{{cn|date=August 2021}}

==== Scope ====
Scope is the ratio of length of the rode to the depth of the water measured from the highest point (usually the anchor roller or bow chock) to the seabed, making allowance for the highest expected tide. When making this ratio large enough, one can ensure that the pull on the anchor is as horizontal as possible. This will make it unlikely for the anchor to break out of the bottom and drag, if it was properly embedded in the seabed to begin with. When deploying chain, a large enough scope leads to a load that is entirely horizontal, whilst an anchor rode made only of rope will never achieve a strictly horizontal pull.

In moderate conditions, the ratio of rode to water depth should be 4:1 – where there is sufficient swing-room, a greater scope is always better. In rougher conditions it should be up to twice this with the extra length giving more stretch and a smaller angle to the bottom to resist the anchor breaking out.<ref>Safety in Small Craft. Ch 2. Royal NZ Coastguard Federation. Mike Scanlan. Auckland. 1994</ref>  For example, if the water is {{convert|8|m|ft|0}} deep, and the anchor roller is {{convert|1|m|ft|0|abbr=on}} above the water, then the 'depth' is 9 meters (~30 feet). The amount of rode to let out in moderate conditions is thus 36 meters (120 feet). (For this reason, it is important to have a reliable and accurate method of measuring the depth of water.)

When using a rope rode, there is a simple way to estimate the scope:<ref>{{Cite web|title=A Simple Way to Check Scope|url=http://cruising.coastalboating.net/Seamanship/Anchoring/Scope.html|access-date=2020-12-26|website=cruising.coastalboating.net}}</ref> The ratio of bow height of the rode to length of rode above the water while lying back hard on the anchor is the same or less than the scope ratio. The basis for this is simple geometry ([[Intercept Theorem]]): The ratio between two sides of a triangle stays the same regardless of the size of the triangle as long as the angles do not change.

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. A 10:1 scope gives the greatest holding power, but also allows for much more drifting about due to the longer amount of cable paid out. Anchoring with sufficient scope and/or heavy chain rode brings the direction of strain close to parallel with the seabed. This is particularly important for light, modern anchors designed to bury in the bottom, where scopes of 5:1 to 7:1 are common, whereas heavy anchors and moorings can use a scope of 3:1, or less. Some modern anchors, such as the Ultra holds with a scope of 3:1;{{cn|date=August 2021}} but, unless the [[anchorage (shipping)|anchorage]] is crowded, a longer scope always reduces shock stresses.{{clarify|stresses in what? |date=August 2021}}

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. As a consequence, in deep water the scope needed will be less, whilst in very shallow water the scope must be chosen much larger to achieve the same pulling angle at the anchor shank. For this reason, the British Admiralty does not use a linear scope formula, but a square root formula instead.<ref>Admiralty Manual Of Seamanship, Vol 1, 1964.</ref>

A couple of online calculators exist to work out the amount of chain and rope needed to achieve a (possibly nearly) horizontal pull at the anchor shank, and the associated anchor load.<ref>{{cite web |url= http://svamanda.dk/anchor/intro |title=Anchor Rode Calculator|author=Bjarne Knudsen}}</ref><ref>{{cite web |url= http://anchorchaincalculator.com |title=Anchor Chain Calculator|author=Mathias Wagner}}</ref><ref>{{cite web |url=http://alain.fraysse.free.fr/sail/rode/rode.htm |title=Tuning an Anchor Rode |author=Alain Fraysse}}</ref>
{{expand section|explain the function of scope|date=August 2021}}