Editing Anchor (section)

==== 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}}