Scope is a ratio of the length of an anchor rode from the bit to the anchor shackle and the depth of the water under the bow of the boat measured from deck height. The anchor, shackle, rode, and bit are a few components of Ground Tackle used in anchoring a vessel.
Or, if you like formulas: S=L/D where L is the length of the anchor rode and D is depth under the bow.
What is the Correct Scope?
The "correct scope" depends on several variables, but there is no need to compute this exactly. Getting close is good enough in this case.
First, it might be best to explain why we want a certain ratio of scope and what will happen if the ratio is much too large or much too small.
Too small is much worse than too large when it comes to scope. Different types of anchors bite into the bottom in different ways, but all have the same property of digging in when they have pulled along at a low angle in relation to the bottom structure.
This dragging is what secures the anchor to the bottom. If the water is 60 feet (18 M) deep and the anchor rode is 120 feet (36 M) then the scope is 2:1 and far too small.
You see, when the boat drifts and pulls the anchor with this ratio of scope it will not drag smoothly and bite in. The result is the anchor being pulled from the bottom with each small wave and bouncing along leaving the vessel far from the intended position.
If the scope is too large, the anchor will bite or set into position but the vessel will most likely surge and drift as forces act on it. In this case, we will use the same water depth of 60 feet (18 M) but increase the length of the rode to 600 feet (180 M). This gives us a scope of 10:1 which is not inappropriate if winds or currents are very strong but is not the best ratio for general anchoring.
The scope that is best for keeping the anchor set and keeping tension on the anchor rode is around 7:1. If we plug our numbers into the formula a water depth of 60 feet (18 M) will require a rode of 420 feet (126 M).
A scope of 7:1 will not pull the anchor free but it will maintain tension for a safe and comfortable stay in the anchorage.
Areas with Strong Tidal Runs
If you do find yourself in an area with a strong tidal run, like you may find in some hurricane holes, it will be necessary to reset the anchor rode scope. Tides of less than three or four meters can come and go as long as the sailor takes this change into account when setting the anchor. In a big tidal run of ten or more meters, it's best to lay out a fore and aft anchor and be generous with the scope. Adjustments should always be made to prevent slack and avoid collision with other vessels or obstacles.
In areas of hard rock or coral, care should be taken with the first thirty feet of rode which should be abrasion resistant Kevlar jacketed line or chain. Chain provides the best protection but it can cause sharp jolts in rough conditions although, in light waves, the weight of the chain will buffer some of the movement. Jacketed anchor rode is generally better because it is lighter and easier to handle plus it offers some shock absorbing properties that could add years of life to the deck and tying fixtures of your boat.