Sailing ABC by Marianne Wheeler
A is for Anchor – Anchoring, VI
Anchors hold a boat best when they are allowed to dig in and stay flat on the bottom while the rode runs far back at a flat angle of pull to the bow of the boat. The longer the rode, the more horizontal it can tug on the anchor keeping it down and buried. A short rode will make a steep angle of pull to the bow and tug on the anchor vertically, pulling it out and letting it drag.
The angle at which the boat pulls on the anchor is determined by scope, the ratio of the length of rode let out to the distance from the boat’s bow to the seabed. Usually, a ratio of 7:1 is recommended for anchoring overnight in normal conditions.
To figure a 7:1 scope, add the depth of the water to the height of the bow above the water and multiply by seven. Estimate the scope including the tidal differences for the time at the anchorage.
4:1 scope may be enough for a short stop if the wind keeps down.
With all chain rode a 5:1 scope is sufficient for anchoring overnight in normal conditions.
My friend Salty Clay says it’s a good idea to have more scope than not enough. In this case, more is better. When in doubt, pay it out and don’t be caught in a crowded cove with your anchor up.