A permanent mooring must remain secure for long periods while unattended, occasionally under adverse conditions. For peace of mind, it should be properly sized for the job, and determining the minimum size has a lot to do with the conditions under which the boat is moored, the amount of fetch for waves to build up, and whether your mooring is a temporary one, designed for overnight use in fair weather, or a mooring capable of riding out a hurricane. Below are the basic components.
Several types are in common use, and we’ll review them in order of their holding power, from the wimpiest to the most tenacious:
Concrete Blocks: Many boats use 50-gallon drums filled with cement, concrete blocks, auto engine blocks and other types of dead weight. This type provides the least holding power, working on the principle of sheer weight, but is reliable if pulled out of the bottom. If they drag, they will resist motion with a constant amount of force. Note that concrete loses over half of its weight when submerged in water, so a mooring designed to withstand a 500lb. pull will need 1,000lb. of concrete.
Mushroom Anchor: the most common type of mooring anchor is the mushroom, which, under ideal conditions, with the right kind of bottom, can dig in, create suction and develop good holding power. Mushroom anchors work best in a silt or mud bottom, and are not as effective in rocks or coarse sand. If a mushroom gets pulled out of the bottom, it is less likely to reset itself completely, and will merely skip along across the bottom. A weight of 5-10 times boat length is a good rule of thumb, as a bare minimum. The heavier the better, as long as you don’t have to move it.
Pyramid Anchor: The cast-iron Dor-Mor pyramid anchor is a superior alternative to the mushroom. Its smaller size, concentrated weight and pyramid shape allows it to embed itself more rapidly, and its holding power (at a scope of 3:1) is up to about ten times its weight. Recommended by Practical Sailor/Powerboat Reports in 2009.
Helical Screw: while the above types rely for holding power on sheer weight or a combination of weight and embedding themselves in the bottom, the helical anchor is screwed into the seabed, usually by a barge-mounted hydraulic device. Helical screws have long, high-tensile steel shafts (8' length is common) with large screw threads (10" to 14" diameter) on the bottom and an attachment eye at the top. These professionally-installed anchors, originating in the offshore oil industry, have gained popularity with recreational boaters since the 1990s, and have the most extreme holding power in relation to their weight.