At some point in your boating life, you will want to stay put in one place. Whether relaxing in the sun, swimming, fishing or enjoying a quiet, peaceful lunch, you will have more fun if you can set anchor. Understanding the dynamics anchor line requirements in relation to the size of your boat and the depth of the water is crucial.
When you set anchor you must have enough line in the water so that it forms a 45-degree angle between the dropped anchor and the bow of your boat. This is "scope." Proper scope keeps the pull on the anchor horizontal instead of vertical. If the boat pulls "up" on the anchor when it moves, it will detach the anchor from the bottom and loosen its grip. A good rule of thumb to get the right scope is 7 to 10 feet of anchor line (known as rode) for every foot of water. The difference depends on surrounding conditions, strength of the current and weather.
Proper anchor rode ends with a section of chain. The chain at the end provides extra weight to the anchor and helps hold it down. It also keeps jagged bottom rocks, or debris from cutting into the anchor rode and possibly losing your anchor. The rule for length of anchor chain is ½ foot for every foot of boat length. Therefore, a 22-foot boat needs an 11-foot anchor chain.
Both rode line and chain come in several sizes. The thickness depends on the length of the boat in this case as well. There is a minor difference between the thicknesses for sailboats vs. power, but the main consideration is length of the boat. The proper thickness will not make a difference when it comes to setting the anchor, but it will make a big difference in durability. A line too light will snap under the pressure of a heavy ship. A 10-foot boat needs an anchor line and chain 3/16" thick. However, that thin line would snap under the strain of a 26-foot cruiser's weight. A 26-foot long boat of any style needs a line at least 3/8" thick, and a ½-inch thick chain.
All anchors come in a variety of sizes to fit different boats, but the two things to consider when thinking anchor size are use and boat. The size of the boat matters when it comes to fitting the right anchor to it. A 10-foot boat will need a lighter weight anchor than a 60-foot boat.
The next thing to consider is use. The three most common uses for anchors are lunch hook, working anchor and storm anchor. For a lunch hook you want to relax but can still pay attention to whether or not you are drifting. A working hook secures the boat so you can go about fishing or working on the boat without worry that it will slide back into the current. A storm anchor is self-explanatory. This is a big, heavy anchor to keep you secure in serious weather conditions.
While conditions and uses make a difference in anchor selection, if you want one anchor to handle all purposes err on the side of plenty. With a grapnel-style anchor, a small 10-foot fishing boat needs only a 1.5 pound anchor while a 26-foot cabin cruiser requires a 13-pound grapnel style anchor.