Anchor Scope

Anchoring is a pretty simple concept. The boat is floating in the water and will float away if not tied down to something. The anchor is a fancy hook that grabs onto the sea floor and you are tied to the anchor, thus tied to the sea floor! 

If only it were so simple. 

There are many different styles of anchors and many different ways to attach the boat to the anchor, but if you comb through all of this information, you will find one common characteristic amongst the variety: Scope. 

Scope is the ratio of line connecting your boat to the anchor with respect to the depth of the water. If you have 10 feet of line and 10 feet of water, you will have 10:10 or 1:1 scope.
If you have 20 feet of line and 10 feet of water, you will have 20:10 or 2:1 scope.
if you have 30 feet of line and 10 feet of water, you will have 30:10 or 3:1 scope.

You get the picture. 

Now, as your scope goes increasing, the angle from the anchor's shank to the sea floor goes decreasing! Anchors are hooks and they are designed to hook into the sea floor when they are slid along the sea floor at an angle. When you pull straight up on them, it is because you are trying to pull them out of the bottom so that you can continue sailing. Therefore, anchors are also designed to be easily retrieved and will pull out if they are pulled upwards. 

With a 1:1 scope, the anchor is dangling vertically, and obviously won't hold the boat to the bottom very well.  with a 10:1 scope (so 100 feet of line for 10 feet of water), an anchor will hold with 100% of its rated holding capacity. So, when you are sold an anchor and told that it will hold the weight of your boat pulling on it, this is only when you have enough scope out, and the scope they calculated it with is 10:1. This means that if you really need your anchor to hold well, you need to let out enough anchor line to give it the needed scope.