Scope is often defined as the ratio of the length of deployed anchor rode to the depth of the water. Wrong! Scope calculations must be based on the vertical distance not from the sea bottom to the surface of the water, but from the sea bottom to the bow chock or roller where the anchor rode comes aboard. For example, if you let out 30 feet of anchor rode in six feet of water, you may think you have a 5:1 scope, but if your bow roller is four feet above the waterline, your scope is actually 3:1.
Scope is required to keep the pull on the anchor horizontal. The more upward pull on the anchor, the more likely it is to break free. Minimum scope for secure anchoring is 5:1. Seven-to-one is better where you have the room. A length of chain between the line and the anchor (at least 20 feet) also helps to keep the pull horizontal.