Anchor holding power depends upon scope, which is the ratio of rode length to depth, where depth is the distance from the deck to the bottom at high tide and might be considerably more than is shown on the depth meter. A scope of 10:1 is always sufficient. If the depth meter shows 12 feet and the deck is eight feet above the transponder, for example, no more than 200 ft of rode is needed. A scope of 7:1 gives 90% as much holding power and is adequate in most situations. A scope of 5:1 gives 75% as much holding and can be used in a crowded anchorage. At 4:1 scope the holding power is only two-thirds of maximum and at 3:1 it falls to half. because the rode lies closer to the bottom near the anchor. Chain has no stretch, however, and relies on the weight of its catenary to absorb shock. If wind or current pulls a chain rode nearly taut, a gust or wave can break the anchor free. It is therefore best to use as much scope with chain rode as with rope.
When alone in a spacious bay, the scope can be ideal. More often, scope is constrained by obstructions or other boats. It is sometimes best to drop anchor at a shallow point and allow the boat to swing into deeper water, provided that the anchor does not drag. Some handheld GPS devices have an anchor drag alarm that provides great peace of mind.