Anchor Scope Ratios Explained: 5:1, 7:1 and 10:1
Why scope ratio decides your holding, how depth, bow height and tide feed into it, and when to choose 5:1, 7:1 or a storm-ready 10:1.
What the scope ratio really controls
Scope is the length of rode you deploy divided by the height from the seabed to your bow roller. Its job is to control the angle at which the rode meets the anchor. At a low ratio the rode rises steeply and tends to lift the shank, so the anchor can trip out. At a higher ratio the rode lies almost flat along the bottom, the pull becomes horizontal, and the flukes bury deeper under load. That angle, not the raw length of chain, is what keeps you where you dropped.
Measuring the total height, not just the depth
A frequent mistake is to base scope on the depth sounder alone. The rode does not start at the surface; it starts at the bow roller, which sits some feet above the water. It also has to cope with the tide still to come in. This tool adds the water depth, the bow height and the expected tide rise into one total height, then multiplies by your ratio. Feed it a 20 ft depth, a 4 ft bow and a 3 ft rise and the total height is 27 ft, so a 7:1 scope calls for 189 ft of rode rather than the 140 ft you would get from depth alone.
Choosing 5:1, 7:1 or 10:1
The right ratio depends on your rode and the weather. All-chain rode in a sheltered, settled anchorage often holds well at 5:1 because the chain's weight does much of the work. Rope or a rope and chain combination usually wants 7:1, the setting most cruising guides treat as a sensible default. When a blow, a swell or a gusty front is forecast, stepping up to 10:1 flattens the pull further and adds a big margin, provided you have the room and the rode to do it.
Turning the number into a safe swing
The rode length is only half of anchoring well. Once it is out, your boat can swing in a circle whose radius is roughly the rode length plus the boat's own length measured from the anchor. Picture that circle against neighbouring boats, moorings, shallows and the shore before you commit, then set the anchor by backing down and watching a transit ashore. If space is tight you may trim the scope, but do it knowingly and back it up with an anchor alarm and a watch rather than trusting a short rode in rising wind.