Why scope matters
An anchor holds best when the pull on it is close to horizontal. If the rode rises steeply, it tends to lift the shank and pull the anchor out. The scope is the ratio between the length of rode let out and the height from the bow roller down to the bottom:
Scope = rode length ÷ (water depth + tide still to rise + bow height)
Recommended scope
| Conditions | All chain | Rope with chain leader |
|---|---|---|
| calm, short stop, good holding | 3:1 | 5:1 |
| normal, overnight | 5:1 | 7:1 |
| strong wind or storm | 7:1 | 10:1 |
Chain is heavy, so it hangs in a curve called a catenary that keeps the pull low and absorbs gusts. Rope is lighter and stretchier; it needs more scope, and a length of chain at the anchor end protects it from abrasion and helps the anchor set.
Example
In 15 feet of water with 3 feet of tide still to rise and a 4-foot-high bow roller, the height to use is 22 feet. Overnight on all chain at 5:1, let out about 110 feet (33.5 m). The boat, 32 feet long, could then swing in a circle about 140 feet (43 m) in radius around the anchor.
Swinging room
When the wind or tide changes, the boat swings around its anchor. The radius is about the horizontal distance covered by the rode plus the boat’s length; this calculator uses √(rode² − height²) + length. Boats on rope swing more, and differently, than boats on chain. Anchor so that your circle does not overlap the shore, shoals or other boats’ circles, and use a scope similar to your neighbors'.
Good practice
- Measure the depth with the sounder and check tide tables for high water during your stay.
- Mark your chain every 5 m or 25 feet with colored markers so you know how much is out.
- Back down gently on the anchor to set it, then take bearings or set an anchor alarm.
- Use a snubber on chain to relieve the windlass and dampen shocks.
- Increase scope before a front arrives, not once the wind is blowing.
Limits
These ratios are rules of thumb. Holding depends on the anchor’s type and size, the seabed (sand and mud hold well, rock and weed poorly) and the boat’s windage. In a crowded anchorage, a shorter scope may be unavoidable: compensate with a heavier anchor, all-chain rode and close watch.