Worked example
- Depth at high water
- 5 m
- Bow roller above water (m)
- 1 m
- Boat length (m)
- 10 m
- Scope ratio (:1)
- 4:1
Rode to let out: (5 + 1) × 4 = 24 m
Approximate swing radius: 24 + 10 = 34 m
How it works
Use the greatest water depth you expect during your stay, then add the height of the bow roller above the water. If you know the depth now, add only the tidal rise still to come; do not add the whole tidal range again. Check whether your sounder measures below the keel, below the transducer or from the surface, and correct it before entering a water depth. Also check that you will retain enough water under the keel at low water.
Multiply that vertical distance by your chosen scope. This tool starts at 4:1 for chain and 7:1 for chain plus rope: common planning rules of thumb for sheltered conditions, not laws or guarantees. Published guidance varies: PBO gives 3 times maximum depth for chain, 5 for a mixed rode and 7 for rope; BoatUS recommends 7:1. Use more scope when conditions, holding or the anchor manufacturer require it, provided there is room. Chain weight does not keep the pull horizontal in every wind. RYA’s Anchoring with Care guide gives 4 times maximum high-water depth for chain and 6 for a mixed rode; the 7:1 mixed-rode default here follows BoatUS’s general recommendation.
In the example, high-water depth is 5 m and the roller is 1 m above water. At 4:1, let out (5 + 1) × 4 = 24 m of rode. With a 10 m boat, a conservative planning radius is about 24 + 10 = 34 m from the anchor, not from your present position. A hull point 40 m from the anchor is outside that circle now, with 6 m of nominal margin. That does not account for the other vessel moving or for measurement error.
Choose a sheltered, permitted spot with suitable holding and space through wind and tide changes. Lower the anchor under control, lay the rode without piling it up, and set it gently in reverse. Check bearings or an anchor alarm and keep a watch. Anchoring and tidal planning are part of RYA Day Skipper seamanship; for exam practice, explain the depth reference and ratio rather than memorising one universal number. Review Rule 30 for lights and shapes and Rule 35 for restricted-visibility signals; vessel-size exceptions matter.
Seabed holding guide
- Sand: often good holding when the anchor buries; compact sand can resist penetration.
- Mud: holding varies; soft mud may need large flukes and more scope.
- Clay: can hold well after penetration, but hard clay can prevent setting.
- Weed: unreliable if the anchor catches vegetation rather than the bottom; avoid protected seagrass.
- Rock: poor, unpredictable holding and a risk of fouling; choose another spot where possible.
Choosing an anchor
- Plough / CQR: versatile on suitable bottoms; the hinged design may take time to set in firm ground.
- Delta: a fixed-shank plough that can set well in sand; not a guarantee in weed or rock.
- Bruce / claw: often sets readily in sand and mud, but holding per unit weight varies with the bottom.
- Rocna / scoop: designed for rapid setting and burial; weed and hard bottoms can still defeat penetration.
- Danforth / fluke: large flukes can work well in sand and mud; less suitable for rock or weed and may need to reset after a change of pull.
