With all chain, let out at least four to five times the depth of water; with rope, seven to eight times. Measure the depth at high water and add the height of the bow above the water, and let out more in strong winds.
The length of chain or rope between the boat and the anchor is called the rode, and the length let out compared with the depth is the scope. Getting the scope right is most of what makes an anchor hold.
An anchor holds by digging into the seabed, and it holds best when it is pulled along the bottom rather than upwards. With enough scope, the weight of the chain makes it lie along the seabed, so the pull on the anchor is horizontal, which digs it in. With too little, the pull is upwards and the anchor breaks out.
Guide figures:
- All chain: at least 4 to 5 times the depth.
- Rope (usually with a length of chain at the anchor end): 7 to 8 times the depth.
- Strong wind or overnight: more than this, if there is room.
Work out the depth carefully:
- Take the depth now, from the echo sounder.
- Allow for the tide: use the depth at high water, not the depth when you arrive. If you anchor at low tide and the tide then rises two metres, your scope shrinks.
- Add the height of the bow roller above the water.
For example, in 5 metres at high water with a bow 1 metre high, the effective depth is 6 metres. With all chain, let out at least 24 to 30 metres; with rope, 42 to 48 metres.
Mark your chain at regular intervals (paint or coloured markers every 5 or 10 metres) so you know how much is out.
More scope means a bigger swinging circle, so check there is room for the boat to swing all the way round without hitting other boats or reaching shallow water. Boats on chain and on rope swing differently, so give others plenty of space.
More in the book: Chapter 9, Anchoring and moorings