The number one cause of false anchor alarms is a watch radius set by guesswork. Too tight and you're woken by normal swinging; too loose and you're already in trouble before it fires. Here's a simple way to get it right from your boat length, depth and scope.
The simple formula
A safe working radius is the horizontal distance from your anchor to the boat, plus your boat length, plus a small margin for GPS accuracy:
- Rode scope — the length of chain/warp you let out (most of it lies along the seabed at anchor).
- + boat length — you swing around the bow, so the stern reaches a full boat-length further.
- + GPS margin — a few meters with a dedicated receiver; 10–15 m if you rely on a phone.
Worked examples
10 m yacht, 4 m depth, 5:1 scope: ~20 m of rode + 10 m boat + ~5 m margin ≈ a 35 m radius (round up to a 50 m preset to be safe).
12 m yacht, 8 m depth, 5:1 scope: ~40 m of rode + 12 m boat + margin ≈ a 55–60 m radius (use a 100 m preset if your device steps that way, or a custom value).
Catamaran: add extra because cats range widely — see anchor alarm for catamarans.
Two adjustments that matter
Tide: if the current will reverse, your boat swings to the opposite side of the circle — make sure the radius covers the whole 360°, not just where you lie now. GPS accuracy: the tighter and steadier your fixes, the smaller a safe radius can be, which means earlier, more reliable warnings — that's the core argument in how a GPS anchor drag alarm works.
For the full setup routine, see how to set an anchor alarm, or step back to the complete anchor alarm guide.
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