At anchor, the boat swings around its anchor. The radius of the swing zone depends on the length of chain paid out, the depth, the length of the boat (the phone’s GPS is not at the bow) and the GPS accuracy margin.
Simple rule:
Radius = length of chain paid out + boat length + 10 m (GPS margin)
Exact calculation, chain taut:
Radius = √(L² − D²) + boat length + GPS margin
where L = length of chain paid out and D = depth + height of the bow roller above the water (≈ depth + 1.5 m).
Important: calculate using the depth at low tide (that is when the boat can drift furthest from the anchor) and centre the zone on the anchor’s position at the moment you drop it, not on the boat’s position once it has settled.
Example: 30 m of chain on a 10 m boat → radius ≈ 30 + 10 + 10 = 50 m.
If you get repeated false alarms, add 5 m to the radius rather than disabling the alarm.