What Is an Anchor Alarm?
A GPS watchkeeper that tells you when the boat has moved outside its anchor circle - the cheapest insurance for a night at anchor, if set up properly.
An anchor alarm is software that defines a security circle around the anchoring point and sounds an alarm if the boat's GPS moves outside it. The check often starts at night, when the wind shifts, and the only watchkeeper for the sleeping crew is the alarm.
The key to setup is the centre and the radius. Set it wrong and the alarm misleads in two ways: a tight circle will sound at every swing, while a loose one will not warn you until long after dragging has started.
Correct setup
Centre: the point where the anchor was dropped, not the boat's position at that moment.
Radius: worked out as rode out plus boat length plus a sensible margin.
Dual watchkeeper: chartplotter plus phone; if one falls silent, the other speaks, and the phone should be plugged in with the settings tested.
Learn it as the last step of the anchoring routine: anchor on the bottom, reverse-thrust test, set the alarm (anchor point as centre, calculated radius), and memorise a bearing. The plan for the 5 minutes after the alarm sounds should already be clear: engine key, floodlight and who does what.