What an anchor alarm does
At anchor the biggest worry is that the anchor drags — that it no longer holds and the boat slowly drifts away. An anchor alarm watches your position via GPS and sounds the alert as soon as you leave a defined area. The principle behind it is a geofence: a virtual fence around your anchor position.
How the geofence works
You set the alarm once the anchor is down and has dug in. The app remembers the position and draws a circle with a radius you choose.
- Too small: the boat swings normally around the anchor as wind and current shift — a radius that is too tight then triggers false alarms.
- Too large: the alarm comes too late when the anchor really does drag.
As a rule of thumb, the radius follows the length of chain paid out plus the swinging circle plus a GPS reserve. Details on the settings are in the Guide: Set up the anchor alarm.
Factoring in GPS accuracy
The alarm is only as good as your GPS position. With a poor view of the sky or in narrow bays, the measured position wanders by a few metres — even when the boat lies still. You have to build this measurement scatter into the radius, otherwise it beeps for no reason in the middle of the night.
Local instead of cloud
A good anchor alarm runs on the device — it needs no signal and does not send your position to the cloud. That is more reliable (it works even in the lonely anchorage with no reception) and better for privacy. How NauticCalc works without a signal in general is described under Offline navigation.
Common mistakes
- Radius too tight → false alarms during normal swinging.
- Alarm set too early, before the anchor has really taken hold.
- GPS scatter ignored → false alarms at night.
- Phone in power-saving mode, throttling GPS/notifications.
Conclusion
An anchor alarm does not replace the anchor watch, but it buys you a calmer night’s sleep. What matters is the right geofence radius: large enough for the normal swinging circle and the GPS scatter, small enough to catch real dragging early — and all of it local, without the cloud.
Read more: Anchor alarm · GPS position · Offline at sea