Get your chartplotter working around the clock
All chartplotters have an anchor alarm that can be set to sound when the boat moves outside a specific radius around a GPS position. The concept is great, but in the real world it is often not all that helpful. The reason is that the anchor alarm's radius is normally set on the boat's position rather than the position of the anchor. If the alarm is enabled when the hook touches bottom it should work as intended. But most of the time there is too much going on while anchoring to worry about pushing buttons on the chartplotter. What usually happens is that the alarm gets set after everything quiets down and the boat stops moving. At this point the skipper or navigator tries to select an alarm perimeter large enough that it won't create false alarms, but small enough to serve its purpose. In most cases a 130-foot radius is enough to avoid triggering a false alarm, but may not be enough to keep you from running aground or bumping into other boats in a tight anchorage.
I've found it is more effective to use my chartplotter to establish what I call a safety perimeter. Basically, this is an exclusion zone centered on a waypoint. If the boat's GPS position moves into that zone, an alarm is triggered.
To establish a safety perimeter I use my chartplotter's waypoint proximity alarm function. If you place your safety-perimeter waypoint far enough away from the boat to create a safety zone directly behind your swinging area, the proximity alarm will sound if the boat crosses that perimeter, which is shown as a line on your screen (Fig 1).
After anchoring, I leave the tracking function enabled on my chartplotter and then zoom in to the screen's closest level. The boat then "paints" a path on the screen as it swings around its anchor (Fig 2). After a period of time, as the boat swings to conform to the varying wind and currents and as its anchor chain stretches out, its natural maximum swinging radius is described on the screen. Then I can set up a safety perimeter just beyond the safe ground of the swinging radius.