Like so much of boating
, the answer to this is "it depends".
What is it that you want the alarm to tell you?
A) You've dragged the anchor
Set the alarm center point to where you dropped the anchor and the radius to be the amount of scope
you've let out. (if you want to be more accurate, set the radius to the square root of rode-length squared minus depth
of anchor squared), but that's a bit much. The only way you can go outside that circle is if the anchor moves - i.e. you've dragged it.
B) You are concerned about drifting into something that is within reach of your rhode even if the anchor doesn't move
Set the alarm center point to wherever you come to rest after setting the anchor. Set the radius to the maximum distance towards the _nearest_ object of concern, minus a safety/warning margin. You may get false alarms due to drifting away from your little area (for example towards the anchor) even if the anchor itself remains firmly set.
However, I've been looking for a project
and I think I'll make an anchor alarm that will allow me to geofence - i.e. set a random perimeter where, if the boat
wanders out of there, I get an alarm. That will let me pull it in close to my stern so I know if I'm dragging anchor, and let me expand the area in other direction to deal with wandering. Maybe add a second mode in which I put a fence around things that are scary so that the alarm goes off if I get near them, plus will alarm if I drag anchor. Hmmmmm