"How about an electronic device that HAS to be WITHIN 20 metres of the boat to KEEP the
autopilot working." If they aren't already on the market, they are easily made.
The simplest would be to use a standard kill switch from a jetski. That's a wrist tether with a 36" long
line that goes to a kill switch in the ignition, pull out the
line and the ignition quits. Install it in your
autopilot's power
line and the same thing happens, the auto will die when you move away.
Next up is to make that wireless, and you can also do that fairly easily using a luggage locater or kiddie tracker. One box goes to each end (you and the luggage, you and the kid, etc.) and when they get 20-25 feet apart, your box squeels an alarm. Tie the alarm to a power relay, interrupt the
autopilot power, again the auto powers gets cut off if you get away.
Ain't rocket science. WILL false if your "box" has a dead battery or gets stepped on. So the more expensive ones that are on the market (I know there's at least one) are MOB
alarms that sound an active alarm when the bearer splashes in the water. Again, just splice the alarm to a reply on the
autopilot power.
If you think stopping an
autopilot is going to save you.