Anchor Watch / Alarm (uk.cr.anchor) on Google Play
This App will allow you to fix your boats location and to set a maximum scope that you want the boat to drift before triggering an alarm. YOu can elect to run in silent mode with just a warning message appearing when the set scope it exceeded or you can elect to use the default alarm sound.The distance drifted and bearing to the set anchor point are shown.There are settings options to enable you to choose whether to use feet or metres and whether to allow the app to run in the background etc