Let Anchor be your anchor assistant. It calculates the necessary cable length and warns you if the ship position drifts out of the accepted radius. Anchor works fine in background if your device supports multitasking.
Be sure to have your device volume on maximum. When in background mode the alert works best with the setting Alert of the device notification setting you find at Settings > Notifications > Anchor.
Continued use of GPS running in the background can dramatically decrease battery life.