NGA is a free anchor alarm for iPhone and Apple Watch.
Set the anchor and a swing radius. If the boat drags out of the circle, the phone sounds a loud alarm, even on silent.
Size the circle from your rode.
Give it the depth, rode and boat length, and NGA works out the scope and the radius. In a tight anchorage, limit the swing to an arc.
How the radius is worked out
With the rode taut, the bow sits √(rode² − drop²) from the anchor. The drop is the depth plus the bow roller height plus the tide rise. The phone can be up to a boat length further out than the bow, so NGA adds the boat length.
30 m of rode over a 6.5 m drop: √(900 − 42.25) = 29.3 m. Add 10 m of boat and the radius is 39 m. 30 ÷ 6.5 gives the 4.6:1 scope; NGA calls under 3:1 insufficient, 3:1 to 5:1 minimal, 5:1 to 7:1 good, and more than that generous.
When the alarm sounds.
- The boat is past the radius plus the GPS error, for 20 seconds or three fixes in a row.
- The boat is 25 m past that. Then it sounds at once.
- A fix worse than 50 m updates the screen but cannot start or stop the alarm.
- It plays through the speaker with the ringer off and the screen locked, and keeps going until you acknowledge it.
Why the GPS error counts
A phone fix can be several metres off, and so can the fix taken when you set the anchor. NGA widens the circle by the larger of the two, so one bad reading cannot fake a drag. It then waits 20 seconds or three fixes. If the boat swings back inside first, it stands down.
The 20 seconds, three fixes and 50 m are defaults, adjustable in Settings.
Check it without unlocking.
The Lock Screen, Dynamic Island and Apple Watch show the status and the distance from the anchor. The age of the last fix sits next to it, so a frozen screen cannot pass for a fresh one.
Below 20% battery and off the charger, NGA warns you. If iOS stops the app mid-watch, the watch restores when you open it again.
What the boat did overnight.
Each watch is saved with its swing trail and a drift graph. Going ashore by dinghy, a compass points back to the anchor.
No account, no analytics, no ads. The watch runs on the phone and the history stays there. Remote watch is off by default; turned on, it goes through your own private iCloud.