Anchor Alarm App — sleep at anchor, actually sleep.
// anchor drag alarm · kalman gps · offline · iPhone & Apple Watch
An anchor alarm app that turns your iPhone into a silent watchkeeper. Drop the hook, set a safety radius, and a Kalman-filtered GPS drift alarm watches all night — firing loud enough to wake a closed cabin if the anchor drags. No internet required, works offshore, with marine weather and an Apple Watch companion.
Screens
Every screen built for 3 a.m.
No clutter, no menus to hunt through half asleep — the safety circle, the radius and the alarm are always one glance away. Real screenshots from the App Store.
What it does
Anchor drag alarm — designed around real failures
Every feature exists because of a real failure mode at anchor — the false alarm at 2 a.m., the dead spot deep in a cove, the phone that locked itself and went silent.
Live drift radius
A visible safety circle drawn around the boat on a real chart. Stay inside it and you're fine; cross it and the watch wakes. The radius is yours to set, from a tight harbour berth to a wide swing.
Anti-jitter Kalman filtering
GPS noise no longer triggers false alarms. A relaxed 500 m reception threshold keeps lock deep in coves, then the filter tightens enough to catch a real 5 m drift on a 30 m radius — and it learns your boat's swing pattern.
Critical alerts
With your permission, the alarm bypasses silent mode and Do Not Disturb so a muted phone never costs you your boat. It keeps monitoring through screen lock, low battery and incoming calls.
Charts & marine weather
An OpenSeaMap overlay shows depths, buoys, hazards and anchorages. A marine forecast brings wind, gusts, pressure and wave height — so you know what's coming before it arrives.
Drift prediction
The algorithm warns you before you cross the radius, not after. Session history records maximum drift and duration, exports CSV logs, and a swing heat-map reveals a dragging anchor early.
Apple Watch & sharing
A watchOS companion taps your wrist silently, while Dynamic Island and a Live Activity keep the status one glance away. One-tap secure position sharing lets shore crew know exactly where you are.
The details
Anchor alarm app specs & privacy.
Free to monitor, no subscription required, no internet required to alarm. Your position never leaves the phone unless you choose to share it.
Privacy first
Your position never leaves the phone unless you share it. There are no analytics tied to your identity, and no ads on the safety screen. The free version covers two full anchor sessions per cycle; Premium (monthly or annual) removes ads and unlocks unlimited sessions. A low-power background mode protects marine batteries.
The collection
More instruments.
Four other apps from the studio, each built around a single job done exceptionally well — and every one keeps working when the bars disappear.
Questions
Anchor alarm app — frequently asked.
Everything sailors ask before trusting an app with a night at anchor.
Drop the hook
Set the radius, then close your eyes.
Free to start, no account, no signal required to sound the alarm. Let the watch keep the watch.