An anchor alarm app is the obvious first choice. It's free or a couple of euros, it's already in your pocket, and for a quick afternoon stop it's genuinely fine. The problem starts when you trust it overnight in an exposed anchorage — because a phone is the wrong hardware for an all-night safety job. Here are the three failure modes every cruiser eventually meets.
1. Phone GPS drifts — so it cries wolf
Consumer phone GPS typically wanders 5–15 meters even when you're sitting perfectly still. To an anchor alarm app, that drift looks exactly like movement. So you set a tight radius to feel safe, and the app screams at 3 a.m. when nothing has actually happened. After two or three false alarms most people do the dangerous thing: they widen the radius until the alarm goes quiet — and now it won't fire until you're already on the rocks.
2. The operating system kills the app
iOS and Android aggressively suspend background apps to save power. An anchor alarm app running behind a locked screen for eight hours is a prime target. It can be throttled, frozen, or killed outright — and you won't know. You wake up, glance at the phone, see the app "open," and assume you were protected all night. You may not have been.
3. The battery you can't afford to lose
Running GPS and the screen all night flattens a phone fast. At anchor, your phone is also your weather station, your chartplotter backup, your VHF DSC fallback and your emergency line. Sacrificing it to the anchor alarm is a bad trade — and a dead phone at 4% means no alarm at all.
What to use instead
The fix is hardware that does only this job. A dedicated GPS anchor alarm carries its own multi-GNSS receiver (1–2 m accuracy instead of 5–15), can't be killed by an operating system because it isn't running one, and has a multi-day battery that leaves your phone free. To understand the accuracy difference, read how a GPS anchor drag alarm works, and for the full landscape see our complete guide to anchor alarms.
None of this means an app is useless — keep one as a convenient backup. Just don't make a consumer phone the only thing standing between your boat and the rocks.
Want a ready-made answer? AnchorKnight is a dedicated GPS anchor alarm built around everything below — 1–2 m positioning, a 72-hour battery and an 85–90 dB cabin siren. See pricing & reserve yours →