An anchor alarm is a deceptively simple tool with very high stakes: it watches your boat's position while you're anchored and wakes you the moment you start to drag. Get it right and you sleep soundly in an exposed bay. Get it wrong — or trust the wrong tool — and you wake to the sound of your keel on rock. This guide explains exactly how anchor alarms work, the three ways to run one, and how to choose the setup that will actually protect you.
What is an anchor alarm?
An anchor alarm records where your boat sits when you set the hook, then continuously measures how far you've moved from that point. As long as you swing within a circle you define — your watch radius — all is well. If your position crosses that boundary, the alarm assumes the anchor is dragging and sounds an alert. It's the maritime equivalent of a tripwire around your safe swinging room.
The concept is centuries old, but the accuracy of the warning depends entirely on the quality of the GPS behind it. That's where the real differences between anchor alarms appear — and why a free phone app and a dedicated device can behave so differently on the same night.
How does an anchor alarm detect dragging?
Every anchor alarm follows the same three steps: it fixes a reference point, sets a radius, and watches for boundary crossings. The nuance is in how it handles two things — GPS drift and your natural swing. A boat at anchor doesn't sit still; it arcs back and forth with wind and tide across a wide circle. A good alarm distinguishes that normal swing from genuine drag. We break the mechanics down in detail in how a GPS anchor drag alarm works.
The three types of anchor alarm
1. A phone app
Free or near-free, always in your pocket, and genuinely useful for a quick afternoon stop. But a phone app inherits every weakness of the phone: consumer GPS that drifts 5–15 meters, an operating system that quietly kills background apps to save power, and a battery you can't afford to flatten overnight. The result is the worst of both worlds — false alarms that wreck your sleep and silent failures that leave you unprotected. We cover this in depth in why anchor alarm apps fail.
2. A chartplotter anchor watch
Most modern chartplotters include an anchor-watch function with excellent GPS. The catch is where the alarm sounds — at the helm, far from your bunk — and the fact that it needs ship's power running all night. For a full breakdown see dedicated anchor alarm vs chartplotter.
3. A dedicated anchor alarm device
A standalone device does one job and nothing else. It carries its own multi-GNSS receiver for 1–2 m accuracy, its own loud siren, and its own multi-day battery, so it never competes with your phone or depends on ship's power. It sits on your nightstand and simply works. This is the category AnchorKnight was built for.
How to choose the best anchor alarm
When you compare anchor alarms, weigh three things above all else:
- Positioning accuracy. Phone GPS drifts 5–15 m; a dedicated multi-GNSS receiver holds 1–2 m. Tighter accuracy means fewer false alarms and a smaller, safer radius.
- Independence. The alarm must keep running all night without being killed by an OS, drained by other apps, or dependent on ship's power.
- Where the alarm sounds. A warning you sleep through is no warning at all. You want a loud siren in the cabin, ideally backed by a push notification.
Price matters too, but it's the smallest variable: the gap between a flaky free app and a €1,500 chartplotter is exactly where a dedicated device lives.
How to set an anchor alarm correctly
Even the best hardware fails if the radius is set wrong. The radius must account for your boat length, your scope (how much rode you let out), and the depth — not just a guess. Our step-by-step guide to setting an anchor alarm walks through the exact method, including the common mistake of setting the radius too tight and triggering false alarms all night.
The bottom line
An anchor alarm is the cheapest insurance in sailing. A phone app is fine for a lunch stop; for a night's sleep in an exposed anchorage you want something that can't be killed, can't be drained, and can be heard from your bunk. Match the tool to the stakes and you'll never lie awake listening to the rigging again.
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 →