Sailors use "anchor watch" and "anchor alarm" as if they mean the same thing. They are closely related, but knowing the difference helps you understand what you are actually relying on when you turn in for the night.
The short answer
An anchor watch is the activity of keeping an eye on an anchored boat to make sure it is not dragging. An anchor alarm is a tool that performs that watch automatically and alerts you if the boat moves outside a set area. In other words, the alarm is one way of keeping an anchor watch — the automated way.
| Anchor watch | Anchor alarm | |
|---|---|---|
| What it is | The act of monitoring the boat's position | A device or app that monitors automatically |
| Who does it | A person, or a device on your behalf | Software, using GPS |
| Effort | Can mean staying awake and checking | Runs unattended and wakes you if needed |
| Example | Taking bearings on a landmark each hour | A phone app that alarms if you leave the radius |
The traditional anchor watch
Long before GPS, keeping an anchor watch meant a person — often on a rota through the night — checking that the boat had not moved. The classic method is a transit: line up two fixed objects ashore, and if that alignment changes, you are moving. Depth, bearings to landmarks and the feel of the boat all feed into the same judgement. It works, but it is tiring, and it depends on someone staying alert at 3am.
The electronic anchor watch
A chartplotter or phone app can keep the same watch using GPS. You mark your anchor position, set a radius, and the device compares your live position to that point. This is what most sailors now mean by an "anchor watch" feature, and functionally it behaves exactly like an anchor alarm: quiet while you stay put, loud when you move.
How an alarm automates the watch
The value of an anchor alarm is that it removes the human cost of the watch without removing the protection. Instead of setting an hourly reminder and squinting at a transit in the dark, you let the app hold the watch continuously and only involve you when something actually changes. A good one goes further than a chartplotter: it keeps watching with the screen off, fires through the lock screen, and can be shared with crew ashore so the watch continues even when nobody is aboard.
Which do you need?
For most cruising, a well-set-up anchor alarm is your anchor watch, and it is a better watch than a tired human at 3am. But the two are not mutually exclusive, and in serious conditions the answer is both:
- Settled night, good holding: an anchor alarm is plenty. Set it and sleep.
- Rising wind or a crowded, tricky anchorage: keep a human watch alongside the alarm.
- Ashore for the evening: use an alarm that shares a live watch so someone can keep an eye on the boat remotely.
Whatever you rely on, remember the alarm is an aid to good seamanship, not a substitute for it. Set the anchor well first — our guide to why boats drag covers how — and let the alarm be your backstop.
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Download on theApp StoreFrequently asked questions
Not quite. An anchor watch is the act of monitoring an anchored boat to make sure it is not dragging, whether done by a person or a device. An anchor alarm is a specific tool that keeps that watch automatically and sounds an alert if the boat leaves a set area. The alarm is one way of keeping an anchor watch.
An anchor alarm greatly reduces the need to sit up all night, but it does not remove your responsibility as skipper. In severe weather, a crowded anchorage or poor holding, a human watch alongside the alarm is prudent. Treat the alarm as a reliable aid, not a replacement for judgement.
An anchor watch on a chartplotter is a built-in feature that records your anchor position and alarms if the boat moves beyond a set radius, the same principle as a phone app. It is always powered but you must be aboard and awake to hear it, and the alarm is often quiet below decks.