How to Set an Anchor Alarm

An anchor alarm warns you the moment your boat drifts beyond a safe circle around where you dropped the hook. Set up well, it lets you sleep. Set up badly, it either cries wolf all night or stays silent when it matters most. Here is how to get it right.

What is an anchor alarm?

An anchor alarm is a tool that watches your boat's position after you have anchored and sounds an alert if the boat moves outside a set area. That movement usually means one of two things: your anchor has broken out and is dragging, or the wind or tide has swung you further than expected. Either way, you want to know straight away — especially at night, in a blow, or when you are asleep.

The idea is simple, and sailors have done it manually for centuries by taking bearings on fixed landmarks and checking them through the night. A modern anchor alarm just automates that watch using GPS, so you are not getting up every hour to check a transit.

How does an anchor alarm work?

When you set the alarm, the app records the GPS position of your boat as the anchor reference point and draws a circle of a chosen radius around it. It then compares your live GPS position to that point, continuously. As long as you stay inside the circle, nothing happens. The moment you cross the boundary, the alarm fires — sound, vibration and a notification.

A few important details separate a good anchor alarm from a dangerous one:

• It must keep running with the screen off. A phone that is left to sleep will often stop updating GPS. A proper anchor alarm uses true background location so it keeps watching all night.
• It should alarm when it loses GPS, not go quiet. Silence is the most dangerous failure mode: you think you are protected when you are not.
• It accounts for swinging. Your boat pivots around the anchor as wind and tide change, so the alarm has to allow a full swing without nuisance triggering.

Phone app, chartplotter or dedicated device?

There are three common ways to keep an anchor watch. Each has a place.

For most cruisers, a good phone app is the practical choice: the GPS in a modern phone is genuinely accurate, the alarm is in your pocket or by your bunk, and the better apps let you share the watch with crew ashore. The catch is quality — this is exactly where a purpose-built app that stays awake and alarms on GPS loss earns its keep.

How to set an anchor alarm, step by step

This works for any GPS anchor alarm app, with notes for HoldFast where it helps.

1. Anchor properly first

   An alarm is a backstop, not a substitute for a good set. Choose your spot, let out enough scope, and back down on the anchor until you are sure it is holding. Only then set the alarm.

2. Drop the anchor point in the app

   Wait until your phone has a solid GPS fix, then record your anchor position. Setting it at your current location is fine for most situations; if you want to be precise, drop the point where the anchor actually lies rather than where the boat is sitting.

3. Set your alarm radius

   Cover your swinging circle plus a small margin (see choosing the right radius below). In HoldFast the radius adjusts from 5 to 200 metres with the circle updating live on the map.

4. Turn on background monitoring

   Make sure the app can keep watching with the screen off. In HoldFast this is background GPS; on the free tier, keep the app open and the screen active. Confirm Location Services is set to Always for the app in your iPhone settings.

5. Set the volume and test it

   Turn your ringer up and make sure a Focus or Do Not Disturb schedule will not silence the alarm overnight. Trigger a test if you can, and ask yourself honestly: would that wake me from a deep sleep?

6. Keep the watch — or share it

   Leave it running through the night. If you are going ashore, share a watch code so crew or family can follow the boat live from any browser, and review your track history in the morning to see how the night went.

Choosing the right radius

Radius is where most anchor alarms go wrong. Too tight and normal swinging trips it every time the wind shifts; too wide and the boat can drag a long way before you hear anything. A reliable starting point:

Radius ≈ length of rode deployed + boat length + a GPS margin of about 10–15 m.

That covers a full swing around the anchor plus the natural jitter in a GPS position. In a tight, crowded anchorage, keep it as small as you safely can and watch your neighbours; in open water with room to leeward, a little extra margin buys quieter nights. Because this deserves its own walk-through with worked examples, we cover it in detail in what anchor alarm radius should you set?

Avoiding false alarms

Nuisance alarms are the fastest way to lose trust in your anchor watch — and a sailor who mutes a noisy app is worse off than one with no app at all. Three fixes solve almost all false alarms:

• Widen the radius to cover a full swing. If the alarm fires every time the wind clocks around, the circle is smaller than your swinging room.
• Record the anchor point with a good fix. Setting it before the GPS has settled bakes in an offset that can trip the alarm later.
• Give the phone a clear view of the sky. Deep inside a steel hull or under a hardtop, GPS accuracy degrades and drift increases.

Background monitoring & battery

Continuous GPS uses power, so the golden rule is simple: keep the phone on a charger overnight. Beyond that, a good app offers a battery-saving mode that eases the update rate while still catching a genuine drag, and records a track so you can scrub back through the night in the morning and see exactly where the boat moved and whether it stayed inside the circle. That track is also the best way to tell a real drag from a wind shift after the fact.

What to do when the alarm fires

Do not panic, but move quickly. Get on deck and check your position against fixed references and your neighbours. Confirm whether you are actually dragging or simply swung to a new part of the circle. If you are dragging, start the engine, take the load off the anchor, and either re-set with more scope or move to a clearer spot. If it was a swing, widen the radius slightly and reset the watch. The alarm did its job — it gave you time.