VaneHow to Set a GPS Anchor Alarm on Your Phone
An anchor alarm watches your GPS position and wakes you if the boat moves further from the anchor than it should. It only works if the radius is right and the phone keeps listening all night.
Updated 1 October 2026
How an anchor alarm works
When the anchor goes down, the alarm records its position. From then on it measures the distance from that point to the boat with every GPS fix. As the wind and tide swing the boat round, the distance moves around inside a circle. If the anchor drags, the boat leaves the circle and the alarm sounds. Everything depends on two numbers: where the anchor is and how big the circle should be.
Step 1: let out enough scope
Scope is the length of rode (chain or line) let out, divided by the height from the bow roller to the seabed. BoatUS recommends 7:1, and 5:1 for a small boat with a light anchor in good weather. The RYA in the UK works from the maximum depth at high water: 4 times that depth for all chain, 6 times for chain and rope. Remember the tide: a scope that is fine at low water may be short at high water.
| Depth at high water | All chain (4×) | Chain and rope (6×) | 7:1 |
|---|---|---|---|
| 4 m | 16 m | 24 m | 28 m |
| 6 m | 24 m | 36 m | 42 m |
| 10 m | 40 m | 60 m | 70 m |
Step 2: work out the radius
The boat can swing anywhere on a circle around the anchor. Its radius is the horizontal reach of the rode plus the distance from the bow to wherever the phone is. At sensible scopes the horizontal reach is almost the whole rode. At 7:1 it is 99% of it. So a safe radius is:
radius ≈ rode out + bow-to-phone distance + GPS margin
A phone in open sky is typically accurate to about 5 m, so a margin of 5 to 10 m keeps it from crying wolf. With 40 m of chain out, the phone 8 m back from the bow, and a 10 m margin, set about 58 m. Smaller than that and the alarm goes off on every wind shift; much bigger and the boat can drag a long way before you hear it.
Step 3: mark the anchor, not the boat
The most common mistake is to press "set" after the boat has settled back on its rode. That records the boat's position, up to a full rode length away from the anchor. When the wind swings the boat to the other side, it is suddenly two rode lengths from the "anchor" and the alarm goes off for nothing. Or, if you made the circle bigger to stop that, it lets a real drag go too far.
Instead, set the alarm as the anchor drops, with the phone at the bow, or use an alarm that lets you enter the distance and bearing from the boat to the anchor.
Step 4: make sure it keeps listening
A phone alarm fails silently in ways a dedicated chartplotter does not. Before you sleep:
- Plug the phone in. Continuous GPS for eight hours uses a lot of battery.
- Allow location "Always" and notifications, so the app keeps working with the screen off and can sound when it is not in front.
- Turn the volume up and check Focus or Do Not Disturb. An alarm you cannot hear is not an alarm.
- Keep the phone where it sees the sky. Deep below decks, under a steel or carbon-fibre deck, the fix can get poor or disappear.
- Prefer an app that tells you when it stops watching, for example if the operating system closes it overnight, rather than one that just goes quiet.
Reading the alarm
- A single jump outside the circle with poor accuracy is usually a GPS glitch. A good alarm waits for more than one fix outside before it sounds.
- A steady track outside the circle in one direction, downwind, is a drag. Go and look.
- A track of your swing that looks like a smeared arc rather than a clean circle segment is normal. Boats sail around at anchor.
An anchor alarm is a second pair of eyes, not a replacement for a proper anchor watch in bad weather or a crowded anchorage.
Sources
- BoatUS Foundation, Anchoring (scope definition and 7:1 / 5:1).
- RYA, Anchoring with care (4× and 6× maximum depth at high water).
- US Government, GPS accuracy (smartphones typically within 4.9 m under open sky).
- Radius arithmetic is geometry: horizontal reach = √(rode² − height²).