Anchor Alarm Apps

Digital Tools · Safety

Dragging anchor is one of the most common serious incidents in recreational sailing, and it almost always happens at night, during a wind shift, or when a storm passes through an anchorage that looked settled at sunset. The anchor alarm is the system that bridges the gap between dropping the hook and waking up on a beach — or worse, on another boat.

Phone-based apps have made anchor alarms accessible to every sailor with a smartphone. The chartplotter anchor watch, previously requiring dedicated hardware, is now standard on every major chartplotter. Understanding how these systems work — and specifically how they fail — determines whether your anchor alarm is a genuine safety tool or a false confidence generator.

Why Anchors Drag

An anchor holds in normal conditions through a combination of scope (the ratio of rode length to water depth), bottom type, anchor design, and set. A well-set anchor in good holding bottom with adequate scope will hold through most conditions the anchorage was designed for. The drag events happen at the margins: a wind shift that changes the load direction and dislodges an anchor set specifically for the original direction; a wind increase that exceeds the holding capacity of a too-short scope; a bottom that looked like sand but turned out to be thin sand over rock; a crowded anchorage where the swinging circles of adjacent boats interact during a change in conditions.

Most drag events develop slowly at first — the boat moves a boat length, then another, then the anchor comes free and the motion accelerates. An alarm that wakes you when you've moved two boat lengths gives you time to assess and react. An alarm that wakes you when you're in the surf does not. The setting of the alarm radius determines which scenario you get.

The Anchor! App: The Most Popular Option

Anchor! (published by Fathom Five) is the most widely used dedicated anchor alarm app on both iOS and Android. The interface is simple: when you drop the hook, tap the app, set your radius, and the alarm activates. The app records your boat's swing over time — a useful feature for understanding how your boat tracks in a current or wind shift, and for reviewing the night's position record in the morning.

The free version provides the core alarm functionality with a fixed radius. The paid version adds the swing log, multiple radius settings, and integration with other boat instruments via Bluetooth. For most sailors, the free version is sufficient for overnight anchoring in straightforward conditions.

Setting the radius: the conventional starting point is 30 meters of alarm radius per 30 feet of boat length, adjusted upward for anticipated wind shifts (a shift of 90 degrees or more requires additional radius to account for the boat swinging to the new wind direction) and for known GPS accuracy limitations in the anchorage. A 40-foot boat in a settled anchorage with accurate GPS might run a 40-meter radius. The same boat in an exposed anchorage forecasting a 60-degree wind shift overnight should run 60–70 meters.

Understanding GPS Accuracy Limits

Consumer GPS — the GPS chip in your phone or tablet — reports position with a typical accuracy of plus or minus 5 meters under good conditions: clear sky, strong satellite signal, no interference. In conditions with poor satellite geometry (when the visible satellites are clustered rather than spread across the sky), tree or cliff interference, or atmospheric disturbance, this degrades to plus or minus 15 meters or more.

A 15-meter GPS error means your reported position can be 15 meters away from your actual position — in any direction — without the boat moving at all. An anchor alarm with a 30-meter radius set on a boat with 15-meter GPS error will trigger falsely whenever the GPS drifts toward the edge of the reported radius. This is the cause of most false alarms on calm nights when the boat is clearly not dragging.

The practical implication: set your alarm radius to account for GPS accuracy, not just scope and swing. If you're seeing frequent false alarms on calm nights, the first step is widening the radius, not assuming there's a problem with the anchor. The second step is examining the swing log to verify the GPS position is actually drifting, which distinguishes GPS drift from real movement.

OpenCPN's Anchor Watch: The Configurable Option

OpenCPN's OpenAnchor plugin provides anchor watch functionality on the laptop or tablet running OpenCPN at the chart table. The anchor watch overlays a radius circle on the chart display, logs the boat's track overnight, and sounds an alarm when the position exits the defined radius. The position log is stored and can be reviewed the next morning as a full chart track of how the boat swung through the night.

The configurability is greater than consumer phone apps: you can set multiple alarm radii, configure visual alerts versus audio alerts independently, adjust the GPS polling interval, and integrate with NMEA instrument data if you have GPS fed through the boat's instrument network rather than from the laptop's internal GPS. For offshore sailors who already have OpenCPN running as their primary navigation platform, the anchor watch plugin adds alarm capability without requiring a separate device to remain powered overnight.

The limitation is the same as any laptop-based system: the laptop must remain powered overnight. A dedicated boat computer running continuously is suitable; a personal laptop running on battery will exhaust its charge before morning in most cases. Many sailors address this with a small 12V-powered tablet running OpenCPN permanently at the chart table.

Chartplotter Anchor Watch: The Most Reliable Hardware

Every major chartplotter manufacturer — Garmin, Simrad, B&G, Furuno — includes an anchor watch function in their navigation systems. Accessing it varies by brand: on Garmin, navigate to Where To > Anchor; on B&G, it's in the Sailing features; Simrad follows a similar path through the menu structure. Set the radius at the drop point and the chartplotter monitors position continuously.

The reliability advantage of a chartplotter anchor alarm over a phone app is hardware: the chartplotter is powered by the boat's 12V electrical system, which means it runs through the night without battery anxiety. The alarm output goes through the chartplotter's external alarm circuit, which on most boats connects to the chartplotter's speaker and can be connected to an external alarm horn. This is a fundamentally more reliable alarm chain than a phone speaker in a cabin berth.

The GPS antenna feeding a chartplotter is typically mounted high on the mast or radar arch, giving better satellite geometry than a phone lying on the chart table. Better geometry means better position accuracy, which means fewer false alarms and more reliable detection of real drag. For anchoring in exposed conditions or unfamiliar anchorages, the chartplotter anchor watch is the preferred primary system.

When Alarm Systems Fail

Single-point failure is the risk. A phone alarm fails when the battery dies overnight — which happens more often than expected, particularly if the app is running continuously with the screen dimmed but not off. A chartplotter alarm fails if the boat's electrical system has a problem, if the chartplotter is accidentally powered off, or if the alarm threshold is set too wide to catch an early drag. GPS goes into degraded mode during solar weather events that reduce satellite signal quality.

The practical standard for exposed overnight anchoring is two independent systems: the chartplotter anchor watch plus either the Anchor! app on a fully charged phone or OpenCPN on the chart table laptop. If both alarms sound simultaneously, the drag is almost certainly real. If only one sounds, the other system provides a cross-check before you come up on deck in a panic.

Solo anchoring in an exposed spot warrants additional caution beyond any electronic system. Many experienced solo sailors set a 20-minute vibrating alarm watch as a backup to everything electronic — waking for a brief deck check every 20 minutes is disruptive to sleep but ensures that a system failure does not translate directly into a grounding. In benign conditions in well-known anchorages, a single system and normal sleeping is reasonable. In new anchorages with forecast wind shifts, redundancy is the professional standard.

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Frequently Asked Questions

How much drag radius should you set on an anchor alarm app? For a 30-foot boat, a radius of 30 to 50 metres accounts for GPS positional uncertainty of ±5 to ±15 metres, scope swings as the tide or wind shifts, and the GPS track accuracy of a phone or tablet. Setting the alarm radius too tight generates false alerts; setting it too wide means the alarm fires too late to prevent trouble.

What is the Anchor! app by Fathom Five and what makes it popular? Anchor! by Fathom Five is a dedicated anchor watch app available for iOS that uses GPS to track your boat's position relative to the set anchor point and triggers an alarm if the boat moves outside the defined radius. It is popular among cruisers for its clean interface, reliable alerts, and ability to run with the screen off while keeping the GPS active.

Can OpenCPN be used as an anchor alarm? Yes, OpenCPN includes an OpenAnchor plugin that provides anchor watch functionality directly within the chart plotter software. The plugin monitors GPS position against the set anchor point and triggers an audio alarm if the boat drags outside the defined radius, making it a useful option for sailors who already run OpenCPN on a dedicated navigation device.

Do modern chartplotters from Garmin or Simrad have anchor alarms built in? Yes, most modern chartplotters from Garmin, Simrad, B&G, and other major manufacturers include built-in anchor alarm functionality that uses the boat's GPS to monitor position. The advantage over a phone app is that the chartplotter typically receives GPS from a dedicated antenna on the mast or deck, giving better accuracy and reliability than a phone's internal GPS.

What is the main limitation of anchor alarm apps on phones? The main limitation is that phone GPS accuracy varies significantly between devices and conditions, with typical accuracy of ±5 to ±15 metres. In addition, some phone operating systems kill background processes to conserve battery, which can prevent the alarm from triggering. Keeping the screen active or using a device specifically configured to allow background GPS operation is important for reliable overnight use.