Anchor Alarm Guide: Phone Apps, Garmin and Navionics

An anchor alarm uses GPS to remember where your boat anchored and sounds an alert if the boat moves outside a circle around that spot, which usually means the anchor is dragging. You can set one up in a phone app, with the Anchor Drag alarm on a Garmin chartplotter, or with the Anchor app on some Garmin watches.

How an anchor alarm works

The alarm saves a position, draws a circle around it (the swing radius), and keeps checking your GPS position against that circle. While the boat swings on its rode inside the circle, nothing happens. If the boat moves past the edge, the alarm goes off.

Because it only measures distance from a saved point, it will not catch:

• A slow drag that stays inside the circle
• A wind shift that swings you toward shallow water inside the circle
• Another boat dragging into you

Garmin's watch manuals describe the Anchor app as a situational awareness tool that may not prevent groundings or collisions in all circumstances. Treat any GPS anchor alarm the same way.

Your options

Phone apps for iPhone and Android

The details below come from each app's store listing or help pages. Check the listing for current prices.

• Anchor Pro (iPhone, also available on Android): free with ads, according to its App Store listing. Anchor Premium is listed at $6.99 a year on the US App Store; prices can change and vary by country. It has an Apple Watch companion and can send alerts by Telegram or email. Pick it if you want a free option.
• Anchor Alarm - Anchor Watch (iPhone): lets you shape the swing area so it allows more room in some directions than others, and can be monitored from a second device. Pick it if you want to watch the boat from ashore or need an uneven swing area.
• Anchor Alarm with Late Set (iPhone): lets you start the alarm after you have already anchored, using a circle or a custom area you draw on the map. Pick it if you usually arm the alarm after the boat has settled.
• DragAlarm (iPhone): its listing warns that background GPS use can dramatically decrease battery life, which is true of every app on this list.
• Navimetrix: the alarm is under Settings > Alarms > Anchor, with a default swing radius of about 130 ft (40 m). You can move the anchor point while the alarm is disarmed. Pick it if you want to correct the anchor position after you drop it.
• savvy navvy: lets you exclude segments of the circle. Pick it if part of the circle covers water you could never swing into.

Garmin chartplotters

On Garmin GPSMAP chartplotters, the Anchor Drag alarm is set like this:

• Go to Settings > Alarms > Navigation > Anchor Drag.
• Select Alarm to turn it on.
• Select Set Radius and enter a distance.

Check your model's manual if the menu differs. Garmin says the alarm sounds if you have moved more than the allowable distance and calls it useful when anchoring overnight.

Make sure Beeper is on (under System Settings or Sounds and Display Settings, depending on model). Garmin's manuals warn that you will not hear any alarms with it off.

Garmin's Cortex V1 and M1 smart AIS systems (originally from Vesper Marine) include an Anchor Watch feature. It combines position with a built-in heading sensor to detect drag and can alert on Cortex handsets and your phone, which helps when you are off the boat.

Garmin watches

Recent Garmin quatix, tactix, fenix and Enduro models (for example the Enduro 4 and quatix 8) have an Anchor app. Check your watch's manual to confirm yours does. Drop the anchor marker at the bow right when the anchor goes down, then adjust these settings:

• Drift Radius: the size of the circle. Use the radius method in step 3 below.
• Update Interval: how often the watch checks your position.
• Anchor Alarm: turns on the alert for when the boat moves beyond the drift radius.
• Anchor Alarm Duration: how long the alert runs.

Does Navionics have an anchor alarm?

The Navionics Boating app does not list an anchor alarm in its App Store description, so run a separate alarm app alongside it. If you use Navionics charts on a Garmin chartplotter, use the plotter's built-in Anchor Drag alarm (steps above).

We found no Garmin or Navionics documentation of an alarm in the app. The closest feature the app lists is placing a marker on a good anchorage.

How to set up an anchor alarm

1. Anchor and make sure it is set

Scope is the length of rode you let out divided by the water depth plus the height of your bow above the water. Allow for a rising tide when you work out depth. Waterway Guide gives these rules of thumb:

• About 5:1 to 7:1 in settled weather. All-chain rode can often use about 5:1.
• 10:1 or more in heavy weather or an exposed anchorage.

Waterway Guide notes the 5:1 and 7:1 ratios only hold in mild conditions, roughly under 15 knots, and you should let out more rode above that. A short scope makes the pull on the anchor steep enough to break it free.

Back down gently in reverse. If your position on the chart or a landmark on shore stops moving while the rode is tight, the anchor is set. Arm the alarm after that.

2. Put the center of the circle over the anchor

Garmin's chartplotter alarm measures how far you have moved from where you were when you armed it. Boaters on Trawler Forum report that the circle is centered wherever the GPS antenna is when you turn the alarm on. If you arm it from the helm after the boat has fallen back on its rode, the center sits away from the anchor. When the wind shifts, the boat can swing to the far side of the anchor, up to about twice the swing distance from that center.

Ways to handle this:

• A 2012 SAIL Magazine article recommends marking the position with a phone at the bow right after the anchor goes down.
• Use an app with a late set option.
• Use an app that lets you move the anchor point. In Navimetrix, disarm the alarm first, then use Move anchor.
• Make the radius larger, as described in the next step.

3. Choose the radius

If the circle is centered on the anchor, the radius should cover your rode length plus your boat's length, with some room for GPS error. Because the rode angles down to the bottom, this is a little more than the actual swing, which gives you some margin. Too small and you get false alarms every time the boat swings. Too large and you get a late warning.

Example: 10 ft of water plus a 3 ft bow height at 7:1 gives about 91 ft of rode. Add a 35 ft boat for about 126 ft, then add room for GPS error.

If your alarm centers on the boat's position when you arm it, after the boat has fallen back, set the radius to about twice the swing distance, or move the center to the anchor.

Some reference points:

• One app maker suggests about 130 to 165 ft (40 to 50 m), and Navimetrix defaults to about 130 ft (40 m). These only suit shallow anchorages. In 50 ft of water with a 5 ft bow at 5:1, you would have about 275 ft of rode out, well beyond those figures.
• One boater on Trawler Forum with 60 to 90 ft of chain out sets the alarm at 120 to 150 ft. That is one person's practice.

Base the radius on the rode you actually let out.

4. Make sure you will hear it

iPhone:

• Set the app's location access to Always so it keeps working with the screen locked.
• Turn on notifications for the app.
• Turn on critical alerts if the app offers them.
• Leave the app running. Do not swipe it closed.
• Check that the alarm sounds with the silent switch on.

Android:

• Set the app's location access to Allow all the time.
• Exempt the app from battery optimization so Android does not stop it in the background.
• Allow alarms to sound in Do Not Disturb.
• Check that the alarm sounds with the screen locked.

On a Garmin chartplotter, confirm Beeper is on.

5. Plan for power and for going ashore

Constant GPS use drains a phone battery, so plug the phone into a charger for the night. If you leave the boat, keep one phone aboard and set up a second device to receive alerts. Anchor Alarm - Anchor Watch and Anchor Pro support remote monitoring or remote alerts.

6. Adjust for lakes and rivers

• In a river, current usually holds the boat downstream of the anchor. An app that lets you shape the swing area can give more room downstream and less elsewhere, which cuts false alarms.
• Leave room for a wind shift. On a lake, a shift can swing the boat to the opposite side of the anchor.
• With bow and stern anchors set, the boat barely swings, so a tighter radius works.

7. Test it

Arm the alarm with a small radius, lock the phone and switch it to silent, then walk it to the stern or along the dock until it goes off. Confirm you can hear it from your bunk. Treat the alarm as a backup to keeping watch.

Helm's anchor alarm (Anchor Watch)

If you use Helm on iPhone, its anchor alarm is called Anchor Watch. You can start it from the Home Screen quick action.

• Suggested radius. It works out a swing radius as an estimate from Helm's depth data at your position, a 5:1 scope, an average bow height and your boat's length, and shows the depth, scope and rode it used. If you let out more than 5:1, such as 7:1 on rope rode, widen the ring with the slider (about 33 to 656 ft, or 10 to 200 m). Where it has no depth data, it says "No depth data here" and uses a rough default, so set the radius from the chain you let out.
• Siren on silent. A looping siren is set to sound even with the silent switch on, along with repeated "Anchor dragging" notifications. The notifications follow your Focus and Do Not Disturb settings.
• Fewer false alarms. It goes off only after several GPS fixes in a row are outside the ring for about 20 seconds, and it allows for poor GPS accuracy.
• Silence and Disarm. Silence stops the siren for 10 minutes and the watch stays on. If the boat is still outside the ring after that, it sounds again. Only Disarm turns it off.
• Works offline. It reads the phone's own GPS and needs no cell signal or internet. See how Helm works offline.

The anchor point is the boat's GPS position when you start setup, so the centering advice in step 2 applies. Keep Helm open in the background overnight. If the app is closed, iOS can only wake it when the boat leaves a wider backup zone of at least 330 ft (100 m), which may be too late.

Anchor Watch comes with the subscription, which starts with a free trial of the whole app. See the Helm FAQ for details. It is an aid and does not replace a proper anchor watch, official charts, required equipment or good seamanship.

Common questions

Is there a free anchor alarm app?

Yes. Anchor Pro is free with ads, according to its App Store listing, and is also available on Android. Its Anchor Premium upgrade is listed at $6.99 a year on the US App Store.

Does Navionics have an anchor alarm?

The Navionics Boating app does not list an anchor alarm, so run a separate alarm app alongside it. If you use Navionics charts on a Garmin chartplotter, use the plotter's built-in Anchor Drag alarm.

Where is the anchor alarm on a Garmin chartplotter?

On GPSMAP models, go to Settings > Alarms > Navigation > Anchor Drag, select Alarm to turn it on, then Set Radius. Make sure Beeper is on so you can hear it. Check your model's manual if the menu differs.

How big should my anchor alarm radius be?

Cover the rode you actually let out plus your boat's length, with room for GPS error. For example, 91 ft of rode and a 35 ft boat gives about 126 ft before the GPS allowance. If the alarm was armed after the boat fell back from the anchor, use about twice the swing distance or move the center to the anchor.

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