Chartplotter Anchor Alarm vs Anchor Alarm App: Garmin, Raymarine & B&G Compared (2026)
Your chartplotter almost certainly has an anchor alarm buried in its menus — Garmin, Raymarine and B&G all ship one. So is a phone app redundant? Not quite: the two alarms fail in different ways, and the differences matter most at 3 a.m.
Quick answer
A chartplotter anchor alarm is a good backup, not a complete anchor watch. It sounds at the helm rather than in your bunk, keeps the electronics powered all night, and on most units the alarm circle is centred on the boat’s position when you arm it — not on the anchor. Most cruisers run a phone-based anchor alarm app such as Safety Anchor Alarm (free on iOS) as the primary watch and keep the plotter alarm on as a second, independent layer.
How a Chartplotter Anchor Alarm Works
The principle is the same on every brand: the plotter takes a GPS position, draws a circle of the radius you choose around it, and sounds a beeper if the boat’s position leaves that circle. It runs on the boat’s own GPS antenna and the boat’s own power — both genuine strengths.
The catch is in the first step. On most units the circle is centred on where the boat is when you arm the alarm, not on where the anchor actually lies. By the time you’ve finished snubbing off and go to switch the alarm on, you’ve usually fallen back a full rode-length from the anchor — so the circle sits offset behind the true swing circle. Swing towards the anchor and you can drift a long way before the alarm notices; swing the other way and you get false alarms. The workaround is to arm the alarm at the moment you drop, or pad the radius generously — both easy to forget after a long sail.
Where to Find It: Garmin, Raymarine and B&G
Garmin — “Anchor Drag” alarm
On most Garmin chartplotters the alarm lives under Settings > Alarms > Navigation > Anchor Drag: switch it on and set your radius. Note this is a chartplotter feature — the Navionics phone app, despite also being Garmin-owned, has no anchor alarm at all.
Raymarine — anchor alarm in LightHouse
On Axiom displays running LightHouse, the anchor alarm is configured from the Alarms menu. Recent LightHouse versions also added a more capable anchoring mode that asks how much rode you’ve deployed and sizes the swing circle from it — worth using if your software has it, because it partially fixes the centring problem.
B&G — anchor alarm on Zeus and Vulcan
On B&G Zeus and Vulcan plotters (and their Simrad siblings) the anchor alarm is switched on from the Alarms settings: enable it, set the radius, and the unit monitors position against the circle. As with the others, the circle is referenced to the vessel’s position at the moment you arm it.
The Three Problems a Plotter Alarm Has at 3 a.m.
- You may not hear it. The beeper is at the helm. Between a closed companionway, wind in the rigging and a tired crew, a plotter alarm two cabins away is a coin-flip. Some boats wire an external buzzer into the network to fix this — most never do.
- It keeps the electronics on all night. A plotter plus its instrument network can draw a couple of amps continuously. Over a nine-hour night that’s a meaningful bite out of a service battery you may want for the fridge and the windlass in the morning.
- The circle is centred on the boat, not the anchor. As above — unless you armed it at the moment of the drop, your safe radius is lying to you a little, in a direction you can’t see in the dark.
What a Phone-Based Anchor Alarm Does Differently
A dedicated anchor alarm app approaches the same job from the other end: the alarm lives on the device that sleeps within arm’s reach of you. Take Safety Anchor Alarm — our free iOS app, built for exactly this night watch — as the example:
- It marks the anchor, not the boat. You set the anchor point where you dropped, then let the boat settle back — the safe circle is centred where it should be, not on wherever the bow ended up when you remembered to arm an alarm.
- It wakes you. Safety Anchor Alarm keeps monitoring GPS in the background with the screen locked, and if the boat leaves the circle it sounds a loud, persistent alarm next to your bunk — heard through sleep in a way a helm beeper isn’t. Pair it with an Apple Watch and the alert reaches your wrist too.
- It costs the boat nothing. The whole electronics network can stay off; the phone runs the watch on its own battery (plug it into USB and it’s a non-issue).
- It works offline. GPS needs no internet, so the alarm works in the emptiest anchorage exactly as it does in a marina — see our guide to GPS accuracy for anchor monitoring.
The honest trade-off: phone GPS in a cabin is a little noisier than a masthead antenna, and a phone can be forgotten unplugged. Neither is disqualifying — you absorb GPS scatter in the radius, and plugging in becomes habit after one flat battery. As a rule of thumb, a safe anchor alarm radius for 5:1 scope in 5 m of depth is ≈ 40 m; our anchor scope calculator does the arithmetic for any depth and scope.
Chartplotter Anchor Alarm vs Anchor Alarm App
| Feature | Chartplotter alarm (Garmin / Raymarine / B&G) | Anchor alarm app |
|---|---|---|
| Alarm circle centred on | Boat position when armed | The anchor point you mark |
| Wakes you in the cabin | Depends — beeper sounds at the helm | Yes — loud alarm next to your bunk |
| Overnight power cost | Plotter + instruments on all night | Phone battery (plug into USB) |
| GPS source | Dedicated marine antenna | Phone GPS (3–5 m in the open) |
| Setup at a new anchorage | Menu-dive on the plotter each time | Two taps on the phone |
| Works with electronics off | No | Yes — fully independent of the boat |
| Extra cost | Included with the plotter | Free (Safety Anchor Alarm on iOS) |
The Setup Most Cruisers Run: Both, With Clear Roles
This isn’t really an either/or decision — the two alarms fail independently, which is exactly what you want from redundant safety gear:
- Primary: Safety Anchor Alarm on the phone beside your bunk, with the anchor point marked at the drop and the radius sized from your scope. It’s free, so the primary layer of your anchor watch costs exactly nothing to add tonight.
- Backup: the plotter alarm, armed as close to the drop as you can manage, with a slightly larger radius so it only speaks up if the primary has somehow missed a real drag.
If the phone dies, the plotter is still watching. If the plotter’s beeper can’t reach you, the phone can. Two independent GPS receivers, two independent power sources, one job. For the broader question of automated monitoring versus keeping a human watch, see anchor watch vs anchor alarm app.
The Bottom Line
Your chartplotter’s anchor alarm is worth switching on every single night — but as a backup. It watches from the helm, on the boat’s power, around a circle centred on wherever the boat happened to be when you armed it. The alarm that actually wakes you is the one next to your head, centred on the anchor you actually dropped — that’s the job Safety Anchor Alarm was built for, and it’s free. For a comparison of the app options themselves, see our anchor alarm app comparison.
Frequently Asked Questions
Do Garmin, Raymarine and B&G chartplotters have an anchor alarm?
Why do sailors use an anchor alarm app if the chartplotter already has one?
Is the chartplotter's GPS more accurate than my phone's?
Where is the alarm circle centred on a chartplotter anchor alarm?
Should I run both the chartplotter alarm and an anchor alarm app?
The Primary Alarm Your Plotter Backs Up
Free to download for iOS. Mark the true anchor point, background GPS monitoring, loud persistent alarm, works fully offline.
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