Underway alerts

How the obstacle alarm and heading reminder work, and their limits.

These are aids with real limits. They don't detect other boats, logs, debris or anything uncharted. They can miss hazards, sound when there's no hazard, and fail if your connection or position drops. Never rely on them to keep your boat safe. Keep a lookout.

Set them up

Go to You, then the gear, then Underway. Obstacle alarm and Change-heading reminder both start on. Set your minimum depth in Edit profile.

What the obstacle alarm checks

While you're moving at 0.5 knots or more, it looks within 70° either side of your course for charted rocks, wrecks and obstructions, land, and water shallower than your minimum depth.

It tells you what and where, for example "Rock ahead, 300 yards NE". As you get closer, it gets more insistent:

Distance	Banner	Sound
500 to 201 yards	Pale	A chime every 12 seconds
200 to 131 yards	Orange	A siren every 2.4 seconds
130 to 71 yards	Red	A siren every 1.1 seconds
70 yards or less	Larger red	A tone every 0.42 seconds

It sounds even with your device on silent, and the screen edge flashes at the closer steps.

When it sounds

OK quiets this step for this obstacle. It sounds again if you get closer.
Turn off alarm turns the whole alarm off, with no confirmation. Turn it back on in Settings, or with the bell in the heads-up view.

If you're going close in on purpose, such as coming into an anchorage, turn it off first.

Test it

Tap Test alarm in Settings. It counts down from 500 to 40 yards in about 22 seconds, then keeps sounding until you answer it. Tap OK to end it. Turn off alarm really does turn the alarm off.

Change-heading reminder

If you've held the same heading, within 10°, at 1 knot or more, it chimes and shows "Time to change heading". The default is 10 minutes, and you can set it from 1 to 120. Tap OK to restart the clock.

Common problems

It says "No chart hazard data here."

There are no NOAA hazard charts for this area, so the alarm uses depth only. That includes Canadian waters.

It says "Can't reach NOAA."

The alarm can't get the data and may miss things. Keep a sharper lookout until it comes back.

Why does it warn about water I know is deep enough?

Depths come from a coarse model and don't include the tide. It can also be set off by land close to your course. Check your minimum depth.

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