Before leaving the dock or mooring, open Boat Knot Meter and confirm your iPhone has a GPS lock. The app should display your current position and speed. In areas with clear sky visibility, GPS locks within 30–60 seconds. If you are anchoring in a narrow fjord, between tall buildings on a marina, or under dense cloud cover, GPS accuracy may be reduced — allow more time for the signal to stabilise.

An anchor watch running overnight will use battery. If your iPhone is at 20% when you anchor, it will not last through the night. Aim for 80% or above before starting a long anchor watch. A 12V USB adapter or power bank eliminates the battery concern entirely and is worth having on board.

Before setting your alarm radius, think about your specific situation:

Use your standard anchoring procedure. Drop the anchor, let the rode out to your chosen scope, back down on the engine to set the hook, and verify the anchor is holding by watching transits (fixed objects aligned on shore) or watching your GPS position for any movement.

After the anchor is set, wait 2–5 minutes for the boat to find its natural resting position in wind and current. The boat will swing back on the rode and come to rest in a predictable position. Setting the anchor watch before this happens will give you a slightly wrong reference point.

Open the app on your iPhone. You will see your current GPS position. If the speed display shows 0 knots or near-zero, the boat has settled and GPS is locked onto your current position accurately.

The main screen of Boat Knot Meter has an anchor watch activation control. Tap it to lock in your current GPS coordinates as the anchor point. The app confirms the point has been set. This is now the centre of your alarm circle — your boat needs to remain within the radius you set from this exact position.

This is the most important step. Getting the radius right requires thinking about your specific situation.

The minimum practical radius should be equal to the length of your rode deployed (your swing radius). If you have 30 metres of rode out, the boat can swing in a circle 30 metres from the anchor. Setting the radius to 30 metres means you will get an alarm every time the boat swings — that is too tight.

A practical formula: Set the radius to the rode length plus 50–100% extra. For 30 metres of rode, set the radius to 50–60 metres. This gives you early warning of genuine drag without constant false alarms from normal swinging.

In crowded anchorages where space is tight, you may need to reduce this margin. In open anchorages with plenty of room, you can set a wider radius to filter out normal oscillation entirely.

For the anchor watch to work while you sleep or go below, the app must be allowed to run in the background. When you set the anchor watch in Boat Knot Meter, the app requests background GPS access. Grant this permission. You will typically see a blue GPS indicator in your iPhone status bar showing the app is actively monitoring.

Before leaving the cockpit, take a moment to verify the alarm is active. You can do this by momentarily increasing the radius beyond your current position and then restoring it — the alarm indicator should respond. You do not want to discover at 3 AM that you forgot to activate background mode.

The anchor watch is active. Boat Knot Meter is monitoring your position continuously. You can put the phone down, lock the screen, and go to sleep. If the anchor drags, you will be woken by the alarm.