DrifterBuoy CMU-30

The DrifterBuoy CMU-30 is a free-floating GPS current meter. It drifts with the water and works out the speed and direction of the current from its own movement, recording every GPS fix to an SD card and showing the result on the device and on your phone.

A green drogue below the waterline is what keeps the buoy following the water rather than the wind — it is the difference between measuring the current and measuring the weather.

Current firmwarev1.6.02026-09-29

At a glance

Measures	Current speed in knots, current direction in degrees
Method	GPS speed and course over ground, averaged over a measurement window
Recording	One row per GPS fix, CSV on SD card
Results	Average speed and direction, deviation figures, sample count
Connectivity	Own WiFi network with a built-in web interface and firmware updates, plus Bluetooth Low Energy 5.0 for the phone app
Display	On-device screen, plus the web page on your phone

Physical

Buoy	125 mm diameter, 90 mm high
Drogue, deployed	Hangs 600 mm below the buoy
Packed	270 × 240 × 175 mm (l × w × h)
Weight, as supplied	2.35 kg, the complete kit in its bags
Battery	Lithium, 3.7 V 1200 mAh (4.4 Wh)
Battery life	Three to four days in normal use
Water resistance	Waterproof, with the charging connector closed and sealed

The drogue is the figure to plan around: it hangs 600 mm below the buoy.

Weight is for the whole kit

2.35 kg is everything as supplied, including both tubes, the drogue and the cable. The buoy you actually lift into the water weighs less.

Travelling with the buoy

The kit is built to travel. Packed, it fits within normal cabin baggage allowances, and at 2.35 kg it is light enough to carry as hand luggage.

The battery matters more than the size. Airlines restrict lithium batteries by watt-hours, and the usual cabin limit is 100 Wh. At 4.4 Wh this battery is roughly twenty times below that, so it does not need airline approval the way a large power bank does. It is smaller than the battery in most mobile phones.

One practical point: carry it in the cabin. Airlines require lithium batteries in hand luggage rather than in checked bags, and the buoy is no exception.

Documentation

Getting StartedFit the drogue, charge, connect your phone and run your first measurement.

User GuideThe measurement cycle, reading results, settings and log files.

TroubleshootingGPS fixes, iPhone WiFi, invalid measurements and battery life.

FirmwareCurrent version, release notes and updating over WiFi.

Android AppControl the buoy over Bluetooth, watch the current live and map your measurements.

DownloadsThe DrifterBuoy Android app.