Rhumb Instruments: Boat Speed

Rhumb Instruments turns your phone into the helm instrument most small boats are missing: course over ground (COG) and speed over ground (SOG) in digits big enough to read from the wheel, with your average speed underneath.

A phone's raw GPS numbers are jumpy on the water. Chop, tide, and the motion of a small boat make the bare readout flick around, and two phones held side by side will disagree. Rhumb Instruments steadies both values the way a real marine instrument does. SOG is a short rolling average and COG is smoothed over the last seconds of track, so the digits settle on what the boat is actually doing.

It works out of the box. Open the app and the instruments are live, in knots and degrees true (°T). You pay once and everything is included.

THE INSTRUMENTS
• COG and SOG fill the screen, sized for a glance from the helm in bright light
• Both values are steadied for the water: a rolling average for SOG, a smoothed track for COG
• The screen stays awake while the app is open

AVERAGE SPEED
• Trailing average SOG over 1 minute, 10 minutes, and 1 hour, computed on the device from GPS
• The 1 minute window shows how the boat is doing right now; the 1 hour window shows what you will really average on a passage
• Useful for passage timing, race legs, and checking a new prop or sail trim

SIMPLE ON PURPOSE
• One screen, ready when you open it, with nothing to configure
• Works worldwide from the phone's GPS, and your position stays on your device
• Auto appearance follows the sun: day, twilight, and a red night face

MADE FOR
Sailors and motorboaters at RYA Day Skipper level and below, racers timing their legs, and anyone in a dinghy, RIB, or kayak who wants a steady speed readout. It is a GPS knotmeter built for the water.

Rhumb Instruments is built by a Yachtmaster Offshore skipper, the maker of Rhumb, the sea chart app. It is a one-time purchase.

Rhumb Instruments is an aid for recreation. It does not replace official instruments or safe navigation practice.

Recent changes:
The average SOG row shows a countdown while it is still warming up.
The settings button follows the face colours, including red at night.
Larger layout on tablets.
Website, privacy policy and support pages open inside the app.