Knowing in real time how many degrees the boom is open relative to the boat's axis is one of the most useful — and hardest to measure well — pieces of data in sail trim. With SailSensor BoomAngle we tackled the problem with an approach that led to a patent filed with the Italian UIBM. This is the technical story of how we got there.
Proprietary Moxá Design project · Domain: marine / low-power IoT · Status: patent filed, product in production.
The classic solution — an encoder or a potentiometer at the base of the boom (gooseneck) — requires wiring along the mast, suffers in the marine environment and wears out at the very point of the rig under the most stress. Unsuitable for a racing boat.
Putting just one inertial sensor on the boom seems obvious, but it measures the boom's orientation relative to the earth, not relative to the boat. When the hull rolls, pitches and heels, that reading gets contaminated: the boat's motion adds to the boom's and you can no longer tell them apart.
MEMS gyroscopes drift over time. On a passage lasting hours, an angle that was correct at the start of the tack becomes progressively wrong. For a sail-trim reading, an error that grows in silence is worse than no reading at all.
No cables along the mast. The sensor lives on the boom and must communicate wirelessly with the rest of the boat.
Battery powered. Replacing the cell every season is not acceptable: it needs an average consumption on the order of microamperes.
Spray, temporary immersion, sun, salt and pounding. The housing must survive on deck for years.
Below this threshold the reading is useful for fine trim; above it, it's only a rough indication.
It must measure the boom angle relative to the hull, whatever the boat does underneath.
The breakthrough was to stop asking one sensor to measure an absolute angle, and to use two: a reference unit fixed to the hull and one on the boom. Both estimate their own orientation as a quaternion; the system then computes the differential quaternion between the two.
The advantage is that everything the two units have in common — the boat's roll, pitch and heel — cancels out in the difference. What remains is the boom angle relative to the hull, cleaned of the vessel's motion. And because the two gyroscope drifts are correlated, the drift too is largely compensated instead of accumulating: this is precisely the heart of the patent claim.
The fusion algorithm (accelerometer + gyroscope + magnetometer) runs directly on the Nordic microcontroller in Zephyr RTOS, in fixed point where needed, to stay within the energy budget.
Instead of maintaining a BLE connection, the sensor transmits in advertising mode: no pairing, no handshake to renegotiate, and the data can be received simultaneously by the app and the onboard bridge. It's the choice that cuts power consumption the most.
The same packet feeds the SailTrim Helper app on the sailor's smartphone and the NMEA2000 bridge, which brings it to onboard chartplotters and displays.
A boom angle sensor with no cables to break, that doesn't drain the battery by season's end, isn't fooled by the boat's motion and doesn't drift over time. The differential fusion technology is now the subject of a patent application filed with the Italian UIBM.
A single IMU measures orientation relative to the earth and includes the boat's motion. With two units — a reference one on the hull and one on the boom — the system computes the differential quaternion: the vessel's common motion cancels out and only the boom angle relative to the hull remains, with the gyroscope drift largely compensated.
The design target is an error below 0.5° RMS, the threshold beneath which the reading is usable for fine sail trim and not just as a rough indication.
Via BLE in advertising mode (connectionless): no pairing to manage and consumption kept to a minimum. The same reading is received both by the SailTrim Helper app and by the NMEA2000 bridge that distributes it to onboard chartplotters and displays.
SailSensor BoomAngle is a proprietary Moxá Design product, part of the SailSensor ecosystem, with technology that is the subject of a patent application filed with the Italian UIBM. More information on the dedicated site sailsensor.it.
Inertial sensing, data fusion, ultra-low-power BLE devices for harsh environments: if you have a physical quantity that's hard to measure well, that's exactly the kind of problem we love to solve. The initial discovery is free.
Tell us about your projectBoomAngle, Pressure, SailTrim Helper, NMEA2000 bridge: discover all the products.
Go to products →