A sail does not drive forward because the wind strikes it: it generates lift thanks to the difference in pressure between its two sides. Measuring that difference, point by point across the fabric, means reading the real work of the sail. It is the physical principle on which SailSensor is built.
SailSensor Pressure Discover the system
A sail in the wind works like an aircraft wing set vertically. The airflow that hits it splits in two: on the leeward side (the convex side, downwind of the direction the wind comes from) the air accelerates and the pressure drops, creating a low-pressure zone; on the windward side (concave) the air slows down and the pressure rises, creating a high-pressure zone.
The force that results — lift — is the direct consequence of this imbalance. In rigorous terms, the aerodynamic drive is the integral of the pressure difference between the two sides across the entire sail surface. Where the difference is large, the sail is working hard; where it vanishes, that portion of cloth is producing no drive or is close to stalling.
Measuring the pressure on just one side of the sail says little: that figure depends on height, on wind speed, on the position along the leech, and does not isolate the contribution that actually generates the drive. What describes the sail's work is the difference between windward and leeward, read at the same point and at the same instant.
It is this quantity — the local differential pressure — that is the physical signal telling you how much and where the sail is loading up. Measured at several points, it draws a genuine aerodynamic load map: the objective portrait of how the fabric is working, moment by moment.
A boat's traditional instruments measure everything around the sail, never the sail itself. The anemometer reports the apparent wind at the masthead; the GPS reports the track and speed over the water; the angle sensors report the position of the rig. These are valuable data, but indirect: they describe the environment and the result, not the work of the sail profile that produces that result.
Until now, the aerodynamic load on the sail was estimated by the racer's eye, by the shape of the telltales, by the feel of whoever is on the helm and the sheets. SailSensor shifts the measurement from the environment to the surface: it brings the sensor directly onto the sail and does so in differential form, capturing exactly the quantity that generates the drive. It is a distinctive, patented approach that opens up a field of measurement conventional instruments do not touch.
The principle is clear; the challenge is to measure it without altering what is being measured. SailSensor solves this with compact, low-profile sensors that read the differential pressure directly on the fabric, keeping disturbance to the airflow to a minimum.
The sensors attach to the sail surface without holes or damage, and are removed without any invasive work.
Each sensor transmits the differential pressure over BLE, with no cables along the sail or on the rig.
The SailSensor app for Android (iOS version in development) turns the readings into a pressure map you can read while sailing.
The result is objective, repeatable data. Where before there was a feeling, now there is a measured quantity: you can compare one adjustment with another, find the setting that works again and understand which part of the sail is delivering. Trim stops being experience alone and becomes measurement too.