open-crew

Free and open source rowing instrument web app

Measuring stroke rate and boat speed for a rowing shell used to require specialized hardware, including a seat magnet, impeller, and a purpose-built electronic device (i.e. the Cox Box, made by Nielsen Kellerman) to receiving the instrument signals over wired connections and translate them into readings on its display. Today the hardware required for the same functionality is packed into every smartphone, and the signals are available to any web app running in a modern browser. This project implements those rate, speed, and stopwatch instruments in a static web app running entirely in the browser. For any rower with a smartphone, having these instruments is now only a matter of mounting the phone and protecting it from the elements, not purchasing specialized electronics or paying for an app.

Instruments

Stroke Rate (SPM)

Reads the phone's accelerometer via the DeviceMotion API and counts strokes per minute.

Algorithm (based on Hermsen 2013, §4.3.2 and §4.4.2):

1. A very slow time-constant filter (τ = 3 s) tracks the gravity vector as the phone's orientation drifts. Seeded from the first sample — no calibration period needed.
2. Linear acceleration = accelerationIncludingGravity − gravity. Works on all devices including Android phones that do not expose DeviceMotionEvent.acceleration.
3. The vertical component is removed, leaving the horizontal (surge) vector — the fore-aft acceleration of the hull, perpendicular to gravity. Assumes the phone is fixed to the boat.
4. The dominant horizontal axis is identified from a slow EMA of each component's absolute value (τ = 2 s). The current sign of that axis is applied to the horizontal magnitude, giving a signed surge signal.
5. The signed signal is smoothed (τ = 80 ms). Strokes are detected as threshold crossings using a dynamic threshold (deepest of the 5 most recent troughs × 0.6, floor −0.3 m/s²).

Divergence from Hermsen §4.3: Hermsen adds a disambiguation window to pin the signal sign (catch = negative peak) for inter-node timestamp consistency. This app does not need that: SPM is a period measurement, so the sign does not affect it. The forward direction (bow vs. stern) is needed for the boat-device heading offset, but cannot be determined from the accelerometer alone — backing strokes produce the same signal as forward strokes with an inverted sign. Instead, the forward direction is derived from the GPS track at the first above-threshold speed after startup or a gyro-detected repositioning. The disambiguation window and signFlip state are therefore removed entirely.

7. Peak time is estimated as the midpoint of the threshold down-crossing and up-crossing, which is more stable than the raw signal minimum.
8. SPM = 60000 / rolling mean of the last 4 inter-stroke intervals. Cleared after 6 s with no stroke.

Tunable constants (src/lib/sensors/motion.svelte.ts):

All time-sensitive parameters use time constants, so behaviour is consistent regardless of whether DeviceMotion fires at 10 Hz or 60 Hz (which varies by device and browser).

Constant	Default	Effect
GRAVITY_TAU_S	3 s	Gravity filter time constant.
STROKE_TAU_S	0.08 s	Surge signal smoothing. Larger = smoother but laggier.
AXIS_TAU_S	2 s	Dominant axis tracking. Larger = more stable but slower to adapt.
SETTLE_TIME_MS	1500 ms	Wait before stroke detection (gravity filter settling).
THRESHOLD_FLOOR	−0.3 m/s²	Minimum dynamic threshold depth (prevents noise triggers at rest).
THRESHOLD_MULTIPLIER	0.6	Dynamic threshold = deepest recent trough × this value.
THRESHOLD_BUFFER_SIZE	5	Number of recent troughs used to set the dynamic threshold.
MIN_STROKE_INTERVAL_MS	800 ms	Minimum time between counted strokes (~75 SPM cap).
NOISE_THRESHOLD	1.5 m/s²	Linear acceleration threshold for "motion detected".
MOTION_TIMEOUT_MS	500 ms	Time below threshold before hasMotion clears.
REST_TIMEOUT_MS	6000 ms	SPM cleared after this long with no detected stroke.
BUFFER_SIZE	4	Intervals averaged for SPM output.

hasMotion flag: set true immediately when linear acceleration magnitude exceeds NOISE_THRESHOLD, cleared after MOTION_TIMEOUT_MS of sustained stillness. Used by the stopwatch to record a candidate start time when motion is first detected.

Boat Speed / Pace

Uses the Geolocation API (watchPosition with high accuracy) and displays speed as a split time in min:sec per 500 m — the standard rowing pace unit.

Algorithm: rolling average of speed samples over a BUFFER_TIME_MS (10 s) window. Each GPS fix reports speed directly via GeolocationCoordinates.speed (m/s). Samples are buffered and averaged; the result is converted to min:sec per 500 m for display. Fixes with accuracy worse than MIN_ACCURACY_M (20 m) or null speed are ignored.

Stopwatch and Distance

The stopwatch and distance counter share a single lifecycle managed by PieceTimer (src/lib/sensors/pieceTimer.svelte.ts). Both instruments start and reset together.

Tap Start to arm. Three auto-start paths:

1. Boat already under way (GPS speed ≥ 0.5 m/s) Rowing is certain — starts immediately with no confirmation wait. Used for the initial start when the boat is already moving, and equivalently for re-arming after a pause (resume). The distance counter seeds its integration from the current GPS speed at the moment of resume, so no distance is lost to the sub-second gap before the next GPS fix.

2. Boat at rest — two-stage detection

• Candidate start — when hasMotion first goes true while armed, the current time is recorded as the candidate start time. This corresponds to the beginning of the first drive off the catch.
• Confirmation — when the stroke detection algorithm completes its first threshold trough (up-crossing of the smoothed surge signal), rowing is confirmed and the stopwatch starts backdated to the candidate start time. This separates stroke confirmation from SPM measurement: SPM requires two troughs for an interval; confirmation requires only one.

If no stroke trough is detected within one maximum stroke period (STROKE_CONFIRM_TIMEOUT_MS = 60000 / MIN_ROWING_SPM), the candidate start is discarded as a false positive. Low acceleration between strokes (e.g. the boat coasting after the release) does not trigger a false positive — only the absence of a confirmed stroke within the timeout does.

Tap Pause to pause; tap Resume to re-arm. Tap Reset to return to the initial state.

Piece timer tunable constants (src/lib/sensors/pieceTimer.svelte.ts):

Constant	Default	Effect
MIN_ROWING_SPM	15 SPM	Slowest expected stroke rate. Sets the false-positive timeout to one full stroke period at this rate.
MIN_UNDERWAY_SPEED_MS	0.5 m/s	GPS speed threshold above which the boat is considered under way and stroke confirmation is skipped.

Distance

Integrates GeolocationCoordinates.speed over time using trapezoidal integration. Stays in sync with the stopwatch: same start time, same reset.

Speculative buffering: when the pending confirmation window opens (boat at rest, motion detected), GPS speed samples are buffered speculatively. On confirmation, the buffer is integrated from the candidate start time as the distance seed — so distance from the first drive is not lost during the ~1-stroke wait for confirmation. The leading interval (from candidate start to the first GPS fix) is extrapolated at the first fix's speed. On a false positive, the buffer is discarded.

GPS fixes with accuracy worse than MIN_ACCURACY_M (20 m) are excluded from integration, matching the pace gate.

Deployment

The app is a fully static SvelteKit build. A GitHub Actions workflow (.github/workflows/deploy.yml) builds and deploys to GitHub Pages on manual trigger. The base path (/repo-name) is injected automatically from the repository name.

To deploy to a custom domain or a root path, set BASE_PATH= (empty) in the workflow environment and configure your hosting accordingly.

Development

yarn dev      # local dev server
yarn check    # svelte-check type checking (run before committing)
yarn build    # production build

Requires HTTPS on mobile for DeviceMotion and Fullscreen APIs. Use npx local-ssl-proxy or similar to test on a physical device against the local dev server.