AC42 Pilot — SignalK Plugin

Control a Simrad AC42 autopilot (and likely other Simrad/B&G NAC-2 / NAC-3 / AC12 calculators) directly over NMEA 2000, from a mobile-friendly web app.

Built for real sailing conditions: big buttons, wet fingers, glanceable at a distance.

Features

• Auto (heading hold), Wind (apparent wind angle hold), Standby
• Course corrections ±1° / ±10°
• Tack, with a long-press confirmation to avoid accidental triggers
• NFU (non-follow-up / direct helm) in Standby
• Animated wind rose
• 1-hour TWD history strip
• SOG / COG / TWS / AWS / TWA instrument tiles

Compatibility

• Confirmed: Simrad AC42
• Likely (untested — feedback welcome): NAC-2, NAC-3, AC12, and other Simrad/B&G NMEA2000 autopilot computers using the same "Simnet" command set
• Requires SignalK with NMEA2000 access over SocketCAN (can0), plus the canboat tools (candump, candump2analyzer, analyzer) and can-utils (cansend) installed on the host. This is standard on OpenPlotter.

If you run this on a different autopilot brand/model, PGN 130850 commands will simply be ignored — it's harmless, but it won't do anything either. Please open an issue with your results either way.

How it works

The AC42 only accepts NMEA2000 command frames (PGN 130850, "Simnet: AP Command") when they're sent from the CAN source address of a currently active B&G controller (a physical MFD or autopilot head that's emitting the PGN 65305 heartbeat). Commands sent from an arbitrary or fixed address are silently ignored.

This plugin detects the currently active controller address on the bus and emits commands "on its behalf" — the real controller head keeps working normally in parallel, nothing is spoofed or disabled. This detection is fully automatic and adapts to your boat's setup (see Configuration below).

Installation

Via SignalK App Store (recommended): in the SignalK admin UI, go to the Appstore tab and search for AC42 Pilot.

Via npm:

cd ~/.signalk
npm install signalk-ac42-autopilot

Then restart SignalK.

Configuration

All settings are optional — the plugin auto-detects everything needed on a standard setup.

| Option | Default | Description | |---|---|---| | canInterface | can0 | SocketCAN interface name | | apAddress | auto-detected | NMEA2000 source address of the autopilot computer. Override only if auto-detection picks the wrong device. | | windDirectionSource | auto-detected | SignalK source for True Wind Direction shown in the app. Falls back to any available source if the preferred one is silent for >8s. | | fixedControllerAddress | auto-detected | Force a specific active-controller address instead of dynamic detection. | | staleMs | — | Timeout before considering a source "stale" for fallback purposes. |

Security / Disclaimer

• This plugin sends real autopilot commands over your NMEA2000 bus. Test at the dock before relying on it underway.
• It does not replace proper watchkeeping. You remain responsible for the safe operation of your vessel at all times.
• If SignalK security is enabled, the API and web app require a logged-in session.
• Use at your own risk. See LICENSE for the full disclaimer of warranty.

Known limitations

• Nav mode (route following) is not implemented.
• Compatibility beyond the AC42 is unconfirmed — feedback from other Simrad/B&G setups welcome.
• TWD history is client-side only (resets on page reload, no server-side persistence).
• Tack: heading target flips with the tack; expect a small rounding gap (±1°) vs. the physical pilot.

Credits

Protocol reverse-engineered from scratch by capturing and decoding NMEA2000 traffic on a real boat. Thanks to the canboat and SignalK projects and communities.

License

Apache-2.0 — see LICENSE.