Nemesis Delta (signalk-nemesis)

A Signal K plugin and webapp that shows how nearby competitors (your nemesis boats) are sailing relative to you during a race. It answers three questions: who is going faster toward where we are heading, who is gaining or losing on us, and who is beating us on handicap. It works from AIS, and for CYCA races it can add the CYCA race tracker for boats out of AIS range.

| Boats | TCF | Vectors | Geo | | --- | --- | --- | --- | | | | | |

Six hours into the 2026 Sydney Gold Coast race, from recorded AIS data.

Demo video: Nemesis Delta on YouTube.

Install

In the Signal K admin UI: Appstore > Available, find Nemesis Delta, Install, then restart the server. Then Server > Plugin Config > Nemesis Delta: enable it, and optionally enter your nemesis boats in advance (MMSI, name, TCF).

The plugin needs Node 22 or newer; the server's Node version is shown at the bottom of the admin UI. Without internet on the boat, see Installing from a package file.

Use

Open the webapp from Webapps in the admin UI, or at http://<boat computer>:3000/signalk-nemesis/, on a phone, tablet or the nav PC. Every screen shares the same nemesis list, TCFs and course, kept on the server.

Across the top:

• Up: what the charts measure toward. Waypoint (the active waypoint from the chart plotter or nav PC), Course (the marks set on the Course tab), COG, North, or Upwind / Downwind (into or away from our true wind direction, smoothed over 2 minutes).
• Mode: Absolute, or Relative to us.
• Time: live, or step back through the race, and the period the charts show.
• Show: which boats appear: nemeses only, sailing boats, all, those in range, or none.

The tabs (each has an i button that explains it):

• Boats: each boat's VMG and lead toward Up as strip charts. Tap a boat's icon to make it a nemesis.
• TCF: time-on-time handicap. Enter each boat's TCF and ours, and see who is beating us on corrected time. The race start comes from signalk-racer, or set it here.
• Vectors: every boat's speed and direction, with Up at the top.
• Geo: the boats on a map, with Up at the top. The OSM and Sea maps need internet. The Course box shows the course from the Course tab, and with true wind the Wind box shows a wind ladder: lines across the wind, in nm upwind of us.
• Course: the marks for Up = Course. Add waypoints made in your chart app, or import a route.
• CYCA: pick a CYCA race to add tracker positions every 10 minutes (needs internet), and link each tracker boat to its AIS boat so they show as one.

A boat not heard for a while (30 minutes by default) is greyed and its lines broken.

More

The wiki has every configuration option, the plugin's interfaces, development and testing, and the project brief. Changes are in ReleaseNotes.md.