Skiff is a server-authoritative sailing simulator for the Lagoon 450S catamaran, built in Rust with an optional 3D React/Three.js cockpit. The core claim is a physics-first system where a headless 20 Hz six-DOF simulation owns all vessel state (pose, velocity, stability, grounding), while the browser contributes optional aerodynamic forces from a cloth-sail solver that gracefully degrades to a coefficient model when stale. It ships with isochrone routing, a native MCP endpoint for Claude integration, Signal K/OpenCPN interop, and command-line tools for polar sweeps and route planning. The source emphasizes it's not certified for real navigation—it's a research platform. The architecture is clean: browser sends control input and sail wrench to Axum HTTP API, server integrates physics, broadcasts state snapshot back. Deployment covers local dev (npm + cargo), Docker, and a Raspberry Pi runbook. Tests, type checking, and documentation exist. It's genuinely functional, not a stub, and the engineering discipline around coordinate frames and staleness-handling is visible.