Fleet racing in the browser, in real 3D, under the World Sailing Racing Rules of Sailing 2025-2028. A sailboat is a two-fluid problem with a closed feedback loop: air drives the sails, water resists the hull and keel, and the boat's own velocity changes the apparent wind, which changes the driving force, which changes the velocity. The apparent wind is computed, never prescribed, and both force balances are solved together, so three results emerge rather than being written in - a boat can sail faster than the true wind on some points of sail, no boat can sail directly upwind, and the best velocity made good sits at a finite angle from which pointing closer always costs progress. Four real boats carry their published ORC certificate measurements and polar diagrams: the J/24, the Melges 24, the J/70 and a named TP52. Steer and trim, tack and gybe, set the spinnaker, and race a fleet through the rule 26 starting sequence, Part 2 right of way, rule 18 mark-room, rule 44.2 Two-Turns Penalties and Appendix A scoring. Sources: the sail lift and drag coefficient tables, the depowering scheme, the Hughes/ITTC-57 skin-friction line and the induced-drag formulae are taken verbatim from the ORC VPP Documentation 2026 published by the Offshore Racing Congress; the rules are World Sailing's Racing Rules of Sailing for 2025-2028, Clean Version v1.1; the boats and their polars come from the ORC public certificate service. Five constants are fitted to five published numbers and 427 numbers across the four polars are predictions, including every beat angle. An independent reimplementation, not endorsed by World Sailing, the Offshore Racing Congress, J/Boats, Melges or any class association; full credits and the honest list of what is reconstructed are in the app's Help panel and in CREDITS.txt. Runs entirely client-side: no account, no server call, no credits.

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