Sailing by the lee

The situation

Running downwind, Rishi heats it up and sails by the lee — the wind is now blowing over the same side as his sail. Another boat runs nearby on the other gybe.

The question

When you're by the lee, which tack are you on for the rules?

The ruling

Your tack is set by the side the wind is blowing across, which is the windward side — defined as the side opposite the mainsail. Sailing by the lee, the wind crosses the leech first, but the definition still puts your leeward side on the boom side — so your windward side (and your tack) is the side opposite the boom, just as it is sailing dead downwind. It's easy to misjudge downwind, and getting it wrong leads to port/starboard fouls under rule 10. If you're not certain which tack you're on, assume the worst and keep clear.