When two sailing boats are on the same tack, the boat to windward keeps out of the way of the boat to leeward (COLREGs Rule 12). The leeward boat is the one further downwind, and is often less able to get out of the way.
Rule 12 of the collision regulations has a second part, for two sailing boats with the wind on the same side: the vessel which is to windward keeps out of the way of the vessel which is to leeward.
The windward boat is the one nearer the direction the wind is coming from; the leeward boat is the one further downwind. Picture two boats both on starboard tack, sailing roughly side by side. The one upwind must keep clear.
There are good reasons for the rule. The windward boat has more freedom: she can bear away behind the other boat, or ease her sheets, without having to tack. The leeward boat may be close-hauled already and unable to head up further without stalling. The leeward boat may also be slowed by the windward boat’s wind shadow. And remember that the windward boat’s own sails hide the water to leeward, so her crew must look under the jib to keep the leeward boat in sight.
How to keep clear when you are to windward:
- alter course early, usually by bearing away to pass astern, or by heading up a little to open the gap if there is room;
- make your change of course big enough to be obvious;
- remember that on a run or broad reach the leeward boat may gybe or luff up, so leave plenty of room.
The windward rule only applies between boats on the same tack. If the boats are on opposite tacks, the port-and-starboard rule decides it instead, whichever boat is to windward. And if one boat is overtaking the other, the overtaking rule (Rule 13) applies: the overtaking boat keeps clear, even if she is the leeward one.
The Racing Rules of Sailing have an equivalent rule (rule 11) for boats in the same race, with extra detail about overlaps.
More in the book: Chapter 13, The rules of the road