(a) When two sailing vessels are approaching one another, so as to involve risk of collisionThe term risk of collision is described in Rule 7. 'Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist.' More, one of them shall keep out of the wayRule 16 Action by give way vessel - Every which is directed to keep out of the way of another vessel shall, so far as possible, take early and substantial action to keep well clear. More of the other as follows;
(i) when each has the wind on a different side, the vesselThe word 'vessel includes every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water'. which has the wind on the port side shall keep out of the way of the other;
(ii) when both have the wind on the same side, the vessel which is to windward shall keep out of the way of the vessel which is to leeward;
(iii) if a vessel with the wind on the port side sees a vessel to windward and cannot determine with certainty whether the other vessel has the wind on the port or on the starboard side, she shall keep out of the way of the other.
(b) For the purposes of this Rule the windward side shall be deemed to be the side opposite to that on which the mainsailThe mainsail is the sail attached to the main mast of a sailing vessel. In most cases, it is the largest sail on the boat. In this image, the mainsail is the large sail aft (backward) of the mast. More is carried or, in the case of a square-rigged vesselA square-rigged vessel uses sails attached to a spar that is positioned perpendicular (or square) to the mast. More, the side opposite to that on which the largest fore-and-aft sailAn example of the largest fore-and-aft sail on a square-rigged vessel is highlighted with a red box in the image. In this example the sail in question is carried on the starboard side of the vessel. More is carried.