Tacking If boats were able to sail only before the wind and off the wind, it would be impossible to reach a destination upwind from the starting point. By sailing on the wind, however, a sailboat can make a course approximately 45° away from the wind direction. By sailing a succession of such courses, first to the left and then to the right of the wind direction, a maneuver called tacking, sailboats can zigzag in an upwind direction. A vessel is said to be on the starboard tack when sailing so that the wind is blowing from the right or starboard side, and to be on the port tack when the wind is blowing from the left or port side. 
To tack, The boat is steered so that its bow points up into the wind and then away from the wind on the opposite tack. As the boat points into the wind, it loses speed, the sails being pressed directly backward by the wind. Then as the bow moves away from the wind on the other tack, the sails fill with wind again and assume a position on the other side of the vessel. During the time of coming about, the boat is receiving no motive force from the wind; it must rely on its inertia to maintain enough speed so that it can be steered onto the opposite tack. When the boat does not have sufficient inertia, and stops with its bow pointing into the wind and its sails useless, it is said to be in stays. For those boats which have a tendancy to be in stays, it is usefull not to ease the genova (or flock) sheet until the boat turns fully to the opposite tack. This helps the boat to turn, as the boat is in the new tack ease the sheet and trim the oppsite genova sheet. Procedures For Tacking
Helmsman gives the command: "Prepare to tack." This gives the crew a chance to get ready and set up for the maneuver. Helmsman puts the tiller over towards the mainsail, (leeward side) and gives the command: "Helms-a-lee". This notifies the crew that the boat is being turned. The bow comes up through the eye of the wind, the sails come across to the other side of the boat, and as the sails fill, the tiller is brought back to the middle of the boat, the new heading is assumed
The other method of changing tack consists of steering the boat away from the direction of the wind (bearing away), until the wind fills the sails from the other side and the boat is on the other tack. 
In fore-and-aft-rigged vessels, this maneuver is called jibing or gybing, and in square-rigged ships it is known as wearing. When running before the wind, a slight shift of wind may cause a boat to jibe unintentionally. Such jibing is dangerous because of the speed with which the heavy booms, or spars, at the foot of the sails sweep across the decks of the vessel from one side to the other, and also because of the danger of breaking spars. In wild jibing, control can be lost momentarily and, if the seas are high, a small boat can broach, that is, veer on its side with danger of swamping or capsizing. An unintentional jibe in a heavy wind frequently has enough force to break the masts of a vessel. When jibing intentionally, careful sailors always haul in on the boom while turning, so that they can travel only a short distance when the wind reaches the other side of the sails. Procedures for Gybing
When the boat is on a broad reach, (wind off stern quarter) the boat will be turned to where the centerline of the boat will pass through the eye of the wind. The helmsman gives the command: "Prepare to Gybe. On that command, both main and jib are sheeted in close to the centerline of the boat. As the boat turns further, the wind crosses to the other side. The sails are carefully eased out to their proper position for the new heading
Note: As the boom nears the centerline, the helmsman commands: "Gybe Ho." Ths alerts the crew that the boom will be crossing over. - Gybing is a maneuver that must be controlled! -
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