Wind: The wind is what powers a sailboat. Both the direction and strength of wind is important in setting the sails and maintaining control of the boat.
True-wind direction is different from apparent-wind direction. The true-wind is the direction of the wind which makes the waves. The true-wind is perpendicular to the waves. The apparent-wind sails the boat. When the speed of the boat and the velocity of the wind are close, the difference between the apparent and true-wind is the greatest. The apparent wind is forward of the true-wind, except when sailing directly into or away from the true-wind. As one sails faster, the apparent-wind is drawn further forward. When sailing with the wind the apparent-wind has less of a force that the true-wind. When sailing against the wind, i.e., in a close reach, the apparent-wind has a greater force than the true wind.
In general, the closer you sail to the wind, the closer the sails are pulled or trimmed to the midline of the boat. As you sail away from the wind, the sails are progressively let out. The exact position of the sails are based upon the direction and speed of the apparent-wind. (The direction of the apparent-wind is determined by the sailboat's tack [or relationship to the true-wind] and the relationship of the speed of the true-wind to the speed of the boat.) For the beginner sailor, it is easy to remember the five basic directions of sail (points-of-sail) each of which has its distinct characteristics of speed, heel and sail position.
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It is easy to think of three basic sailing conditions
Finally one may turn the boat directly into the wind or In Irons: Since a sailboat cannot sail directly into the wind, this is usually done to stop the boat. In this position the wind cannot power the sails. A sailboat cannot sail directly into the true-wind. Most boats can only sail 40 degrees to the wind but some boats can sail as close as 30 degrees.
When sailing on a reach, the experienced sailor, trims the sails based upon the apparent-wind. This is important because sails cannot be efficiently positioned based upon the point-of-sail or the direction of the true-wind. An extreme example is ice-sailing on a broad reach, a sudden strong gust of wind can cause the craft to reach high speeds and cause apparent-wind conditions similar to a close reach.
Trimming the Sails on a Close Reach
On a close reach, one method of testing mainsail and jib position is to let the sail out until it luffs (the leech starts flapping) then pull it in (or trim the sail) until the sail is steady. It is easier to tell if the mainsail and jib are let out too much, than if the sail is over trimmed (pulled in). Telltales may also be used as a guide. Telltales are small pieces of yarn attached to the sail. Although telltales are helpful when sailing on a reach, they are not useful when running (sailing directly downwind). In a run, the wind should be perpendicular to the sail.
Whenever the sails are trimmed using the sheets a change in sail shape occurs. Other lines and the "fair lead" may have to be adjusted to maintain sail shape. See section on sail shape.
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