As a boat sails into the oncoming wind, the sail splits and bends the airstream. As long as the boat does not turn too close to the direction of the wind (the eye of the wind) the sail can bend the flow of air towards the stern of the boat. The wind's energy is utilized when the flow of air is bent this way resulting in forward movementof theboat.
As the windstream separates at the leading edge of the sail, (the luff), the flow of air passes along both the windward and leeward sides. While the flow of air tries to go straight, it is forced to follow the contour of the sail. The wind blowing across the windward side produces a pushing force referred to as drive. The wind flowing across the leeward, (back side) tries to pull away from the surface of the sail creating a lower air pressure area along the back of the sail. This pulling effect is called lift. The lifting force accounts for as much as 70% of the sailboat's power when sailing to windward.
The jib in the illustration is used to increase the flow of air across the back side of the mainsail, helping to create even less pressure. This produces more lift, increasing the pulling force to windward. The use of the venturi princi- pal in this way on sailboats is called the slot effect.
A strong aerodynamic force is exerted in a sideways direction by the wind as shown in the illustration. The keel, situated under the boat, prevents the boat from moving sideways by creating a lateral resistance force. These two forces combine to create the resultant force which moves the boat in a forward direction. The interaction of forces is what propels the boat up wind.
The sails should be trimmed so as to create the maximum lifting force. (See Trim) If the sail is out too far, the flow of air will not be bent as much as possible. If the sail is in too far, the flow of air will break away from the back side of the sail. Either case will result in less than maximum lift and less than optimum performance for the boat.
A different set of forces come into play when the boat is sailing downwind with the wind aft of the beam. The lifting effect is minimized since little wind travels across the leeward side of the sail. Most of the drive now comes from the wind acting as a pushing force on the sail.
Whether sailing up wind or down, the boat's performance depends on Sail trim. To achieve correct trim the Sail must be adjusted so it is neither over trimmed or under trimmed. Over trim and under trim are explained here So they can be avoided.
In the illustration the wind is coming directly across the beam. Here the Sails are in too tight resulting in a condition called overtrimmed. The wind is creating a pushing force against the sails but very little lift is created because the air can't flow smoothly across the sail's back side. As a result, the forces tend to heel the boat and push it sideways but do not provide much drive to propel the boat forward.
The sails should be eased out so that the air can travel across both sides of the sail smoothly. Then they will be able to generate lift and the boat will perform properly. The way to find the right trim is to ease the sails until they begin to luff. (The luff or forward part of the sail will start to puffin.) Then trim back in just enough to fill the sails and stop them from luffing.
Another way to correct for an over trimmed situation is to head up. When you get the boat pointed in the right direction, it will accelerate and heel over a bit more. This is due to the increased lift generated by the sails. Correct by heading up if you are beating. If you're reaching in the direction you want to go, ease the sails.
The sails are under trirnmed when they are eased out to far. The sail will luff (flutter) and the boat will slow down, standing up straighter in the water. Here the sails need to be trimmed in, to be filled, in order to get the boat moving. Luffing the sails can be done to slow the boat if desired, but should not be done too much or too long. Sails are damaged by extensive luffing.
Sailing Trim Guidelines.
Sailing Upwind: "When in doubt, ease it out."
Sailing Downwind: "Sails ninety degrees to the wind."
TACKING TO WINDWARD (BEATING)
The only way a sailboat can reach an upwind destination is through a series of tacks referred to as beating, or beating to weather. Tacking, or coming about, is changing the boat's direction so that the bow comes through the eye of the wind.
Since a 45 degrees is about the best a sailboat can point into the wind, tacking requires a turn of about 90 degrees.
Procedures For Tacking
I. Helmsman gives the command: "Prepare to tack." This gives the crew a chance to get ready and set up for the maneuver.
2. 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.
3. 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.
Note:When sailing closehauled, the mainsail is sheeted in tight so little or no adjustment of the mainsheet is necessary when tacking. As the boat begins turning up through the eye of the wind, the sails begin to luff. When the main fills on the other side, the tiller is straightened, and the boat assumes the new heading.
Gybing is the maneuver of changing tacks downwind It involves taking the stern of the boat through the eye of the wind. As the boat turns and the stern comes through the eye of the wind, the sails must be brought across to the opposite side of the boat.
Gybing is a maneuver that must be controlled! As the boat is turned, the boom must be controlled as it passes over the boat. If there is more than very little wind, the boom can swing across the boat with tremendous force. Crew members could be injured or the rig could be damaged in an uncontrolled situation. You can prevent mishap simply by pulling in the mainsheet and easing it out again as be boom passes from side to side.
Procedures for Gybing
Figure 1. 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."
Figure 2. On that command, both main and jib are sheeted in close to the centerline of the boat.
Figure 3. 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 in Figures 4 and 5.
Note: As the boom nears the centerline, the helmsman commands: "Gybe Ho." Ths alerts the crew that the boom will be crossing over.
Caution: The sails should be controlled while crossing over the boat. They should not be allowed to swing on their own.