Point of sail - WikiHQ

300px|thumb|Points of sail and approximate [[apparent wind for a conventional sailboat on starboard tack]]

A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over the surface.

The principal points of sail roughly correspond to 45° segments of a circle, starting with 0° directly into the wind. For many sailing craft 45° on either side of the wind is a no-go zone, where a sail is unable to mobilize power from the wind. Sailing on a course as close to the wind as possible—approximately 45°—is termed beating, a point of sail when the sails are close-hauled. At 90° off the wind, a craft is on a beam reach. The point of sail between beating and a beam reach is called a close reach. At 135° off the wind, a craft is on a broad reach. At 180° off the wind (sailing in the same direction as the wind), a craft is running downwind.

A given point of sail (beating, close reach, beam reach, broad reach, and running downwind) is defined in reference to the true wind—the wind felt by a stationary observer. The motive power, and thus appropriate position of the sails, is determined by the apparent wind: the wind relative to an observer on the sailing craft.

The points of sail

The recognized points of sail are judged relative to the true wind direction. The subcategories of these two situations include:

Into the wind where a sailing craft is pointed directly upwind in the middle of the no-go zone, where sails cannot generate power.
Close-hauled means a boat is sailing at the sharpest angle possible toward the wind without entering the no-go zone, where sailing isn't possible.
Reaching, including:
Close reach: between close-hauled and a beam reach.
Beam reach: the craft has the true wind at a right angle to its direction (on its beam).
Broad reach: the true wind is coming from behind, but not directly behind.
Running downwind where a craft has the wind coming from directly behind.

File:Shrike-port-beam.jpg|Close-hauled

File:Shrike-reaching.jpg|Reaching

File:Shrike-running.jpg|Running downwind

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Into the wind

thumb|Iceboats parked in irons with sails loose and not generating power, but flapping like a flag.

The range of directions into the wind, where a sailing craft cannot sail is called the no-go zone. A sailing craft cannot sail directly into the wind, nor on a course that is too close to the direction from which the wind is blowing, because the sails cannot generate lift in this no-go zone. A craft passing through the no-go zone to change tacks from one side to the other, must maintain momentum until its sails can draw power on the other side. If it remains in the no-go zone, it will slow to a stop and be in irons. This is called missing stays. To recover, that craft typically must return to its original tack and pick up sufficient speed to complete the maneuver. The span of the no-go zone depends on the efficiency of a sailing craft's sails and its resistance to sideways motion in the water (using a keel or foils) on ice or on land, typically at an angle between 30 and 50 degrees from the wind. or laid aback if deliberate. In either case, the stopped vessel will be blown backwards, which with proper positioning of the rudder allows the vessel to point outside the no-go zone and resume forward motion, once the sails can draw power. Iceboats are often parked in irons with a brake applied to the ice to prevent motion. To commence sailing, the craft is guided to one side and boarded, once the sail can draw power.

Close-hauled

A sailing craft is said to be sailing close-hauled when its sails are trimmed in tightly and are acting substantially like a wing, relying on lift to propel the craft forward on a course as close to the wind as the sail can provide lift. This point of sail lets the sailing craft travel upwind, diagonally to the wind direction. Pinching occurs as a craft's point of sail approaches the no-go zone and its speed falls off sharply. Beating upwind, a vessel alternates between having the wind come on the port and starboard sides (the port and starboard tack). Changing from one tack to the other, by steering through the wind direction, is called tacking, or going about.

Reaching

A craft sailing with the true wind on its side (within limits) is reaching.

Craft running downwind increase power from the sails by increasing total area presented to the following wind, sometimes by putting out sails that adapt well to the purpose, such as a spinnaker on a fore-and-aft rigged vessel. Another technique is to place the jib to windward (opposite to the main sail)—called "wing on wing" or one of several other terms—for a fore-and-aft vessel going dead downwind.

File:Amante Choate 48 photo D Ramey Logan.jpg|Spinnaker

File:Training ship 1902 (cropped).jpg|Studding sails

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True wind versus apparent wind

True wind (VT) combines with the sailing craft's velocity (VB) to be the apparent wind velocity (VA); the air velocity experienced by instrumentation or crew on a moving sailing craft. Apparent wind velocity provides the motive power for the sails on any given point of sail. The apparent wind is equal to the true wind velocity for a stopped craft; it may be faster than the true wind speed on some points of sail, or it may be slower e.g. when a sailing craft sails dead downwind.

File:Forces on sails for three points of sail.jpg|Apparent wind and forces on a sailboat. As the boat sails further from the wind, the apparent wind becomes smaller and the lateral component becomes less; boat speed is highest on the beam reach.

File:Ice boat apparent wind on different points of sail.jpg|Apparent wind on an iceboat. As the iceboat sails further from the wind, the apparent wind increases slightly and the boat speed is highest on the broad reach. The sail is sheeted in for all three points of sail.

On ice boats and sand yachts, lateral forces are countered by the lateral resistance of the blades on ice or of the wheels on sand, and of their distance apart, which generally prevents heeling.

References

Bibliography

Rousmaniere, John, The Annapolis Book of Seamanship, Simon & Schuster, 1999
Chapman Book of Piloting (various contributors), Hearst Corporation, 1999
Herreshoff, Halsey (consulting editor), The Sailor’s Handbook, Little Brown and Company, 1983
Seidman, David, The Complete Sailor, International Marine, 1995
Jobson, Gary, Sailing Fundamentals, Simon & Schuster, 1987