Recreational sailing in the Sound of Mull, Lochaline, Scotland. SCO 7742
Sails propel the boat in one of two ways. When the boat is going in the direction of the wind ( downwind - see Points of sail), the sails may be set merely to trap the air as it flows by. Sails acting in this way are aerodynamically stalled. Drag, always parallel to the wind, contributes the predominate driving force.[9] The other way sails propel the boat occurs when the boat is traveling across or into the wind. The sails acting as airfoils propel the boat by redirecting the wind coming in from the side towards the rear. By the law of conservation of momentum, the wind moves the sail as the sail redirects downwash air backwards. Air pressure differences across the sail area result in forces on sails including drag and lift. A component of the lift is the main driving force.[10][11][12][13] The sails can also act as airfoils in some downwind situations, spinnakers and square-rigged sails can be trimmed so that their upper edges become leading edges and they operate as airfoils again, but with airflow directed more or less vertically downwards. This mode of trim also provides the boat with some actual lift and may reduce both wetted area and the risk of 'digging in' to waves. In stronger winds, turbulence created behind stalled sails can lead to aerodynamic instability, which in turn can manifest as increased downwind rolling of the boat. Sail are often equipped with lightweight tapes or strands (tell-tales) to indicate the airflow in their area. They may be on both sides near the leading edges of the sail, or at the trailing edge of the sail. Horizontal strips sewn into fore-and-aft sails and V-shaped markings on spinnakers assist with judging their shape from on deck. These may even glow in the dark, using luminous tapes. On a sailing boat, a keel or centreboard helps to prevent the boat from moving sideways. The shape of the keel has a much smaller cross section in the fore and aft axis and a much larger cross section on the athwart axis
Size: 3940px × 5766px
Location: Sound of Mull, Lochaline, Argyll, Scotland. UK.
Photo credit: © David Gowans / Alamy / Afripics
License: Licensed
Model Released: No
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