Forces on sails to stay within the limitations of a craft's stability and power requirements, which are functions of hull (for boats) or chassis (for land craft) design. Sails derive power from wind that varies in time and with height above the surface. In order to do so, they are designed to adjust to the wind force for various points of sail. Both their design and method for control include means to match their lift and drag capabilities to the available apparent wind, by changing surface area, angle of attack, and curvature. Wind speed increases with height above the surface; at the same
Scarlet Sails (tradition) of free entertainment provided by the city of St. Petersburg. Entertainment also includes appearances by popular rock-stars, as well as the St. Petersburg Symphony Orchestra, ballet and other classical acts, performing on several stages simultaneously during the event. The show also includes a series of large-scale events on the waters of the Neva River, such as rowing and motorboat races and a massive battle with pirates culminating in the appearance of a tall ship sporting spectacular scarlet sails. The show has become the main part of the White Nights celebration. The 1961 release of the film "Scarlet Sails" boosted the