angle of heel at which a vessel becomes unstable and does not bob back upright, does not exist; a self-righting boat will return to upright from any position, including completely upside-down. A self-righting vessel must be positively buoyant when swamped. There are three methods of making a vessel self-right: careful distribution of stationary weight and buoyancy, inflatable airbags, and movable ballast. A basic tool for calculating a vessel's stability is a static stability diagram, which plots the angle of heel on the horizontal axis and the righting lever (GZ) on the vertical axis. (see metacentric height for details). If the static stability curve never crosses the x-axis, the boat is not stable upside-down. This is not sufficient to built a boat with good stability at sea, as it neglects the effects of wind, waves, and human occupants, but it is a simple, powerful way to analyze the stability of a vessel. See also primary stability and secondary stability. Self-righting through distribution of weight and buoyancy requires the weight low-down, and the buoyancy high up. It is often accomplished with a self-sealing superstructure, such as the large deckhouses on modern rescue boats. Most small craft intended as lifeboats with rigid (rather than inflatable) hulls designed since about the middle of the twentieth century are self-righting. Small radio-controlled boats may also self-right. This is particularly useful for racing. Notable capsizings , 19 July 1545, capsized and sank, English carrack, 380 dead , 10 August 1628, Swedish warship, maiden voyage, 30 –50 dead RMS Empress of Ireland, 19 May 1914, capsized and sank in the Saint Lawrence River after colliding with the Norwegian collier Storstad, 1,012 dead , 1915, excursion boat, 845 dead, greatest loss of life on the Great Lakes , 1915, German armoured cruiser, sunk at the Battle of Dogger Bank (1915), 770 dead , 1918, Austro-Hungarian capital warship, torpedoed, 89 dead Scuttling of the German fleet at Scapa Flow, 21 June 1919, several German battleships and battlecruisers scuttled , 7 December 1941, U.S. battleship torpedoed at Pearl Harbor, 415 missing or killed , formerly , 9 February 1942, at dock while being converted to a troopship, one casualty , 24 August 1942, sunk by American air attacks, 120 casualties , 15 November 1942, 212 casualties , 24 October 1944, sunk during the Battle of Sibuyan Sea, 1,023 casualties , 25 October 1944, sunk during the Battle of Surigao Strait, approximately