value that serves as an exponent for wind shear, serving as a gust factor. So, one can expect gusts to be about 1.5 times stronger than the prevailing wind speed (a 10-knot wind might gust up to 15 knots). This, combined with changes in wind direction suggest the degree to which a sailing craft must adjust sail angle to wind gusts on a given course. Hull physics Waterborne sailing craft rely on the design of the hull and keel to provide minimal forward drag in opposition to the sails' propulsive power and maximum resistance to the sails' lateral forces. In modern sailboats, drag is minimized by control of the hull's shape (blunt or fine), appendages, and slipperiness. The keel or other underwater foils provide the lateral resistance to forces on the sails. Heeling increases both drag and the ability of the boat to track along its desired course. Wave generation for a displacement hull is another important limitation on boat speed. Drag Drag from its form is described by a prismatic coefficient, Cp = displaced volume of the vessel divided by waterline length times maximum displaced section area—the maximum value of Cp = 1.0 being for a constant displace cross section area, as would be found on a barge. For modern sailboats, values of 0.53 ≤ Cp ≤ 0.6 are likely because of the tapered shape of the submerged hull towards both ends. Reducing interior volume allows creating a finer hull with less drag. Because a keel or other underwater foil produces lift, it also produces drag, which increases as the boat heels. Wetted area of the hull affects total the amount of friction between the water and the hull's surface, creating another component of drag. Lateral resistance Sailboats use some sort of underwater foil to generate lift that maintains the forward direction of the boat under sail. Whereas sails operate at angles of attack between 10° to 90° incident to the wind, underwater foils operate at angles of attack between 0° to 10° incident to the water passing by. Neither their angle of attack nor surface is adjustable (except for moveable foils) and they are never intentionally stalled. Heeling the vessel away from perpendicular into the water significantly degrades the boat's ability to point into the wind. Hull speed and beyond Hull speed is the speed at which the wavelength of a vessel's bow wave is equal to its