a steam turbine exhaust hood includes an exhaust hood section , and a butterfly plate arranged in the exhaust hood section . the butterfly plate includes a complex curvilinear cross - sectional profile having a first section that extends between a first end portion and a middle portion , and a second section that extends between the middle portion and a second end portion . one of the first and second sections is formed from at least two curvilinear segments including at least one curvilinear segment having a positive curvature and at least one curvilinear segment having a negative curvature .