The dynamic stability of a surface-piercing plate, advancing with high forward speed in the horizontal plane, is investigated in the scope of linear theory. The hydrodynamic forces on the plate in sway and yaw are presented in terms of frequency and forward speed-dependent added mass and damping coefficients. Flow separation from the trailing edge of the plate is considered. A time-domain boundary integral method using linear distribution of Rankine sources and dipoles on the plate, free surface, and vortex sheet is used to calculate these hydrodynamic coefficients numerically. Comparison between the current numerical results and previous numerical and experimental results is presented. Using linear dynamic stability analysis, the influence of hydrodynamic coefficients on the plate's stability is investigated as a simplified alternative to a semidisplacement vessel.