A dynamic model for the motion of planing craft on the vertical plane was developed; the motions ofsurge, heave, and pitch are coupled. Critical conditions that produce the inception of instability are evaluated. The Wagner model (1932) for 2D impact is extended for section with knuckles. Planing hullswere analyzed through the application of slender body theory. The results are compared with Tveitnes(2001), Peterson (1997), Savitsky (1964), Troesch (1992) and Celano (1998).