A small planing craft that has V-shape hull and hard-chine is good for running in a little rough sea at high speed. But it has been known to have various motion instabilities. From these, we may choose so-called corkscrew motion, which was supposed to include roll oscillations (chine walking) mainly and heave oscillations (porpoising) partly at high speed turning, as one of the most difficult problems to predict its occurrence at design stage and one of the most serious problems for safety. If it happens, the driver is hard to maintain his position due to large lateral acceleration and may lose craft control. This report shows that this turning instability at high speed comes from vertical dynamic instability at heeled attitude at turning, and proposes some criteria that are useful to design a new boat that has enough turning stability. These criteria are obtained from the turning motion simulation program that is newly developed as a design tool, which can judge the degree of turning stability at early design stage. Adding extra keel to hull bottom is a very effective measure for turning stability. Though its effect has been known from experience but it has been confirmed quantitatively by this simulation for the first time.