In this paper, the authors present a new method for calculating vertical motions of large-sized high-speed ships with hydrofoils. It is very important for this calculation to predict correctly the dynamic lift of the submerged hydrofoils which travel close to the free surface under the waves. Measurements were made of the dynamic lift of the hydrofoils moving at constant speed in regular waves. Using these experimental results, we propose a method for estimating the dynamic lift of the hydrofoils travelling beneath the waves. The ship motion is predicted by the strip method taking account of the effects of the nonlinear hydrodynamic forces and dynamic lift of the hydrofoils. To verify the present method, the authors compare the computational results with the experimental ones of HC2000. HC2000 is a new type of hydrofoil catamaran having a displacement of 2000 tons and composed of twin V-hulls and four rectangular hydrofoils. The computational results in still water and head seas agree very well with the experimental ones of the 1/50 model of HC2000.