A field test of ship motion was carried out on the investigation and observation ship OOTORI, of which ship type is Semi-submerged catamaran.
　The equation of motion was examined by step response test. It shows that the coefficient of viscosity is very small. The proper period of pitching is 8.73 second and that of rolling is 13.5 second, which are rather longer than those of ordinal ship type.
　The stem trim angle, the fluctuation of motion angle, and the difference between the motion on head sea and that on following sea tend to be maximum when ship velocity is between 12-14 knot.
　The rapid change of propelling force caused the change of pitching angle first and then ship velocity started to change.
　Theoretical response to periodic wave and single wave was made clear.
　Calculation shows that trim angle can be kept at about 0 degree if the angle of front fins are changed properly.