In this study, the effect of underwater hydrofoils was analyzed through model tests on a motor boat with a total length of approximately 8.5 m and a maximum speed of 25 knots(approximately 12.9 m/s). The degree of hull sinkage and resistance performance according to the size and angle of attack of the foils with the same cross sectional shape were analyzed and the required horsepower was estimated. Through this study, it was confirmed that the area and angle of attack of the foils had a great effect on hull sinkage and that a rapid decrease in resistance and change in required horsepower occurred depending on the degree of hull sinkage.