Although there are many investigations on the relations between fouling of vessel and her speed, we have not yet ample data of the similar problems on small ship, In this paper the authors attacked this problem by using the S.S. "UMIKAZE MARU" (Gross 70 tons.) of Nagasaki Marine Observatory, and obtained following results: (1) The relation between the speed and the periods from the day of cleaning of the bottom was found as shown in Fig.1. (2) The amount of decreasing of speed is rather small for the period ranging from Jan. to May, but in June (rainy seaon) the speed suddenly decreased and then the amount of decreasing became nearly same to that of Dec., that the total amount of decrement of speed was nearly 40% at the end of year.(3) The frictional resistances were calculatedform above data. (4) The time of bottom cleaning should be decided on the economical points.