To enable travel at high speed on the mater, while making a propeller in a nearly fully submerged state at the time of acceleration for a high speed state and making the propeller in a nearly half-submerged state at the time of high speed navigation, by making the distance from a transom to the rotary surface center to a specific range.
When, at the navigation time, a distance from a transom A3 which is the most rear end of a boat bottom contacted to the water to the rotary surface center of a propeller 10 is L (m), the diameter of the propeller is D (m), the maximum boat speed at a trial operation time is V (m/sec), a type draft is d(m) and a gravity acceleration is g (m/sec2), L is shown by the following formula; D/2≤L≤Vd1/2/2.5g1/2 and also the cross line formed by the crossing of the vertical section passing the axial center of the propeller shaft 13 with a boat bottom surface is extended to a rearside and the propeller 10 is arranged to the position in which the extension line U passes the nearly center of the rotary surface of the propeller 10. Thereby, at the stop or low speed navigation time of the boat, the entire propeller 10 is nearly submerged and also at the high speed navigation time, the propeller 10 is made in a nearly half-submerged state.
NAKAMURA YUKITOSHI
NOGUCHI KEN
