Propulsion system for boat

A propulsion system for a boat, the propulsion system comprising:

• an engine;
• a propeller arranged to be rotated by the engine;
• a transmission mechanism arranged to operate in at least a low speed gear reduction ratio and a high speed gear reduction ratio, and arranged to transmit a driving force of the engine in a state where a speed of the driving force is changed to one of the low gear reduction ratio and the high speed gear reduction ratio;
• a control lever section arranged to be operated by a user when the engine drive is controlled;
• a control unit arranged to output a signal to control a gear change in the transmission mechanism on the basis of an amount of a lever opening, which is based on the user's operation of the control lever section, and a speed of the engine;
• and
• a cavitation detecting section arranged to detect cavitation generated in conjunction with rotation of the propeller;
• wherein
• the control unit is arranged to control an output of a signal to the transmission mechanism to change the gear reduction ratio to the high speed gear reduction ratio when cavitation is detected by the cavitation detecting section.