A propulsion system for a boat includes a motor, a connecting arm, a fairing portion intended for being mounted on the boat, at least one propeller, a belt for transmitting the torque from the motor to the at least one propeller, the belt forming two belt sections between the motor and the propeller. The propulsion system further includes a first rotary assembly with the belt passing therethrough and capable of pivoting relative to the fairing portion about a geometrical axis directed towards the top, so as to orient the at least one propeller relative to the boat in order to turn the latter, and a second assembly capable of being translated relative to the first rotary assembly in order to retract the at least one propeller.