A boat propelling system capable of determining an abnormality regarding steering of a propelling system main body includes an outboard engine main body, a steering section arranged to steer the outboard engine main body, a steering angle sensor arranged to detect a steering angle of the steering section, a pivot sensor arranged to detect an actual pivot angle of the outboard engine main body, and an ECU. The ECU calculates a target pivot angle change amount based on a target pivot angle derived from the steering angle and a baseline target pivot angle, calculates an actual pivot angle change amount based on the actual pivot angle and a baseline actual pivot angle, and obtains a gap amount between the target pivot angle change amount and the actual pivot angle change amount. Then, the ECU determines an abnormality regarding steering of the outboard engine main body based on a result of comparison between the gap amount and the first threshold value. Setting of the baseline target pivot angle and the baseline actual pivot angle is based on a result of a comparison between the gap amount and a second threshold value.