Boat propulsion unit

A boat propulsion unit comprising:

• a power source;
• a propeller arranged to be driven by the power source to generate a propulsive force;
• a shift position switching mechanism having an input shaft connected to the power source, an output shaft connected to the propeller, and a hydraulic clutch arranged to change a connection state between the input shaft and the output shaft, a shift position of the shift position switching mechanism being switched between forward, neutral, and reverse by engaging and disengaging the hydraulic clutch;
• a control device arranged to adjust and maintain an engagement force of the hydraulic clutch to be any value in a range of 0% to 100%, including any value between 0% and 100%, of the engagement force of the hydraulic clutch by adjusting a hydraulic pressure applied to the hydraulic clutch and thereby vary a rotational speed of the output shaft with respect to a rotational speed of the input shaft;
• and
• a retention switch connected to the control device;
• wherein
• when the retention switch is turned on by an operator, the control device controls the engagement force of the hydraulic clutch to assist a hull being retained at or propelled toward a predefined position;
• and
• the control device calculates the engagement force of the hydraulic clutch based on a boat speed integral value, which is a speed of the boat integrated over time since the retention switch was turned on.