Horsepower is a measure of energy used to generate torque.
The typical sailboat achieves hull speed with RPM at the prop about 900-1200. It is the job of the motor to generate the RPM at the prop. An illustration of typical horsepower and torque ratings for diesel motors is shown below. The typical diesel, gasoline or electric motor requires time to build up to peak torque levels. For a boat operator, this means full power is available as soon as forward or reverse motion is chosen.