That depends on the type of yacht you have. The most frequently used system for motor yachts is a twin-propeller setup. This is for ships up to 35 to 40 knots.
For displacement yachts, an alternative for the twin-propeller and rudder setup, is a so-called podded propulsion system, or a rudder propeller. These units are primarily aimed at displacement yachts up to app. 15-16 knots maximum speed.
For high speeds, say above 30-35 knots, a waterjet propulsion system is the most common choice.