Spade or balanced rudder:
A rudder has 20-40% of the area forward of the stock, similarly there is no torque on the rudder stock at certain angles and it is called balanced. The axis of the rudder is placed near to the center of gravity, so torque required to move the rudder will be very less.
A rudder with 20% of its area forward of its stock is called semi-balanced rudder. It is often found in twin screw ships. In this at no angle, rudder is balanced.
A rudder with whole of its area aft of its stock is called unbalanced. In this at no angle, rudder is balanced