The surface piercing or ventilated propeller (image) is a propeller

Propeller

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

 that is designed to intentionally cleave the water and entrain

Entrainment (engineering)

Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. For example:* The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke....

 atmospheric air to fill the void, which means that the resulting gas layer surrounding the propeller blade consists of air instead of water vapour. Less energy is thus used, and the surface piercing propeller generally enjoys lower drag than the supercavitating propeller

Supercavitating propeller

The supercavitating propeller is a variant of a propeller for propulsion in water, where supercavitation is actively employed to gain increased speed by reducing friction...

. The surface piercing propeller also has wedge shaped blades, and propellers may be designed that can operate in both supercavitating and surface piercing mode.