Piranha offers 3 and 4 blade options. Three blades are the most economical. Four blades typically offer better hole shot and lower vibration.
Pitch is one of the fundamental characteristics of a propeller. Props are usually sized by their diameter and their pitch. A 14 x 17 would describe a 14 inch diameter with a 17 inch pitch.
Pitch is not an angle, it is a distance traveled. The prop acts like a screw being driven into a solid material. As the propeller rotates it moves forward. How far the propeller moves in one revolution is the pitch. The 14 x 17 would move 17 inches forward in one revolution of the propeller.
The Splines are the teeth on the motor out put shaft. The motor spline count matches the propeller spline count. The spline locks the propeller to the drive shaft and transmits the engine torque to the propeller.
Not every propeller is designed to turn the same direction when the boat travels forward. There are always two possibilities.
When the propeller is viewed from behind the boat looking forward: left-rotation (counter-clockwise) or right-rotation (clockwise).
Diameter is one of the fundamental characteristics of a propeller. Props are usually sized by their diameter and their pitch. A 14 x 17 would describe a 14 inch diameter with a 17 inch pitch.
When choosing a propeller matching propeller diameter is less important than matching the propeller pitch as compared to your old prop.