Given, for example, a 10 x 6 prop, the first number is the diameter in inches (10"). The second number is the pitch. That is the distance in inches (6) that the prop will move through a solid surface in one revolution. This is theoretical, since the prop will slip in the air. GIVEN THE SAME RPM, more pitch, less diameter equals more speed, less pull. Larger diameter, less pitch gives more pull and less top speed.

Dr.1