Charles, please look in your PM inbox where I replied to your question some days ago!

Basically, all propellor blades have a twist on them, called pitch.

GENERALLY speaking, high speed props have more pitch than low speed ones.

Two blade plastic props come in two main pitches.. "S" and "X".

S pitch has less twist on the blades than X pitch. In point of fact X pitch pops refers to the general design of the entire prop, encompassing pitch and blade shape.

For high speed craft, racing boats and so on, an X pitch would normally be used. For a PT boat, an S pitch.

This is not to say that an X pitch prop is soley for high speed use, as the larger ones develop good thrust at low revs as well. X50 (50mm diameter) props are specified for MFA 800 and 850 motors, and they work very well at all speeds with these motors, the 800 only revving to some 5000 rpm…which in any case is a reasonable speed for such a large prop (model wise)

Ashley