On Fri, 15 Jun 2007 10:57:42 GMT, Short Wave Sportfishing
wrote:

The four blade has a cup or more of a twist in the blades than the
three blade.

Is there a reason for that?

Cupping is a way of increasing the effective pitch. If you have
enough torque to spin them, the cupped props should give you a bit
more speed at the same RPM.