I have a 18' Nitro 180FS with an Evinrude 150 HP on it, I am trying to determine the speed of my boat, the prop is a three blade 14 ½ x 24, I went to this site, http://www.rbbi.com/folders/prop/propcalc.htm, but not sure Im doing something right.<br /><br />1)What is "Propeller Slip"? How do...