You mean “prop slip”, not gear slip, and here is the link to calculate. http://basscatowners.yuku.com/topic/1555 The four blade will lift the bow better(Trophy), and hold the water better than the three blade(Tempest), although at those altitudes you may have to go to a 23p to get the gains youre looking for. The three blade is a more of a surfacing prop, which is where you lose some of the rough water bite, or feel,and the four blade should be easier to drive. When you get the rpms youre looking for you may need to lower the engine a tad to compensate for the differences in the props, as you should get more lift by doing so. Good luck, D.