say for instance you have two props, we will use the mercury laser II for example. if they are both 21p but one has a diameter of 13 1/2 and the other is 13 7/8, what is going to be the difference besides losing rpm with larger diameter?

A bigger prop will (theoretically) be able to move more water and generate more thrust. A smaller prop will not use as much horsepower to turn a given RPM. Basically you can match the diameter (actually it is blade area but that is directly related to diameter) to the weight of the boat. So if you have a heavy boat a larger diameter prop is better. My boat is fairly light and I found that reducing the diameter of my stock Hydromotive Q IV made a 3-4 MPH difference on the top end with no other changes. The stock prop was larger than I needed. This is a very simplified explanation and a lot of other factors will come into play.

cool deal, thanks for quick reply

Bigger diameter usually adds lift to the stern.

And the decrease in the stern lift is what made laster's boat faster. More stern lift can drop the bow and wet more of the hull on certain applications. His hull doesn't want extra stern lift the diameter created. Also, the R's might have grown a touch to give more speed.

Brett