83 15hp prop question

[EZAKMC]

I need to replace the prop on my 15 hp motor. question is I see many differant prop sizes to chose from. What is the best size prop to be max. speed? I have the boat on a 11.5 b*** tender. It is a heavy boat and when I have two people in it it seems that it just cant get the boat up to enough power to get it to stop plowing through the water..it seems like it needs more power.. i am guessing that it is a prop problem. Pitch? 3 or 4 blade? what is the differance?

[boats.net]

There are alot of variables when choosing a propeller. There usually are not 4 blades offered for such a small motor.