I have a 1986 16' Sarline with a merc 120 I/O with an alphaone. I was looking to buy a new prop and then decided that I have no clue on which one I need, or how to decide. I think the stores sale guys want to sale me the most expensive **** they've got. My boat does not have a tach or speed so I don't know anything really. The prop I have now is damaged so I need to do something. Thanks