I recently acquired a 1972 6HP 2CYL Johnson outboard. I had to put on a new prop as the other one had a piece missing. I replaced the 2 blade with a 3 blade and the boat seemed to go slower. Had the prop shop "tweak" the prop for less pitch and it ran faster but he said to get the maximum efficiency from the prop, you need to know what the RPMs are with the throttle wide open under a load. It was suggested that I need to hook up a tachometer. How can I do this and is it possible? Would appreciate any information. Bob H.