I have a 15' Wilker Galaxie with a 1986 Mercury 75HP engine. Here is the picture of what the boat looks like:<br /> http://www.shipsahoy.biz/boatframes/15FtWilkerGalaxie.html <br /><br />With the current prop I only get 4100RPM, but I am not sure if the tach is reading properly or not. I know...