I need to figure out what rpm I'm running because I'm in the market for one or two new props. My boat setup is a 18' flat bottom with a 35hp 1978 Johnson seahorse. I've heard to just wrap the tach wires around the spark plugs but I have no idea what this means.. Can someone walk me through what I need to do specifically and maybe give me a link as to what specific tach will be the best to buy for this application? Thanks for any and all advice!