Funny this has come up today. Yesterday I called my dealer to inquire and negotiate a deal on a terrova 55 12 volt for my G3 pro 175. The subject of shaft length come up. I told him I hadn't measured the shaft on the existing troller. He immediately said I needed the 54". I told him I could launch the boat and take a measurement for the formula that's used to determine the proper length on the Minn Kota website. He said don't bother get the longer 54.