Some of the old ones back in the 70's and 80's had up to 70's on them. It was a popular boat with the drug runners. They would use run out to the drug boats. If the CG or Sheriff's boats chased them they could run into the shallows or creeks to escape.
One builder back then was Edwards, it was built by a dispatcher for the Sheriffs Dept for the drug runners. His were 15" shaft & rated for a 60 I believe. That was the mold that my dad used to build the plug boat and new mold out of.