sorry about the jargon. I've been reading up on marine steam powerplants for the last year or so and found that the cheapo way to reduce drag for a propeller driven steamer when under sail was to have a clutch inline with the drive-shaft, allowing the propeller to pinwheel. The clutch tended to have 6 to 8 staggered raised sections that allowed them to engage. These clutches were frequently made of up laminated sections with spacers. This may have made them stiffer for the weight. I get the impression they may have been made of ligen vitae, a hardwood so dense that it doesn't float. My father was a marine engineer and told me as a child that this wood was used for drive-shaft bearings also. Apparently, larger naval and transport vessels were technically auxiliary steamers and were provided with full sail rigs and lifting propellers. When raised, the vessel sailed with little drag other than the form drag of the hull. The downside was the complexity of the lifting mechanism and the framing of the stern to allow the detached prop to be lifted vertically into a recess in the stern. Unless greatly reinforced, this layout weakened the stern.