A custom-built, half-sized antique curved dash Oldsmobile weighing 400 lbs., whose license plate read “HALF” built by Dick Francis of Philadelphia in his bicycle shop. It had a Cushman engine, two speeds forward, one reverse, transmission with automatic clutch.

It could be swung over the side on davits, so that the travelers could putt-putt around various ports of call.

http://jamesmeltonmusicalcareerantiquecars.blogspot.com/2010/07/more-on-half-and-boat.html

I’ve posted a couple yachting cars in the past, Fiat Jolly and such, but this is the first time I’ve found a photo of one actually on a yacht