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.
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
Many Popular Mechanics type magazines had plans to show you how to build your own, or, you could buy them already made from the Horseless Carriage Company in Ft Lauderdale