Surprisingly, it took just a bit of
research, aided by fxykty's information, to find a picture of the "yacht ensign." I found it NOT in Chapman's book but rather on Wikipedia.
Prior to flying this ensign realize there are restriction other than the vessel MUST be in fact be documented and you better be prepared to produce the necessary paperwork to back up your claim of
documentation. Read up on the restrictions, they are interesting.
There is one additional advantage of being
documentation. Because the Federal Govt. 'has an interest' in the documented vessel, the Feds' will assist, in theory, in locating your vessel if it disappears.