Found this in a search regarding the late-'20s vintage Packard 24 cylinder 1500 hp engines-yikes!
If the engine essentially looks like a V-12 engine with another V12 engine bolted below it upside down (forming an X configuration of the cylinders) you've got your boat.
Hope the links work:
http://papers.sae.org/280064/
edit: one of the original links for the x24 engine seemed to have become unavailable and was removed.