This concept of what looks like a large flying boat probably shouldn't been taken
literally. Christian Volf filed a patent for a propeller, which should have been more
efficient, than a standard prop. I'm not sure, that this design really was drawn by
himself, or just by an artist from the popular science magazine, who tried to visualise,
what an aircraft with a "volfprop" could look like.
But it would be interesting to know, if his propeller really is as efficient, as claimed,
and if so, why this idea wasn't developed further ?