If the two halves of a mold have some helicity, I suspect it is possible to cast such a shape. After all, the water also needs to flow through the prop, so too can the halves of a mold.
Even if the geometry doesn't quite work out, it's also possible you could cast a shape that can be "unscrewed" from the mould and then machine a relatively thin layer from the outside of the cast part (relative to how much you have to machine from a billet). It'll need some level of machining anyway.
Binder-jet printing of sand cores is pretty cool too.
