[Tobra]

I was not hip to the magic of a toroidal prop until recently.

The shape of the blade is such, that, rather than bubbles tending to form in a concentrated area at tip of the blade, it spreads out this area, and greatly reduces bubble formation and cavitation. I watched a video of a prop in the water going 40 mph. The difference in cavitation was striking. Upshot of this is that a toroidal prop is greater than 100% more efficient that a regular prop. From a practical standpoint, you get double the thrust from the same power, which also would be expected to result in much greater fuel economy.

Downside is cost, as they are much more expensive to manufacture. Over time, I would expect to recover this expense through improved fuel economy.

Standard prop $500

Toroidal prop $5000

I don't know if this would scale up from a 1' diameter prop to a 30' diameter prop. If it would, seems like the Navy would be all over this. Oh yeah, they are super quiet too.

I have no doubt someone will be along presently who either wrote the book on this, or edited the book for the guy who wrote it.