Cool topic, must have been here before I entered this forum as I hadn’t seen it before.

One thing they all don’t mention. These “ducts” are basically constructed like an aircraft wing, take and aircraft wing, bend it downwards until you have a cilinder and there you have your duct. What is so special about this? Well, as you can see with aircraft, the air is bent down, pushing the wing up with a high pressure field below the wing, above the wing you have a low pressure field. As you can now see, the inside of the duct will have a much higher pressure inside, which allows the propellor to be much much much more efficient and turn on a much lower rate. So, instead of 87rpm, you will only need 60rpm (just an example) to have the same speed. This, together with the propellor diameter determin your water acceleration. Cavitation is created by the acceleration of water, this acceleration means a reduced pressure, and if you lower the pressure enough (below vapor pressure), water evaporates. When the water leaves the propellor, it slows down, and eventually the pressure rises again and the bubble collapses, creating noise.
There you have the other advantage of these propellors, you can go at much higher speeds without creating a huge amount of cavitation and at low speeds even prevent cavitation. Add to this that you have an engine running much slower and hence lower noiselevels and you have quite an idea why everyone is designing pumpjets.
The disadvantage is of course that it it’s not as easy as designing propellors because the parts have to fit quite well mechanically (otherwise it’s really screwed up).

On the other hand, what a scewed propellor does, is just stressing the blade tips of a propellor, making it more efficient once again, to lower the rpm. The bending “inward” of the wingtips is to reduce the diameter hence acceleration again. The blades can’t be built too “fat” though as the stresses would become too large then. But now everyone can build these as once you have seen one, you can give that image to some scientists and they will come up with the answer to what it does, just a matter of invention as is mostly the case. But sometimes the problem is deeper, like with a pumpjet, you can’t design one because you’ve seen it…