The coolest recent development in marine propellers is toroidal propellers which are now commercially available and seem to perform significantly better than standard propellers: https://www.sharrowmarine.com/
For the most part the Sharrow props have not proven to be much of an improvement, particularly for the high price.
The tests that have shown "significant" improvements have frequently compared the Sharrow to a sub-optimal prop. Feedback from many actual users is that the gains are moderate over a narrow RPM range.
And much harder to repair.
That's pretty cool, but I wonder if they can get stuff tangled in there.
That website seems to no useful information; only marketing speak about how great it is... Do you know of a good source on how toroidal propellers work and the engineering behind them?
Here you are: https://en.wikipedia.org/wiki/Toroidal_propeller
Really enjoyed this:
> In the centuries after Archimedes invented the Archimedes' screw, developments of propeller design led to the torus marine propeller... it was invented in the early 1890s
"the centuries" indeed. :)
Uhm... The article lacks quite a few citations.
I didn't even see a picture of the propeller, if there was one. There was a giant, white, blank space.
That space is blank for me too.
This page seems to have one halfway-functional photo for me: https://www.sharrowmarine.com/store/p/sharrow-by-veem-ds9sw
This URL may work: https://images.squarespace-cdn.com/content/v1/560055b1e4b017...
If that's machined, that's impressive. I've seen some crazy 5-axis CNC examples, and it's usually some bladed turbine fan. Not sure how many axis (axii??) this would require, but it looks cool nonetheless.
It looks bent to me, or potentially stamped.
Propellers (both marine and aero) are just spinning wings. If you picture a 2D airfoil like you might see in one of those "intro to lift" diagrams, all the flow is in what we call the chordwise direction, that is the flow is entirely along the axis of the wing's chord (leading edge to trailing edge).
A real 3D aircraft, however, has a fuselage. Similarly, a prop has a hub and the tips of each blade are spinning faster than the roots. The tl;dr of this is that real 3D lifting surfaces typically exhibit a mixture of chordwise and spanwise flow, which causes wingtip vortices to form[0], resulting in induced drag/induced power loss.
For a given amount of thrust the total amount of momentum that the prop transfers to the fluid is fixed. The tip of a conventional prop ends abruptly which causes a large pressure gradient and a strong vortex. A toroidal prop's shape causes the pressure gradient to be broader and less concentrated, therefore the wake vorticity is distributed over a larger region, reducing peak swirl velocities and lowering the kinetic energy lost to vortex formation (and to cavitation).
