Toroidal fans and closed wings

I came across this one recently - somebody was hacking around in FreeCad trying to duplicate the geometry:

MIT researchers claim these propellers (fan blades), because they don’t have conventional blade “tips”, generate less acoustic noise from tip vortices and also because they are structurally more rigid blades.

Perhaps more interesting are these guys (, who claim their props are quieter and more efficient than open-blade props:
Standard vs Sharrow MX™ Noise Comparison - YouTube

Ok, so the idea has been around awhile, see wikipedia article for closed wings linked below - apparently Prandtl suggested it back in the 1920s.

Others point out you really can’t eliminate vortices arising from lifting bodies - see

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Very interesting. I’d seen circular wings before, certainly hadn’t considered using them as rotors. P99 in that thesis is the money shot.

Most of his work blows.

Mint, quit it - I had to wipe the coffee off my screen because of you :grinning:

I’m not a boat guy, so some of the stuff on the Sharrow website is somewhat baffling, e.g. what is “advance rate”. I’m speaking of the website content here:

They claim their props don’t cavitate at rpm’s where open blades do, and this gives the props better performance (more efficiency). Not sure how the closed rotors prevent cavitation, but is it just that having more blades at lower pitch per blade is helping? Then again, not all their reports compare a smaller and lower pitched closed rotor to a larger/higher pitch open blade.

One would hope for some independent lab testing, not just some pretty Excel charts and graphs. Oh wait, here:

Scroll down a bit - they claim independent testing at U of Michigan confirms a 9 to 15% improvement in propeller efficiency. Being a U Wash. grad, I automatically question anything from UMich…didn’t even know they had a hydrodynamics lab…but still, makes me go “hmm”. I have an issue with the dye injection video and commentary there - you can see that the dye injection rake is moved upstream for the Sharrow prop, which influences the “streamline capture” of the two videos. But the visual evidence of that video does suggest the Sharrow prop is generating a less severe wake vortex, and other videos do seem to suggest lower cavitation levels.

Looking for somebody here who might have some Marine experience (and no way am I going to the old site and asking Mr. Tugboat about it). I wish Mike Halloran was still around, may he RIP. I may have to go bug some friends who have boats and see what they say…

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Ok…looking into it more - the Sharrow props, in comparison to “standard” outboard marine props, have more physical blades (6 vs. 3 typically in the examples they show) and thus a lower blade loading (theoretically) for the same thrust…thus lower potential for slip/stalling/cavitation. Maybe. But they should also have more form drag, thus lower efficiency at lower speeds where no cavitation occurs, which the data do not suggest.

Other studies are being done for aerial helicopter-type drone propellers, where the technology is called a “loop blade” or “loop type airfoil”. Results there are more mixed - generally the multiblade profiles will have higher drag and thus lower efficiency compared to a two-blade profile…but they show benefits for noise production. Maybe. Hard to tell how well optimized the “loop blades” were, most likely they were optimized for ease of production on a 3d printer, though that’s just a personal opinion.

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Interesting. If I had a boat, I’d try a Sharrow prop. What is puzzling is listening to the boat video comparison, the engines sound “loaded” for normal prop, but not for the Sharrow. So, less power consumption at same speed for different props?

As aside, while working on a CAES design project, we pursued using trim coolers in lieu of cooling towers to reduce water consumption. what killed that idea/concept was the noise created by the trim cooler fan blades. Cooling tower fans were higher in elevation, so less noise at grade. I search my employers library for any literature on quiet fan blades - nothing. I even went to the local university to see what I could find - nothing. I thought if submarines had quiet blades, the same technology/idea could be used for fan blades. I gave up after a few days looking into this matter thinking this data was kept away from the public. Then again, Dick Rotan (?) had quiet props on his experimental planes as I remember seeing a few fly around Alb., NM.