hckrnws
Cool, similar to helicopters which can also control direction indepentendent of thrust, which leads to RC helicopters being able to pull of crazy, physics-defying moves like here: https://www.youtube.com/watch?v=QSiwyoQldfo
Thank you for that. That was the most astonishing display of precision (in any field of endeavour) that I have seen in quite some time.
That's the worst lawnmower I've ever seen. It didn't even cut the grass as much as it just bothered the blades of grass. 0 out of 5 stars. What's that? It's not a mower. Oh, well then, that thing is cool as hell, but not as cool as the pilot that looks like he's just casually standing there.
This made me chuckle:)
That is bizarre and amazing. I have never seen any kind of aircraft that resembled a dragonfly as much as that.
The maneuvers are so extreme and come so fast that I would not have been able to say for certain that this wasn't just a very nasty crash in progress. But they were, in fact, completely controlled and intentional.
Incredible.
Thanks! I’ll keep this on hand for next time I hear that a UFO proof video comes out saying that an object can’t move the way it does on screen.
Wing area (roughly) scales by n² and mass by n³, so size matters for this kind of thing.
See https://en.wikipedia.org/wiki/On_Being_the_Right_Size (PDF at https://www.phys.ufl.edu/courses/phy3221/spring10/HaldaneRig...)
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.
Do you have any more information/sources to share on this? I have an Eastern 18 powered with a Yamaha 60hp 4 stroke and I've been struggling to dial the prop right. I don't know off the top of my head what the specs of my current prop are but basically I feel like I'm not taking advantage of the engine's torque at less than WOT, so I basically just run it flat out. If I could extract just a little more thrust out of the prop at lower RPMs it feels like the engine would have enough grunt to make the boat plane in the mid-high 4000s instead of 5000-5200rpm where I currently run it. Ideally, given the bsfc/hp curves, I'd like to run the engine at a bit lower RPM, but the way it's currently set up at ~4600rpm it's not fully up on the step. I was (perhaps wishfully) thinking a little more efficient prop design might help.
The other thing I was thinking of trying is swapping in a different "high torque" lower unit with a lower gear ratio and running a significantly larger prop.
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.
Comment was deleted :(
It looks bent to me, or potentially stamped.
It's a casting which was machined to final dimension
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).
The same article in german has some nice animations: https://de.wikipedia.org/wiki/Voith-Schneider-Antrieb
This reads a lot like an advertisement. The linked page [[Cyclorotor]] is more neutral and has more information on the design and applications outside of marine vessels:
This is conceptually significant because the article links the mechanisms used in watercraft and aircraft.
These were used on three car ferries in Scotland, mostly between Gourock and Dunoon but the same vessels were sometimes used on other routes. The Saturn, Juno and Jupiter which were quite fast and incredibly maneuverable: "...service speed was around 15 knots (although she could also achieve 13 knots astern and 3 knots sideways" with just those drives. No separate screws, bow or stern thrusters. Same drive was used on a few smaller local ferries, too.
If I remember right what they didn't do was go exactly straight. You could see a (very modest) s-shape in the wake over distance.
ref: https://www.shipsofcalmac.co.uk/fleet-features/the-streakers
See a tug showing off here: https://www.youtube.com/watch?v=u6uNECa_X8Q Voith is the only company producing those, even though the patent has expired.
Please excuse me while I rock your boat up against the pier unnecessarily as I show how to do burnouts and donuts with my boat for some viral content.
I recommend this video with animations : https://www.youtube.com/watch?v=QrBF9BWYAZY
They're really useful for tugs and other specialty applications that need the ability to have differential thrust in arbitary directions with lots of thrust at low speed, but loose to Azipods on faster and larger ships.
From a resilience POV, my guess would be that failure of any one blade would botch the system overall. Maybe that is why many diagrams show them installed in pairs. (I would guess each operates in a different direction for angular momentum reasons.) I have no idea about overall reliability.
For marine applications dual drives are common as it enables better rotational control for maneuvering. The redundancy aspect is also a factor, but moreso for applications where you are going to be far from shore. For tugboat and ferry type applications, where these drives are most common, that is less of a concern.
I think you are right, One only provides directional thrust, a pair would be needed for rotational thrust.
Most traditional tugs have a pair of screws for just this reason. Not so much to turn but by applying differential thrust they can pull sideways. A vector drive like this will vastly increase the envelope of possible pull conditions.
They are installed in pairs and work together with bow thrusters to allow for dynamic positioning, the RV Falkor (too) has them for this reason.
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