Voith Schneider Propeller | Hacker News Reader

Voith Schneider Propeller

en.wikipedia.org

140 pointsLuc42 comments

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

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.

is mowing grass with a helicopter an actual commononly attempted trick like a kickflip with a skateboard?

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.

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.

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...)

Acro RC helis are an amazing sight, but it’s not really related to the article. They’re just rotors that can reverse their thrust entirely by changing the pitch angle below zero (i.e. lower than regular helicopters can). Many prop planes use that for braking

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.

Most boats operate 95%+ of their engine hours in a very narrow RPM range.

Yes and that range is at higher RPMs where the Sharrow props have reduced benefit.

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.

Sources are primarily boating forums, dockside conversations, etc.

In theory your boat in right in the sweet spot of recommended power range at 60HP. I don't know all the background on it, so all kinds of potential problems, but I would wager that "propped wrong" is unlikely to be the core culprit.

I'd start by getting it weighed and comparing your loaded weight to manufacturer specs. USCG requires positive buoyancy for hulls under 20'. This is typically achieved with using expanding foam in hull cavities, and that foam can have a tendency to absorb and hold water if the boat develops any failure of the seals around the bilge areas that are foamed. Reports of poor performance are very common for these sub 20' hulls because of waterlogging. If not a waterlogged hull, you might also just have too much stuff on-board.

To a lesser degree, a bimini can also have an adverse affect on speed/planing, if it's acting like a parachute. Not sure if you have a bimini, but if so it's worth trying a run with it up vs. down.

I'd also look at how your outboard is mounted. It's not clear if it the outboard from the factory, or if the boat has been repowered. Outboards being too high, too low, etc. are pretty common issues that can also majorly impact performance.

That's a few thoughts that comes to mind off-hand.

Yeah, you're likely right it probably makes sense to just run it rather than trying to optimize further. As for excess weight, there is some--mine is a rare variant that has a small cuddy cabin forward, and that thing is really wet and needs to be completely rebuilt. A previous owner's refit of the decks removed any flotation there may have been originally, but also introduced a lot of unsealed wood which is now wet and heavy. It needs a deck job soon.

The boat was re-powered under my ownership. I'm pretty confident the motor height is correct based on a variety of observations and measurements, so I don't think there's really anything to adjust there.

I wouldn't say I have a complaint with the boat's performance, more like trying to get the engine to run at cruise in a more efficient range of the bsfc/hp map, which may be a tall order at 60hp. To your point, though, if I can shed a couple hundred pounds in the refit that could very well do it.

And much harder to repair.

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.

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).

[0] https://www.youtube.com/watch?v=duSZ1hyK7sY

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.

It's a casting which was machined to final dimension

That's pretty cool, but I wonder if they can get stuff tangled in there.

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:

https://en.wikipedia.org/wiki/Cyclorotor

This is conceptually significant because the article links the mechanisms used in watercraft and aircraft.

The same article in german has some nice animations: https://de.wikipedia.org/wiki/Voith-Schneider-Antrieb

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.

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.

I recommend this video with animations : https://www.youtube.com/watch?v=QrBF9BWYAZY

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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