Downwind faster than the wind. Poll

[Keen_Ed]

Practical? No. But I'm sure it could be done. Ellison recently achieved a downwind VMG of close to 3X wind speed on the water by tacking.

Yes, allright clever clogs. You know what I mean. Rather than a 120' monster, would something about 20' be possible? Is it possible to build props efficient enough?

 

[spork]

Yes, allright clever clogs. You know what I mean. Rather than a 120' monster, would something about 20' be possible? Is it possible to build props efficient enough?

I suspect it would be possible to build a prop that's efficient enough. I have less experience with the turbine which would be below the waterline. That might be the tough part. And of course you'd want the hull to be very slippery - perhaps a foil.

 

[no_way]

Force x distance moved = work. Work x time = energy. Basic physics.

Basic physics yes, but Work x time most definately does not equal energy ! That's pretty meaningless product.

Ummm... work = energy, work x time = power?

Work x time most definately does not equal power !
That's pretty meaningless product.

Force x velocity = work
Force x distance moved = work done (energy)

Force x velocity most definately does not equal work !
Force dot velocity equals power though !
Or the rate the work on the cart is done by the force in IRF where the velocity is measured.



And Force dot distance moved equals work done (energy).
cross product of vectors just isn't the same as dot product, but that could just be a side issue possibly having to do with limited allowable symbols in this forum making it appear like BH & Uber... meant cross product.

 

[no_way]

Ellison recently achieved a downwind VMG of close to 3X wind speed on the water by tacking.

Basic logic & video evidence suggests it had VMG around twise windspeed, not three times WS.
Just calculate how narrow the awa would have been at such vmg and it's easy to see the wing would have not provided lift forward at all at angles it was used if there were that narrow awa and the head sail would have been luffing without upwash from the wing.

Even easier to see on the reaching legs on the second races that v_boat was around or just above 3 times WS, and simple logic suggests v_boat was not any higher in downwind legs and thus vmg down was significantly less than 3 times WS.
It's too easy to be fooled by the look of water surface and not take into account how much more wind was at the top of the wing around 230ft above the sea than at the surface. But the awa can be deduced from the sheeting angles of the headsail and the wing, to reveal the truth.

 

[RAI]

We contemplated using a spool and takeup reel as a transmission. This has the benefit that it can be extremely efficient, and helps demonstrate that the wheels turn the prop (since you can't push on a rope). We may well do that before it's all over.

But then you would have range limited by rope length. Is a chain drive really so less efficient?

 

[spork]

But then you would have range limited by rope length. Is a chain drive really so less efficient?

Actually, it seems the chain has proven to be quite efficient. Normally roller chain is one of the most efficient ways you can transmit power - some percentage in the high 90's if I recall correctly. We worried about losing efficiency as we had to go through a 90 degree twist. Apparently, this can be fairly lossy. But it seems we didn't suffer much penalty because we make our 90 degree twist over such a long span.

The spool and take-up reel wouldn't suffer from the 90 degree twist at all, but does add other complexities and constraints. The limited length is one issue. Also the need for an accurate level-wind, and the need for a mechanism to slow the prop if necessary to avoid developing slack.

The line we bought for this was ideal for our initial design goal. 600 lb test, and very minimal stretch. But we've gone significantly past our original goal and we're now getting into the neighborhood of 600 lbs tension with our sights set at higher numbers. For now it looks like we'll just be reinforcing the replacement chain wheel and see what fails next. There are a couple of trolls on talkrational that get no end of pleasure watching us break components (as we achieve better and better results).

 

[RAI]

I had read somewhere that the chain is the most efficient transmission method, much better than gears. Although a couple of crown and pinions would look much more elegant.

I wonder really if a rope and spool system is very efficient at high loads, rope has quite a high internal friction under load, I believe.

Based on your current experience, do you think it would be worth going for a proa like asymmetric design (main hull and out-rigger). That is, placing the pilot over the wheel that the propeller is trying to lift. and moving the mast over it too. Use small racing motorcycle wheels at the rear instead of mountain bike tyres for more grip and less slipping.

 

[spork]

I had read somewhere that the chain is the most efficient transmission method, much better than gears. Although a couple of crown and pinions would look much more elegant.

Agreed. This is normally the case. However, a twisted chain typically loses a lot of efficiency as it effectively produces stretch. It seems the amount of twist/foot we require has minimal effect fortunately.

For our current archtecture we'd require two sets of 90 degree bevel gears to do the job. We did look into that approach. There is one outfit that makes very efficient bevel gears for human powered craft - but they're pretty expensive. We were also very worried about them taking the design loads. Now that we're putting far more than the design loads on the system (already going quite a bit faster than our original goal) I'm glad we didn't try it. Those particular gears wouldn't have stood a chance.

There are also some timing belt solutions that I really like.

I wonder really if a rope and spool system is very efficient at high loads, rope has quite a high internal friction under load, I believe.

I'm not sure what you mean by "internal friction". Is that the resistance to bending and straightening? The line we have was selected for it's excellent properties, and we had the manufacturer leave the "armored" coating off the line for a short production run. This makes it very pliable.

Based on your current experience, do you think it would be worth going for a proa like asymmetric design (main hull and out-rigger). That is, placing the pilot over the wheel that the propeller is trying to lift. and moving the mast over it too. Use small racing motorcycle wheels at the rear instead of mountain bike tyres for more grip and less slipping.

Of course there are all sorts of interesting possibilities if we were going to start over or do a really major overhaul. In the past couple of days we've mostly been debating the ideas of increasing the left axle length and potentially adding ballast to the right axle. Currently the driver sits so far from the rear axle I think he'd have minimal effect on roll stability.

 

[ThinAirDesigns]

Nalsa has now ratified a new World Record for the Blackbird of 2.8x the speed of the wind while traveling dead downwind using only wind power and no stored energy.

Record ratification reports at: http://www.nalsa.org.

And a great article by Kimball Livingston, an editor at Sail Magazine:

http://kimballlivingston.com/?p=3971

JB