https://www.youtube.com/watch?v=q2il8Fagbyk

All it needs is a little bump start to get the apparent wind going

https://m.youtube.com/watch?v=MYyLlX5YOMY
Apparently with batteries, not just apparent wind?

Andinista

https://m.youtube.com/watch?v=MYyLlX5YOMY
Apparently with batteries, not just apparent wind?

No the batteries are only used to power motors to move the boat quick enough to generate the initial apparent wind if the water current/lower wind doesnt get the boat going initially on its own. The motors then become hydro generators to recharge the batteries.

No mention of batteries in the video. They are relying on the 10kts of current from the river to start moving the boat at 10kts downriver, which then generates apparent wind and she'll get up and fly. That all make sense, well sort of, as you start generating apparent wind you are forced to crack off so your 10kts of current will effectively turn into 6kts. Still that is believable, the next bit isn't. They are arguing that you can then turn around and go back up river at 30 kts with 10kts of current against you and no wind. The math on that doesn't work out as eventually that current will degrade your velocity over the surface to the point that you are going backwards. Sailboats are close to perpetual motion machines but the laws of physics still apply and drag will eventually decay your velocity without additional energy, and if that additional energy is opposite your direction of travel that adds to drag to further decay your speed.

A sailboat relies on the difference between the density and the velocity of two flow medium (fluids) in order to generate motion. I.e., you have a relative difference between the velocity of water and air. The boat doesn't differentiate between which fluid is moving relative to the ground, only that there is a difference in velocity.

We generally consider that the water is more or less stationary and the air is moving. But in this case, the air is stationary and the water is moving, so there is still a difference in the relative velocities of the two mediums. So what is being stated in the video seems entirely conceivable. It's a little hard to wrap your head around, but the ability of the boat to sail up-current in this example should be no different than a sailboat's ability to sail upwind under "normal" conditions. That being said, I think the video is a little misleading since it shows the boat sailing directly up current. I the induced wind direction would be exactly opposite the current and the boat would still have to tack back and forth (actually jibe... I think).

sm

What dogboy said about the 2 fluids and their differentials +1. Still, the final claim in the video is... theoretical at best, veering into misleading.

The efficiency you need to make anything like that viable isn't at hand, IMO. It's getting closer...

.



Edited by traphappy on Dec 17, 2018 - 05:32 PM.

Idiotic, it states that the 30 mph is upstream, thus in calm/flat water the boat would be doing 40 mph in dead air. So why not just pull a sailboat with a power boat to 10 mph to "bump" start it then sail at 40 mph in calm wind and flat water. These guys have had too much to drink.

--
Hobie 16 (3 formerly)
MacGregor 25 (formerly)
Chrysler Dagger 14 (formerly)
NACRA 5.0 (currently)
High Point, NC
--

How does any sail work at 0-degrees (directly into) the true or apparent wind? I know these things are more efficient than our beachcats, but how can forward velocity be generated without some angle to the wind?



Edited by tominpa on Dec 23, 2018 - 12:31 PM.

--
Tom
NACRA 5.7 (1984 Sail 181)
Pennsylvania
--

leeboweffect

Idiotic, it states that the 30 mph is upstream, thus in calm/flat water the boat would be doing 40 mph in dead air. So why not just pull a sailboat with a power boat to 10 mph to "bump" start it then sail at 40 mph in calm wind and flat water. These guys have had too much to drink.

That’s sorta what the boats designed to do. On the end of the foils they have electric props to bump start the boat and generate the required apparent wind. Then they shut off.

tominpa

How does any sail work at 0-degrees (directly into) the true or apparent wind? I know these things are more efficient than our beachcats, but how can forward velocity be generated without some angle to the wind?Edited by tominpa on Dec 23, 2018 - 12:31 PM.

I’m curious as well because in 0 wind wouldn’t the apparent wind from the forward motion all be directly in front? Maybe it has to have the 10 mph current for the model to work?

leeboweffect

Idiotic, it states that the 30 mph is upstream, thus in calm/flat water the boat would be doing 40 mph in dead air. So why not just pull a sailboat with a power boat to 10 mph to "bump" start it then sail at 40 mph in calm wind and flat water.

You’re missing the point. It is the difference in velocity between air and water that matters. The boat can not tell the difference between whether it is the air or water that is moving (that movement is only relative to a “fixed” point on land. As long as there is a difference in velocity between air and water, lift can be generated by both the rig and the underwater foils. (In zero wind and zero current, or if wind and current were the same velocity, the boat would not be able to maintain any velocity.)

This is not unique to the americas’s cup cats. What is unique to the cats is their efficiency and thus their speed and angle relative to the flow.

sm