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John Sohl says Greenbird can sail upwind at faster than windspeed.
It is certainly true that Bernard Cleyet, Greenbird and almost any
sailing dinghy can go faster
than the actual windspeed. What NONE of them can do is sail faster than
the windspeed DIRECTLY DOWN WIND! <g>
If you need to do that you need to extract wind energy and apply it to
wheels or water propeller!
Just as, in David's case one would need to extract river stream energy
compared to a stationary feature in windless conditions.
Close hauled, one comes closer and closer to the apparent wind angle, so
low drag and high efficiency are vital to
sailing just 3 degrees off the apparent wind as Greenbird has done at
about 4X actual windspeed, so perhaps 12 degrees
off the actual wind direction.
On Thursday, June 3, 2021, 11:19:26 AM CDT, John Sohl via Phys-l <
Moving faster than the wind is possible and not just done by the
"Blackbird" here is the "Greenbird." The Greenbird has no prop, only a
rigid sail (a wing really).
True Wind at 110°
Apparent Wind at 3°
Speed = 3-5 times wind speed
Location: Ivanpah Lake bed, Mojave, CA which is a lot firmer than any sand.
I've never been there, but I have driven on the Bonneville Salt Flats many
times, including at speeds in excess of 100 MPH, it is surprisingly smooth
and it does not feel like 115 MPH. (Indeed, I was startled when I looked at
the speedo on my motorcycle and started to back off. A speedometer that I
have calibrated with GPS and measured miles both.)
If you get rid of hull drag and (mostly) skim across the surface, you can
do the same thing with a sailboat.
Details for the video:
Winds = 25 kts
Avg. Speed = 59.38 kts
Peak Speed = 64.78 kts = 74.55 MPH = 120 KPH
This is the "Vestas Sailrocket 2" boat that broke the
world speed sailing record for a mile in 2012: 78.26 mph. It is pretty big:
Length = 40’, Beam = 40’
On both of these it is worth noticing where the sail is located and how the
outriggers are designed. Both of these are WAAAAY more professionally built
than the Blackbird.
In all of these the key issues include reducing any drag as much as
possible and using aerodynamic lift rather than drag. Lift produces
significantly more "thrust" (yeah, I know, poor word choice) than drag can.
There are other ways to get rid of drag. Go out on the ice!
If you have the guts (I don't), you can do it on a windsurfer ("ice
This guy averaged 52.65 kt in 20 kt winds:
There are arguments over the world speed record for ice boating including
values of 155 MPH and 143 MPH. These boats routinely move a FOUR times the
- - - -
John E. Sohl, Ph.D.
WSU Brady Presidential Distinguished Professor
Department of Physics and Astronomy
Department of Environmental Science
Weber State University
1415 Edvalson St., Dept 2508
Ogden, UT 84408-2508
Office: TY 326
Office phone: (801) 626-7907
cell: (801) 476-0589 (Text me, I don't answer the phone if you are not in
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