[MHml] Would a Von Karman Vortex Street have saved the capsized catamaran?

[Alan Christoffersen]

Hi John,

I have some difficulty in understanding exactly how this will work.

1.) To generate a Von Karman Vortex Street  the "bluff body" needs to be 
symmetrical to a certain degree. Then the velocity around the body needs to 
be fast enough to generate and release votices. When this happens, is a 
function of Reynolds Number, and thereby size of the body, speed and 
viscosity. Laying the symmetrical hull of a boat at 50 degrees will 
probably enhance shedding to some degree.
So according to this, the greater the speed difference between the vessel 
and the water, the better vortices can be generated. If the boat is 
drifting,most of the drift will surely be wind generated, as the water is 
probably not flowing that fast unless the waves are breaking. So  unless 
the waves are actually breaking, and thereby creating a velocity relative 
to the hull, there is no effect??

Check out this link for more vortex basics: 
http://en.wikipedia.org/wiki/Vortex_street

2.) Vortices are generated around the bluff body/hull, and at some point 
are shed from the hull (released) they will then travel further downwind. 
 How is a "slick" downwind of the boat going to smooth breaking seas on the 
upwind side?


Regardless of this, I'm pretty sure that running under the conditions 
mentioned isn't the best tactic.

Anyone out there have some more info?

Regards

Alan Christoffersen


-----Oprindelig meddelelse-----
Fra:	John Foster [SMTP:jfoster at pacifier.com]
Sendt:	24. februar 2007 02:33
Til:	undisclosed-recipients:;
Emne:	[MHml] Would a Von Karman Vortex Street have saved the 
capsized	catamaran?

Cat being delivered in Atlantic flips, reminiscent of Catshot delivery
capsize in Pacific storm.

Would a Von Karman Vortex Street have saved the capsized catamaran?
/

First lets be sure we are both discussing the same things when we use
certain words. For me:

"Heave To" has the ship in an attitude about 50 degrees to the wind.
(That 50 degree angle seems to be important in the generation of the
protective slick upwind) Headway is stopped and the ship is making a
drift (at a rate of 1/2 to 2 knots) to leeward, straight down wind. At
that angle of heading, the flow around the hull and keel is stalled and
a turbulent wake will appear on the weather side. This turbulent wake
has the effect of smoothing down breaking seas on their approach

The scientific name for the turbulent field caused by your hove to ship,
with or without a para anchor, is a Von Karman Vortex Street.


All the best

John
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