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

[John Foster]

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

Ketchum man rescued after boat flips in Atlantic 
<http://www.mtexpress.com/index2.php?issue_date=02-23-2007&ID=2005114300> 
- *Idaho Mountain Express* /23/02/2007

<http://www.mtexpress.com/index2.php?issue_date=02-23-2007&ID=2005114300>

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.

To quote from Pardey's book (Storm Tactics Handbook (Modern Methods of 
Heaving-To for Survival in Extreme Conditions) (ISBN:0964603667) Pardey, 
Lin/ Pardey, Larry. Approximate Price: US$ 21.93 ) on page 83:

" I have sat on deck during Force 10 winds and watched while almost 
pipeline like waves toppled onto our slick, then crumbled into heavy 
foam coming close to the boat. Yet the same pipeliners, with their 
overhanging crests, kept their shape and power as they broke fore and 
aft of where our boat lay. To write this on paper does no justice to the 
drama of watching the slick sap the power of the waves"

Although a boat produces its own Von Karman Vortex Street, the parachute 
anchor, upwind some 300 ft of so, also produces a Von Karman Vortex 
Street so the protection from breaking waves is far better than without 
the parachute anchor.

The 4th edition of Drag Devices Data Base ( by Victor Shane. ISBN 
1-878832-03-4 Approximate Price $36.95 ) has a lot of multihull case 
histories should you need additional opinions besides those reported in 
detail by the Pardeys.

Oh yeah, one more thing, I have not been misled into thinking that a 
bigger sea anchor would be better than one that is the correct size for 
your boat. The generation of the protective Von Karman Vortex Street 
depends on the boat drifting slowly, quite slowly actually, straight 
down wind. Too large a chute, too slow a drift, and hence, no protective 
Von Karman Vortex Street.

See also the Earl Hinz book "Heavy Weather Tactics using Sea Anchors and 
Drogues", printed 2000, ISBN 0-939837-37-4

Earl spent a lot of pages on the need to have the para anchor and the 
boat about one wavelength apart.

All the best

John