As you mention there are gradients caused by upwash. The head falling off
accommodates this phenomena.



All seamless string sails do have seams. Molded sail technology relies on
flat mylar in a roll form, they broad seam the mylar for the initial shape.
Ain't marketing a great?


"Garland Gray II" wrote in message
...
And it is desirable for the top to fall off some. The wind gradient
results in apparent wind aloft being some degrees more off the bow than
the apparent wind closer to the deck.

"Bryan" wrote in message
...
Square top mains have been around a long time. VOR 70s carry them as
well as maxi cats, open 60's and windsurfers. Nothing new here. They
are made from battens and the top twists off as the breeze comes on. (ie
self reefing since they twist)


"Wilbur Hubbard" wrote in message
...

"Gogarty" wrote in message
...
In article ,
llid says...



"Gogarty" wrote in message
news I have been doing a lot of Googling but I can;t find an answer.

You have no doubt noticed that the sails on the current crop of
America's Cup challengers are not triangular -- even with a huge
roach
-- but trapezoidal. How do they maintain that shape at the head? Is
there a gaff at the head parallel to the boom? How do they keep the
head
of the sail from falling way off to leward? Is there some arrangment
of
masthead sheets that keep the head parallel to the boom?


How about using your friggin' eyes for once? Sail shape is held by
battens. Doh!

http://media3.washingtonpost.com/wp-...6/PH2007051601
455.jpg

Wilbur Hubbard

Doh! yourself. According to the North Sails artricle that top batten
has a
square or rectangular section. It is might stiff and bends very little.
But
what keeps it from going to leeward?

Tension along the leech.


How is that "gaff" kept parallel to the
boom? The article also said the ideal sail would be a rectangle. Wait a
minute...

Tension along the leech. Caused by either the mainsheet or the vang or
both.

As for a rectangular sail being optimal. It might well be for the light
winds those boats sail in and the height restrictions on the mast. They
won't even go out in anything above 18 knots and that's sad. Imagine
that. Boats costing millions and so unseaworthy a Catalina 25 takes
higher winds in stride.

Wilbur Hubbard