So, I was thinking about sail shape and floppy heads and twist off and I came to the conclusion in my thought experiment that when the head of the sail is *fully* twisted off, and in essence the head is fully de-powered, it is doing nothing but dragging along in the slipstream.
But it's not doing "nothing" because similar to a tail fin on an aeroplane, or streamers on a kite, it is providing stability. It's part of if not entirely what makes your sail so stable. There is some drag but the stability it returns more than compensates for that.
So what if you had a really, really large head on your sail, or for that matter, dare i say it, streamers trailing behind the sail? : \
Perhaps a long, floppy clew?
It seems like this would provide some lateral stability allowing you to use a smaller fin, giving you much less drag in the water.
Am I onto something or am I still high from the weekend? Does it all just equal out in the end anyway or something?
No, it does do something, the apparent wind is a different direction up there.
Still remember my dad watching me rig up and scream 'stop stop you're bending it'. 
So a floppy leech is good cos it creates interest from old folks. 
It twists off to cater to the apparent wind being a different direction at the head of the sail, but also to have less power up high and more, relatively, down low. And if it's really loose it's effectively depowered completely at the head and therefore just trailing in the wind; providing stability up high. You can feel it more on larger sails. Too much downhaul and you need to put your masttrack forward to keep the nose of the board down there's so little drive at the top of the sail.
Anyway, maybe I was just too high and looking for an excuse to put streamers on my sail.
I think its about providing a smooth transition for the wind from the high pressure at the load area to ambient pressure as it leaves the sail.
Old fashioned sails created a heap of turbulence at the top particularly when the sail deformed under load.
To avoid the hook the sail has to be flat at the top in the minimum loading condition. When its further downhauled for strong wind it has more flat area at the tip than it needs. Even though you may be carrying more area than you need the drag accruing to it is much less than the alternative of a hooky turbulent tip.
If I recall correctly it was Bruce Peterson who discovered the significance of a twisty unloaded tip when he was working for Gaastra. He was out in the test rig, a van with the facility to mount a sail on the top and load cells to detect the lift developed. The van was stopped with a test sail mounted. The wind was blowing and he poked the tip of the sail with a stick and watched the lift reading go up.
Don't use a smaller fin. Nothing you do to the air has any effect on the lateral resistance in the water.
Select to expand quoteNotWal said...
I think its about providing a smooth transition for the wind from the high pressure at the load area to ambient pressure as it leaves the sail.
Old fashioned sails created a heap of turbulence at the top particularly when the sail deformed under load.
To avoid the hook the sail has to be flat at the top in the minimum loading condition. When its further downhauled for strong wind it has more flat area at the tip than it needs. Even though you may be carrying more area than you need the drag accruing to it is much less than the alternative of a hooky turbulent tip.
If I recall correctly it was Bruce Peterson who discovered the significance of a twisty unloaded tip when he was working for Gaastra. He was out in the test rig, a van with the facility to mount a sail on the top and load cells to detect the lift developed. The van was stopped with a test sail mounted. The wind was blowing and he poked the tip of the sail with a stick and watched the lift reading go up.
Don't use a smaller fin. Nothing you do to the air has any effect on the lateral resistance in the water.
I may be off the mark but I hate big brands taking a previous idea and saying it is their own brain child. There is way too much of that these days.
I remember Midget Farrell espousing the virtues of a flexible sail head for wavesailing in the early eighties (when the current trend was for tall, thin, stiff sails) and using them himself. This predates the current modern sails by 20 odd years. I am guessing Bruce Peterson made his discovery 15 odd years ago?
I had a photo of him on the south coast using one of his "twisting head" sails.
