Arne wrote:
Kurt, I am looking forward so reading a more detailed article about how to achieve camber through twist.
I answered:
Arne: About getting camber through twist - a picture is worth a thousand words, thus the photos in the 'Merits of...' article. It was easy to get that camber. It isn't much, but it's enough.
But I think, a little more detail...
If yard and battens are horizontal or there is no twist allowed, there's no fan camber. If the yard is quite steep and the battens rise at increasing angles, each batten up from the boom, and some twist is allowed, the air finds a curve to follow while travelling across the upper battens roughly parallel to the water.
It's easy to simulate this with my hand held up like a fan, with twist. Actors do it often in Bollywood movies.
(Yard and battens could all rise at the same steep angle, but not in a practical sail.)
Relative to Hasler & McLeod's recommended sail, there'll be more fan camber if :
- the yard is high(er)-angled
- the upper battens of the 'parallelogram' are kicked up above parallel, each one up, a little higher
- only one batten meets the yard at the throat, thus giving a higher rise angle to one more of the battens (transitional panel?)
(Practical point - got to check effect on sheet-snagging and batten stagger, using H-M's methods.)
David Tyler's Fantail sail design emphasises these features, without reference to the H-M parallelogram shape.
I'm pretty sure you knew all that...
It's when we're reefed that the fan camber in our sails is most obvious and probably makes the most difference in drive. That's either when the wind is strong, or we're controlling our speed, or when I couldn't be bothered to raise more sail for a short skip across the bay.
Other kinds of camber, like cambered panels, display full shape in lighter conditions, when camber is most needed.
David's recent design lets the full sail 'fan' better than mehitabel's full sails. The rise angles are higher, and they grade up from the very bottom of the sail.
Cheers,
Kurt