David Tyler wrote:I've taken a little gentle mental exercise, and done some number-crunching on this question:

Camber	Growth of diagonal, when measured along the cloth as opposed to the straight line distance, corner to corner
4%	0.45%
6%	1.0%
8%	1.8%
10%	2.7%

Camber;     P/B	Rise of batten above horizontal, to give 1% positive stagger
4%;     0.2	9.6 degrees
4%;      0.35	12.3 degrees
6%;     0.2	11.5 degrees
6%;     0.35	13.4 degrees
8%;     0.2	13.8 degrees
8%;     0.35	14.7 degrees
10%;    0.2	16.0 degrees
10%;    0.35	16.3 degrees

So as the camber increases, so does the need for a shorter diagonal, but so also does the differential between the diagonals needed for narrow and wide panels diminish.

For comparison, the rise of the sheeted battens in the fantail sail is  8.2, 11.5, 15.6, 19.6, 26.8 degrees. The geometry of the panels is very different, of course, from that of a HM sail.

A great thread; good, useful information here.

For completeness, in case anyone is interested, I came up with a formula recently for calculating the diagonal of a cambered sail when its flat dimensions were known:

Stagger (%)= [{B/[Sq rt(P^2+L^2-2L*cos(90+rise)]}-1]*100

where B is the length of the panel measured along the batten (i.e. the length of the lens), P is the length of panel luff, and L is the length of the lower edge of the panel from luff to leech.

It's interesting to see that the stagger may or may not necessarily need to be a consideration. Thanks to all for contributing to this.

Later: it occurs to me that wider panels might have an advantage when the boat is pitching, as the air flowing over the sail would see camber over a greater range of pitch; though I suspect this may be one of those factors that don't apply in the 'real world'.

Karlis, I tried that once, but then it appeared that the luff of panel 3 went a bit slack. Then I terminated the THP in the yard and fitted the only block at the tip of batten 2. This is as close as I feel I need to have a true THP.

Arne

Gary K.

Your described luff hauling parrel via just 1 block, is what I call a Throat Hauling Parrel. It works well. See this little Write-up

Arne

Barry & Meps / Stellrecht & Schulte wrote:David,

Meanwhile...when you speak of how the fantail sailform reefs with nice positive stagger...Do you think that is only true at the amount of camber you have cut in it? Would it need to be tweaked if sewn with significantly more or less camber in it?

Barry

Barry,

David,

I wish I had thought about this diagonal length over the camber issue when I designed and sewed my sails. Of course I did it a over a year ago and this discussion happened a couple months ago. Looking at this table, I would get 1% positive stagger at a boom angle of around 14 or 15 degrees, instead of the 10 degrees I designed.

As my sail has gone up and down while I work on it, I've found it wanting to show negative stagger. I bet this would have helped a lot. It also would have given me a little more room for my sheet spans, which would might be helpful too.

Meanwhile...when you speak of how the fantail sailform reefs with nice positive stagger...Do you think that is only true at the amount of camber you have cut in it? Would it need to be tweaked if sewn with significantly more or less camber in it?

Barry