Gary King wrote:I've been messing around with Arne's chain calculator and some numbers.
Does this mean a shelf footed sail with 5% camber have the same aerodynamic lift as 10% barrel cut? Because, it looks like they both use the same amount of sail fabric.
edit:
Arne, on Johanna's sail, does the "R" value at 26cm give an actual 8% camber?
R=(chainlength-batten distance)/2
Gary,
A barrel cut panel with 10% camber in the middle of the sail will have much less camber towards the top and bottom of the sail. The cross section through the panel is intended to be a catenary curve, such as a chain takes up. Thus, it's not accurate just to say "10% camber" with this type of construction. An average might be 6 - 8%?
A shelf footed/headed panel, with horizontal shelves, will take the designed camber close to the top and bottom of the sail (I think 6% is optimum), and will blow out under wind pressure into a slightly greater camber (8%?) in the middle of the panel. Slieve believes that this type of construction is slow to inflate in light airs, but I must say that I didn't find it so on Footprints' first sail with her new rig, in light going. At it's best, this construction provides the most even depth of camber over the full width of the panel.
Slieve prefers to put in the extra work to set the shelves at an angle to the horizontal, to reduce the amount of cloth in the sail. This tends towards the round, barrel cut, in providing a trapezium-like cross section, rather than the "three sides of a rectangle" of the horizontal shelf foot/head.
If you take Slieve's point about the head of a horizontally-shelved panel not lifting easily to a light wind, you would, ideally, put a shelf here at 45 degrees. But the same argument cannot apply to a shelf foot, where a horizontal shelf does not need to be lifted by the air pressure.
Broad-seam in a vertical clothed panel, or tucks in a one-piece, barrel-shaped panel, can duplicate the effect produced by either a horizontal or an angled shelf. My preference will remain for an even depth of camber over the full width of the panel, if I can achieve it, but there's no way of proving that it produces more aerodynamic lift than a curved cross section, barrel-shaped panel. In the end, it's about what kind of appearance you want your sail to have, as you sit at the tiller and look up at it. As long as you have an average camber somewhere in the region of 6 - 10%, you'll get the performance you're looking for.