Gentlemen,
I am extremely sorry that I have not responded to this thread earlier, but other pressures have kept me occupied.
I find it very interesting to see the diagrams that you are producing, and am also seeing that my explanations are not necessarily making my thinking clear to everyone. Chris's rotating rigs look great, yet they show the jib shape as it would be if made of tin plate. With a cloth construction the leech of the jib would be more like the arc of a circle, and on the jib centreline the sheeting angle would be wider than we get in my diagrams. Equally, there would be less camber in that cross section of the sail. I haven't tried to draw cross sections from luff to leech for quarter height of the panel as well as half height with the curved leech, probably because I feel it would be based on too many guesses, and I was taught that that was not the way to approach a design problem.
For the first SJ on Poppy I played around with my 'sheeting angle/ camber drawings' until I thought I was getting somewhere and than made a full size panel in wall lining paper, which I fitted between two battens and laid the luff horizontally. Gravity pulled the leech into the shape that the wind would eventually blow it, and I was able to measure the achieved sheeting angle and camber along the centre line, and as I said above, the angel was bigger and the camber less.
One thing I have always done when 'eye balling' a cross section was to only accept diagrams where the after 40% of the sail shape was no tighter than parallel to the batten. In other words, the after part did not curve in towards the batten. This was to stop the drag of a curved in leech. Now the question arises, am I wrong in making this cut off point? In my present way of thinking I believe I might be just a little restrictive.
The reason I think this is that I am still convinced that the camber in the first third of the rig is the real work horse for high lift and low drag.
Although not fully happy with the jib shapes I built the first rig, and the performance was fully up to my wildest dreams. As far as I was concerned I was equal to or better than the Bermudan sister, so would I be greedy to ask for more? If it ain't broke, don't mend it. If it works as planned, leave it alone. On the other hand I now want to ask the question, could it be even better? Frankly, The cruising Bermudan rig is not the be all and end all.
In one post Scott suggests that every degree we give away in sheeting angle takes away one degree of pointing. My answer is, 'Not necessarily.' Most people ignore 'up wash', and I have even read that one sail maker does not understand it. Ouch! It is critical to getting a boat to windward. With it the wind bends to give the boat a lift before it even reaches the luff of the jib. So how does the wind know the sail is coming, and bend in the right direction? Simple. The low pressure that develops to the lee of the luff sucks the airflow to bend it and the lift lets the boat point higher. That is why it is so critical to get camber working right up to the luff. That is also why the majority of junk rigs, which don't have camber right to the luff don't perform to windward, and give the rig such a bad reputation.
Scott asked, 'Wouldn't we want a sheeting angle of zero?' I don't think so, as the after part of the jib would have it's thrust at right angle to the sail and pointing aft, which makes drag, our great enemy. It may be that I have gone up in sheeting angle and should have gone down or stayed the same and just increase camber, but Poppy went so well that that I didn't want to experiment too much. I feel that I have made significant discoveries, and as I get older and having got a good lead in the first leg am happy to hand the batten on to the next runner in this particular race.
In some of the posts there are comments of getting the camber forward in the jibs, and here again I scratch my head. I know that I have written words to this effect, but I wonder if I should be talking about the camber shape of each individual sail, jib and main, or the camber of the overall rig, from jib luff to main leech. I really don't know the answer to this one, and in recent drawings I've tended to move the jib max camber point aft to around 40% (if I remember correctly) to try to slightly tighten the 'arc of a circle leech' of the jib and therefore not require the little sheetlets we have been playing with. I do feel they are unwanted complications and not in the KISS way of things.
Scott's drawings at the bottom of post #4481644 look very 'tight' to me, with the jib leeches coming in and then flaring a little close to the leech. I see your point about the air flowing across the split on the windward side, but I'm not comfortable with that (but who am I to say?). The air flowed freely on Poppy and it went well. Blocking or restricting the air flow just doesn't seem right.
Also, on Bermudan rigs the camber in the main is often further back at near 50% on some sails which are foresail driven, but further forward on mainsail driven boats with tiny (token) self tacking jibs.
There are so many questions that the theoretical design can easily take over, and will never result in a practical boat. Is it better to take the best of the proven knowledge and get sailing, and then play about bit a replacement rig? That was my theory, but the first rig was so good and so idiot proof and easy to sail that I was only prepared to tweek it a little (to what end?)
Once again, some possible answers and more possible questions. Sorry.
Cheers,
Slieve.