Hi Scott,
Your comments in your mail on 23rd about napping strikes a chord. In the 2009 Island Race in Poppy I told my daughter and her husband off for working their way backwards through the fleet while I had been below and she replied that I should not have had such a long siesta. Ungrateful child (aged about 35 at the time). You just can't get the staff! In the 2007 race I was sailing master on a Hunter Pilot 27 called Romarin, and we came 6th out of some 700/800 boats entered, so it can be done, but I didn't allow myself a nap in that race.
Your model rig looks very good, and it should help you envisage how the full sized will work. Please don't take my comments as criticism, but as observations based on other viewings. The batten rise of 10° looks high to me now that I've seen Edward's 5° rise battens which is easy on the eye and looks as if it encourages airflow in the right direction. The yard angle looks very high and I don't think it will control the tip vortex. The split junk is an effort to get a high Lift/Drag ratio, so where it is possible to improve the lift with camber it is equally important to minimise the total drag. Total drag is made up from a number of components, and the major one is 'Induced Drag' (or tip drag or vortex drag which are not quite the same but are similar). This is one area we must try to get right.
The elliptical Spitfire wing was supposed to be the most efficient shape possible, and aesthetically is most beautiful, but since the 1940's no-one has used it as better results can be achieved more simply. Look at the trouble aerodynamicists (spelling?) have put into tip design, and the structures on the tips of modern airliners. Arne's tip plates on rudders is a simpler but also useful effort to help the L/D ratio, so we should also look for gains at the head of the rig, or to be more precise, less losses with inefficient sail profiles. As tip plates seem to be impractical though some have tried them, the best simple solution is to sweep the tip up at an angle that will discourage excessive airflow rolling over the sail and letting the vortex form further down the leech. It would appear to be best if the rotating air is pushed right up to the tip of the yard, and in the absence of wind tunnel information my best guess has be as used on Poppy and Amiina, at around 40-30°.
I now prefer the lower angle and feel the lower angle gives a better shape to the top tapered panels when it comes to defining the material patterns to get the desired camber. I feel the 30° yard gives good control of the tip flow and vortex.
I didn't worry about the angle between the yard and the luff of the top panel. The model will show that you do need the tapered luffs to balance the tapered leeches. Any cambered sail drawn with straight luffs but tapered or fanned leeches will end up requiring parrels to maintain the straight leech or have high stress at the leeches of the upper panels, as the angled leeches push the battens forward like a bow pushing an arrow. The model helps you get a balance between the forward pressure on the battens at the leech and the aft pressure at the luffs.
Yes, the yard has to be stiff. The weight of the whole rig is being carried by the halyard close to the middle of the yard, but the actual weight is carried at the tips of the yard, putting it under heavy bending moments. I tend to start with the sling point at 50% yard, but it usually ends up further aft, with the adjustable yard hauling parrel peaking it up and keeping it snug. A simple yard parrel keeps the yard against the mast, but lets it fall aft when lowered. You don't want anything tight when hoisting. Try hoisting the model rig from the lazy jacks.
At one stage I referred to Amiina's rig as being 'brutal' as it was almost a parallelogram and aesthetics weren't considered, but when sailing with the cambers filled it looks powerful and doesn't look bad at all. The small amount of taper in the top panel softens the shape nicely in practice.
Arne doesn't worry too much about the orientation of the cloths as it is not that critical with lowly stressed material, but I have always tried to follow normal sailmaking practice and place the thread line parallel to the leech. My reasoning is two fold. Firstly I want a clean run off for the air with no risk of a hooked leech, which will cause significant drag. And secondly, I form the leech with normal leech tablings and leech lines as is standard sailmaking practice. This way the cloth itself starts to take the structural loads towards the leech and must not develop bias stretch just inside the tabling. With a sail of any size I make the individual panels up from vertical cloths which is economical in material and with careful planning can provide the seams to use for broadseaming, killing two birds with one stone. It is really very easy when you can picture it. Try to make a model test panel with vertical strips of paper. Even angled shelf foot can be made up with vertical cloths, and be economical. There's not that much sewing involved, and when you use basting tape it's dead easy.
Enough for now. Does this make sense?
Cheers
Slieve.