Pete,
In order for your other control lines to work properly, your spreaders and diamond wires must be close to right for your weight and sail shape. Get these in the right ballpark and then the downhaul and rotation will work for you. You guys really need to look to the A’s for rigging advice. Here is a quick synopsis from AHPC, Nacra, and Landy (as I understand it).
First, your sail maker has a hard time cutting a luff curve that is not smooth and a constant arc. Your mast will have a bending curve depending on the combination of the spreader rake and diamond wire tension. When you sail you must determine if you have good height (pointing) but poor speed (other boats squirt forward while your boat wants to heel), or if you have good speed, but lack height (everyone seems to point higher than you).
To go forward in a puff you need a flatter sail, to point higher you need more power (fuller sail). This is where the amount of pre-bend comes in.
The hard part comes when you realize that if you only adjust one parameter, your mast will not bend in the constant curve you sail has. Too much spreader rake and your mast will bend low and be straighter up higher, and conversely spreaders too far forward will give you a straight mast low and a curve up higher. So, first measure your pre-bend, spreader rake and diamond tension. For pre-bend string some fishing line from mast base to top and measure the distance to the track. (I place the mast on its side to negate gravity). For spreader sweep, place a batten or something across the wires at the spreaders and measure to the mast. A good sail maker can tell if your mast is not a good arc, but most of us cannot.
Tip: lay the boat on its side with the main up but NOT in the mast groove. Med sheet pressure and some downhaul and med rotation. Now look at the curve of our sail and the mast curve. Are they the same? (Play with sheet tension to get the sail close to the mast) If your mast has more curve down low your spreaders are too far back for your sail. If you just bring them forward you will fix the curve, but now your sail will be fuller (less pre-bend)…did you want or need that? (See above). If not, adjust your diamond wires to get back to the same pre-bend.
The goal is to have good power…first on the wire when the wind picks up, and still be able to flatten things out when it really starts blowing.
Hope this helps some of you.
Ken