Is there a logical sequence of events in sail design or is it a hit and miss type thing. By this I mean that if a designer has a good working set of sails(no obvious defects) how does he take the next step towards making them better. Is there certain boat manuvers that can be made to show weaknesses in design or does he just say," I wonder what would happen if—."
I think what it really boils to is to take a look at the airflow over the sails under different conditions. Can you get the twist you want for lighter winds while maintaining good shape in heavier winds? How does the camber distribution look for various settings of outhaul and mast bend? Are there any hard spots in the sail? where is the camber placement at each shaping stripe?
If you have room to vary the sail dimantions, then you might look at the balance of the jib and main. Do you need a bigger jib and smaller main or vice versa?
Some other things to consider are the jib luff sag and mast bend allowances. How does your jib shape look when loaded? Did you leave enough allowance in your jib luff for sag?
There are a few different points of sail to consider. For example, when going downwind, how much does the leach twist off when propertly set up for upwind? You might have a set of really great upwind sails that are so mis-shapen downwind that you loose all your drive. But in many cases, the sail shape for different settings is more controlled by the rig stiffness than by sail shape. But you might need to cut your sail shape differently if you are having trouble getting the shape you want…
We were camping and sailing last weekend and were sitting around the campfire. There was a bit of a breeze blowing and I, as usual, was poking the fire with a stick. This stick, for some reason, put out a very thick stream of smoke when the flame died. The boats were on their stands not far from the fire and on a whim, I held the smoking stick up in front of the jib. The smoke boiled around both sides of the sail,obviously stalled. I turned the boat into the wind and now the smoke streamed nicely around the sail. It was very interesting. I’ve seen wind tunnel tests before but doing it myself was kind of neat. I think it could be useful if I could get a better supply of smoke. Does anyone know what they use in wind tunnel testing?
Wind tunnel smoke is made by burning Ethelene Glycol (automotive anti-freeze). This may not be the easiest way to make the smoke becuase you need a wand with a heating element in it to burn the fluid. But it produces a very good smoke with the right consistency and density to allow for good visualization.
I would say that an easier way to visualize the flow would be to load up the sail with tons of telltales. Rod Carr recommended strips of Sig Silk. I have been using these on my sails and it works great. Also, you should use the Accumeasure program to measure the sail shape. The thing about the Accumeasure is knowing what you want the sail shape to look like. but the great part about it is that you can make small changes and take a picture and compare the result. So you can really get a good sense of how much of a particular adjustment (vang, backstay, outhaul, etc) you need to get your desired result…
Take a look at this site, it shows some interesting info about the airflow close to the water. http://www.onemetre.net/Download/Gradient/Gradient.htm