In the spirit of Christmas I placed some tinsel on the mylar sails of my remote controlled sailboat. A wind tunnel was set up in my laboratory using an air mover fan set on a cooler. The sailboat was placed in various orientations.
I am trying to load some videos but not having great luck.
The wind was surprisingly flat and well redirected. By 15 feet away the air had only spread out from a 6 inch nozzel to about 2 feet. This was measured with tinsel 3M'd to a kabob stick.
The tinsel was about 6 inches long glued on both sides of the luff and 6 inches of tinsel glued to the leech. I got to see air moving from the luff to about twice the foot out.
Some interesting observations.
1. On average, as air flows around the jib it leaves the jib at pretty much the direction as it is trimmed, for a long distance out. If the jib is trimmed to 45 degrees it leaves 45 degrees, if it is trimmed in 80 degrees it leaves at 80 degrees. This lookes like a true deflection/redirection.
2. Upwind. I started out with the boom on centerline. It was sort of stalled. Lee telltale looked great with the boom to lee a little. This is boat specific, but a start position. I centered the boom again and accepted poor telltales.
The jib is slowly trimmed in. As it is pulled in the trailing airflow comes with it. At some point the airflow trailing the jib is directed to the lee draft of the main. The main lee telltales pick right up. This must be the "slot". There is no veturi or anything like that. Trimming the jib simply redirects more moving air over the lee main(lift surface). With more trimming the jib redirects the airflow right into the leading edge of main. The telltales and main flop. This is the backwind. Over trimming the jib has the same effect as decreasing the angle of attack on the main.
3. On a broad reach. The main sail splits the wind up in a stall. Much like a bird coming in for a landing. There is a cool effect where the air flow does a 80 ish degree turn between the mast and the jib. Its really noticable mid jib. the air then deflects out past the jib as if on a beam reach/close reaching, at whatever the jib is trimmed at, boosting the main lee telltales some. Main luff windward telltales are sucked up around the mast to lee.
Take-a-ways (from a noob point-of-view). Assuption is flowing telltales equal best sail performance.
The headsail is magical. The jib affects the main. The best trim is when the airflow from the headsail is directed to the deepest camber of main. When the jib is trimmed right the main can be trimmed to a lower angle of attack. Adjust main to take advantage of this.
A well trimmed and cut jib increases pointing AND a poorly trimmed/cut jib devastates upwind pointing.
Jib helps main even downwind. It redurects main turbulence to back over main lift surface.
The well trimmed/cut headsail isnt just extra sail, its a mainsail force multiplier.
