In my quest to understand and appreciate sail trim I've set up a test rig to record telltales in various angles of attack.
Its hard to find good videos of foils underwater. Most videos are of the smoke streams or are computational cgi.
I found a broken rudder in the garbage and figured it's size might be perfect for testing. It appears to be from some hobie or small dinghy. It is actually designed for water so tests in water should alleviate concerns about scale.
Got it home, cleaned it up and then placed telltales on one side of the bottom half. The telltales are a combination of cassette tape and the outer shell of sta-set double braid. The doublebraid fibers were flea combed to spread the fibers out. The mylar is taped on, the fibers are hot glued.
I choose the Sarasota Hydrodynamic Testing Basin, aka, the community pool. The video was shot with the potato 2000 gopro knockoff in 720p.
What I observed was interesting and informative with a couple applications.
1. The rudder cut through the water like nothing, literally a blade. The blade could be directed up or down and the response was instant. While slicing through the water up and down the telltales lay flat everywhere.
2. If while moving the rudder is angled 20 to 45 degrees the rudder simply changes direction. There isnt any noticable effort to twist the blade, just a quick deflection and the blade is going in the new direction. This is what i would expect a rudder to. If however, I angle the rudder, but do not allow it to go off in the new direction it starts acting more like a lifting wing. This is a common scenerio when the sail plan is not balanced or too much rudder is applied given the hullspeed. The rudder is turned to compensate, but the hull is providing resistance.
3. With the rudder at about 30 degree angle of attack the leading edge telltales luff first and as the rudder slows down or angles up higher the rest of the telltales all jump up and luff.
4. Just like a boat coming off a plane, there is a 3D wake behind foils. As the wing slows down abruptly from the drag, the chaos behind the wing catches up and overcomes the telltales, reversing the flow momentarily.
5. The same experiences were observed with an air mover fan in the garage.
Lessons learned: Turbulent air is as good as laminar air, if the turbulance is small, big swirls are bad.
Stalls are first noticable at the leading edge. There is almost no grace period between turblent leading edge and turbulent trailing edge. Its laminar, some leading edge stall or complete stall.
Seems telltales really only need to be on the immediate area 0-10% from leading edge of sails. If these leeward telltales are luffing, disaster is imminent. Once stalled, the wake of air from behind will rush in. Do everything to keep the boat on course and lighten up the angle of attack to prevent the wake from catching up and making it difficult to retrim and get going again with that big turbulence surrounding the sail.
I question if leech telltales have any value. If they are luffing the sail was stalling a long time ago.
Keep rudder and sail movement consistent with speed. If boat is moving slow, let rudder carve a clean line, quick movements that dont allow the hull to respond will result in drag and stalls.