In the late ‘60s when I started sailing, I saw that many racing boats used a pair of telltales on their headsails, about 24 to 36 inches back from the luff depending upon the size of the boat. When I asked my sailmaker about them he said:
“When the leeside telltale is wiggling wildly, the sail is stalled. When the windward telltale is streaming upward, you are close to luffing. If you put them too close to the luff, you will get false readings because of the turbulence behind the headstay. To see the leeside telltale, we often install a plastic window.”
A couple of years later I was studying the airflow about very thin fighter airplane airfoils (I was working at Douglas Aircraft at the time). I found that as the angle of attack was increased slightly, a small separation bubble would form on the top side of the airfoil near the leading edge. The outside airflow would then reattach to the airfoil and continue to the trailing edge without stalling. Increasing the angle of attack caused the bubble to get longer until finally the bubble would burst and engulf the entire upper surface. The airfoil would then be in the stalled condition with less lift and higher drag. As I made the airfoil thinner, the separation bubble would form sooner.
I suddenly saw the light! The sails on a boat were the ultimate thin airfoils. For wind tunnel testing, we often attach very short pieces of string all over our models as a visual means of studying the airflow. In the aircraft business we call them tufts. That weekend I taped about 500 short telltales all over both sides of my jib and mainsail and went sailing. It looked weird and a few people scratched their heads.
I stood at the bow watching the lee side of the jib as my wife, Pat, steered the boat. I could clearly see the leading edge separation bubble form as she came off the wind. The bubble grew in length as she sailed wider until it finally burst and all the tufts on the lee side were twirling wildly. It was obvious that a system of short tufts near the luff would be a great tool in steering the boat. I removed the 500 tufts and installed a short series of tufts on my jib. They dramatically improved my windward performance.
I then searched through the sailing books and magazines. All the pictures showed just a single telltale well back from the luff as my sailmaker had described. With my new Tuft System, I had discovered something new in sailing!
As part of my series on How Sails Work in SAIL magazine, I wrote an article describing my tuft system for the November 1973 issue. A few sailors picked up on the idea and eventually I started to see pictures of my system on a few top boats. On the bigger boats the tufts are usually placed further apart then in my original SAIL article.
In a series of rewrites of my sail theory material for a Dutch sailing magazine, Waterkampioen in 1985, the author, Jan Slijper, calls them Gentry-verklikkers. In the U.S. they are known as the Gentry Tuft System.
The first link below describes my system in more detail. The other link contains related articles on the use of tufts and telltales.