I would place tell tales on the leach of the sail, maybe 4 along the length of the leach. When they are streaming, they are indicators of smooth flow off the leach. When fluttering, they indicate turbulence at the leach. Stable flow off the leach is one indicator of proper sail trim for the conditions..In response to Don's message on fractional rigs and tell tails, I would like to know your opinion on telltales for gaff rigged cat-boat with a single sail. I have difficulty using the telltales as as good of an indicator of sail trim.