three sets
Telltales show attached flow. You should use three sets - one 1/4 from the top; one 1/4 from the bottom; the third in the middle. The bottom is the easiest to see from the windward rail, but the middle are the most important. The top tells are only useable to establish correct lead position for the sheet (all 3 sets should break evenly when the helmsman luffs into the wind). Locating tells too far forward is a mistake. The headstay and jib wires can interfere with the flow. 12" frum the luff is about right for a 25' boat. Lose those store-bought ones. They'll fray to nothing in no time. Plus, they stick to the sail. Cotton yarn is by far the best stuff, and cheap too. Get green for starboard and red for port. Classy! By convention, the port tell is located just below the starboard so you know which is which w/o having to think about it. While you're at it, don't forget the telltales on the upper shrouds. Put them at about shoulder height. On the backstay too, for when you're sailing with the wind behind 100 degrees. Billy is right on the mainsail leech location. The only ones I ever pay any attention to, though, are those at the top batten. When you're on the wind, they should just be stalling.
