It is nice to get the telltales going on both sides of the sail but not always possible. Remember that it is the one that is harder to see that is much more important. Of course the flow might be attached in one part of the sail and not in another, for which reason some people put so many telltales on their sail it looks like a part made rag rug. It is difficult to argue against this as such people also tend to sail better than me, but to begin with, a pair of telltales a few feet up and about a quarter of the way back should suffice.
You may come across experts or literature that suggests that telltales should be attached to the leech but I have not found this to be useful. I have also been told, by an eminent sailmaker, that there is no point putting them where we do because the flow has not reattached to the sail at this point, after having been disrupted by the mast. This is evidently untrue but I am unwilling to discount such opinions and I wonder if we enjoy some advantage with our size or type of rig, which is denied to larger sails.
Telltales can be bought in a little packet at an inflated price from the chandler. Alternatively you can make your own, which might be just as good. Telltales are so important that I would always improvise rather than do without. (They are much more important than a burgee.) The easiest way to make them is with darning wool. Middle six or eight inches, knot it, sew it through the sail with a big needle, put a knot on the other side and that is it.
If, in these degenerate and thriftless times, you have no darning wool, coloured polythene strips cut from printed carrier bags, or cassette tape work perfectly well. Either may be stuck with duct tape or, if you have left even that at home, there is usually something in the first aid kit. Make sure the bottom end of your telltale cannot snag on a seam as this may stop it working. I cannot think of anything else you can make for a boat that is so cheap and that will make it go so much faster.
Sri Lankan Sailing Canoe (DaveP)