Wool telltales
Proper sail trim is an important part of sailing. It allows your sail to work efficiently, so you can make the most of the wind. Pulling in the mainsheet or “Sheeting in” too much will stall the sail, causing it to loose power. This leads to slower boat speed and increased heeling. On the other hand, not sheeting in enough will allow too much wind to spill from the sail also resulting in slower boat speeds. So, How do you know if a sail is sheeted in properly?
For “soft” sails, or sails that don’t have full length battens, the basic procedure is relatively simple. Hold your boat on course, then sheet in the sail in until the leading edge of the sail, called the “luff”, stops fluttering or “luffing”.
With fully battened sails that don’t flutter, like the one supplied with the Kayaksailor, determining proper sail trim can be a bit tricky. An experienced sailor can trim the sail until it “feels” right. But even they can have difficulty when the wind is light or shifty. This is why we now include a set of telltales with each rig. These are the small lengths of red and green wool yarn attached to the sail.
By learning how to read the telltales and adjusting the mainsheet accordingly, it’s easy to find proper sail trim. You can’t actually see the wind, so the telltales allow you to see the effect of the wind as it moves around the sail. The wind should flow smoothly on both sides of the sail. So, if the sail is trimmed properly, the telltales should also flow smoothly on both sides of the sail.
If your sail is sheeted in too much, the downwind or “leeward” telltale will move erratically, while the upwind one or “windward” one will flow smoothly.
If your sail is not sheeted in enough, the windward telltale will move erratically while the leeward one will flow smoothly.
It’s basically a matter of sheeting in or out until both telltales are streaming equally.
The telltales should be placed on the sail near the area of deepest draft. For our own sails, we chose an open area between the battens and above the reef points so we can see them when the sail is reefed. 
Some people put theirs facing forward, claiming they don’t get snagged by the batten stitching and fly better.
Be sure not to get too caught up staring at the telltales too much. They can be a bit hypnotizing. Plus, there is all that beautiful scenery out there to enjoy.
Here is a link to a nice website from WB Sails, describing the use of telltales. Check out the cool animation they came up with for showing windward and leeward side bubbles. http://www.wb-sails.fi/news/95_11_Tellingtales/Tellingtales.html
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