UK Sailmakers' Encyclopedia of Sails
5.3 - Sailing to the Telltales
No indicator has been developed that approaches the sensitivity and effectiveness of telltales along the luff of the jib. These yarns or “ticklers” monitor the flow of wind across the sail.
Telltales are used for fine tuning your genoa sheet trim and to fine-tune the course you are steering. Telltales are only an aid when the sail has wind flow across both sides, i.e., when sailing angles between beating and beam reaching. When sailing lower than a beam reach, the sail is catching wind instead of working like an airfoil.
Telltales come in pairs, one on each side of the jib. The best set up is to have three sets evenly spaced along the luff so that you can see how the wind is flowing over all heights of the sail. Telltales should be anywhere from six inches to 18 inches aft of the luff, de- pending on the size of the boat — the bigger the sail, the further aft the telltales should be. Also, the farther forward they are, the more sensitive they are.
All UK Sailmakers sell inexpensive packets of green and red telltales that stick onto your sail. When applying telltales, always put the starboard one of each pair higher than the port one. This will help you figure out which is which when reading the telltales from the cockpit.
The most basic way to read telltales is to get the yarns on both sides of the sail streaming straight back. If the leeward one stops streaming, head up and sail closer to the wind. If the windward one stops streaming, fall off. When the yarn streams straight back, it tells you that there is attached flow over the sail at that particular point. If a telltale stops streaming and hangs limp, there is no air flow and that side of the sail is stalled. If the telltale dances around, flow is turbulent.
By concentrating further on interpreting the telltales, you can get even more information. The following are four different “settings” for telltales:
When sailing to windward, the degree of course change between “Bow-down Speed-building Mode” and the “Pinching Mode” should be about 5 to 7 degrees. If the telltales go from one extreme to the other after a two-degree course change, put a bit more tension on the luff. The rounder entry angle will broaden the gap between stalled from sailing too high and stalled from sailing too low.
Remember not to sail with the telltales in the pointing or pinching mode in light winds or in wavy conditions. The boat will lose too much speed. In heavy winds when the boat is overpowered, you can reduce heeling by heading up into the pointing mode.
Use your telltales to fine tune your lead position. With three sets of telltales along the luff, you’ll be able to shape the jib like a trimmer on an America’s Cup contender. In general, on a beat the lead is in the right place when the lower two sets stream straight back and the inside upper telltale dances about 45 degrees above horizontal.