Telltales are short lengths of light yarn or ribbon attached to a sail so that the sailor can see how the air is flowing across it. Most commonly they are fitted in pairs on either side of a headsail luff and singly along the leech of a mainsail, though they also appear on shrouds and backstays as wind indicators. To a beginner, telltales are the simplest trim aid there is: if they stream aft, the sail is working. To an experienced sailor they are a flow-visualisation instrument, and reading them well is the difference between a boat that is merely sailing and one that is being sailed.
How telltales work
Air flowing smoothly over a sail is attached to the surface, and a telltale sitting in that flow lies flat and streams straight back. When the flow separates – because the angle of attack is too great or too small – the air behind the separation point becomes turbulent and slow, and the telltale lifts, stalls or spins. Because the two sides of a headsail see different pressures, the windward and leeward telltales tell different stories.
On a headsail luff, the leeward telltale is the more important. If it stalls or droops, the sail is over-trimmed or the boat is sailing too low for the trim: the angle of attack is too great and the flow on the low-pressure side has separated. If the windward telltale lifts and dances while the leeward one streams, the sail is luffing slightly: the boat is too high or the sail is eased too far. Both streaming aft means the sail is at the right angle to the wind at that height.
Leech telltales on a mainsail work differently. They show whether the flow is exiting the sail cleanly at the leech. If the top leech telltale streams most of the time, the leech is open enough. If it disappears behind the sail and stays there, the leech is too closed – too much sheet or vang, or not enough twist – and the back of the sail is stalled.
Placement and the sailmaker’s view
Where telltales go is a design decision. UK Sailmakers fits headsail telltales a short distance behind the luff, typically at about a quarter, half and three-quarters of the luff length, placed far enough aft that they sit in the region where separation first shows but not so far back that they read the disturbed flow near the leech. On a genoa with a foam luff or a furling pad, they are set aft of the reinforcement so they read the true entry.
The windward and leeward yarns are offset rather than directly opposite one another, and often coloured red to port and green to starboard, so you can tell which one you are looking at through a translucent sail.
Reading telltales upwind
- Steer to the headsail telltales in steady conditions: Aim for both streaming, or for the windward one lifting occasionally. The helmsman uses the telltales; the trimmer sets the sail so the helmsman can steer to them.
- Check the vertical spread: If the top telltales stall before the bottom ones, the sheet lead is too far forward and the top is too closed; if the top luffs first, the lead is too far aft or the halyard is too loose. Move the lead until all three break together.
- Use the mainsail leech telltales for twist: Trim until the top telltale streams most of the time and folds behind the leech occasionally. Always stalled means too much leech tension; always streaming can mean the top of the sail is too open and not working.
Cruising and racing use
A cruising sailor uses telltales to answer one question: is this sail doing its job? On a long passage, a glance at the headsail telltales tells you whether the autopilot has wandered or the wind has shifted, long before the speed log does. Telltales also make reefing decisions easier, because a sail that will not fly its telltales at any trim is telling you the angle of attack has become impossible.
Racing sailors use telltales at a finer level. Trimmers use the leech telltales to set twist to the exact edge of stall for the conditions, and they read the differences between top, middle and bottom telltales to diagnose lead position, halyard tension and sheet tension. Telltales tell you about flow, not about speed or heel, so read them alongside the boat, not instead of it.
Frequently Asked Questions about Telltales
Which telltale should I steer by upwind?
In steady conditions, steer so that both headsail telltales stream aft or the windward one lifts occasionally. The leeward telltale stalling is the most costly error because it means the sail has separated on the low-pressure side.
Why do my mainsail leech telltales always disappear behind the sail?
The leech is too closed. Ease the mainsheet or vang, or add twist by easing the sheet and moving the traveller up, until the top telltale streams most of the time.
Telltales turn an invisible flow into something you can see and act on. Learn what each one is telling you – luff, leech, top and bottom – and your trimming and steering will improve faster than with any electronic instrument on the boat. Every sail that leaves a UK Sailmakers loft carries them for a reason.