Learn · 7 min read
Reading telltales and trimming the sail
Air is invisible, but a few short pieces of yarn on the sail make it visible. Once you can read them, you always know whether to sheet in, ease, or steer.
You cannot see airflow, but you can see what it does to something light. Telltales (also called tell-tales or woolies) are short strands of yarn or ribbon stuck to the sail. They stream in the air right next to the surface, so they show what the flow is doing at that exact spot.
This guide explains where telltales go, how to read them, and how to turn what they tell you into trim. The simulator draws telltales on its sail, so you can practise reading them before you go out on the water.
Where telltales are placed
A typical sail has two kinds:
- Luff telltales: near the front of the sail, about a hand’s width to a third of the way back from the leading edge, with one on each side. You see the windward one directly, and the leeward one through the sail (or through a small clear window).
- Leech telltales: on the back edge of the sail (the leech), usually where the battens end. They stream off the edge itself.
The luff telltales tell you about the angle of attack at the front of the sail. The leech telltales tell you whether the air is leaving the back of the sail cleanly.
In the simulator the leeward luff telltale is red, the windward one green, and the leech telltale is dark. Tick Zoom in on the sail for a closer look.
The reading table
| What you see | What the air is doing | What it means | What to do |
|---|---|---|---|
| Both luff telltales stream straight back | Air follows both sides smoothly | Attached flow: good trim | Hold it |
| Windward telltale lifts, flutters or spins | Air hits the front of the sail from the wrong side | Angle of attack too small: the sail is starting to luff | Sheet in, or turn away from the wind (bear away) |
| Leeward telltale lifts, flutters or spins | Air on the leeward side is breaking away | Angle of attack too large: the sail is stalling | Ease the sheet, or turn toward the wind (head up) |
| Leech telltale curls behind the sail and disappears | Air leaving the back edge has separated | The back of the sail is stalled, often over-sheeted | Ease the sheet a little |
A simple way to remember it: the telltale that misbehaves is on the side that has a problem. Windward trouble means too little angle; leeward trouble means too much.
Seeing it in the simulator
Here are three settings on the same boat, wind and course (12 knots, wind 60° off the bow). Only the sheet changes.
- Boom at 45°: angle of attack about 1°. The windward (green) telltale lifts and flutters, the front of the sail ripples, and the boat makes only 4.0 knots.
- Boom at 23°: angle of attack about 15°. Both telltales stream straight back and the boat makes 7.3 knots, its best speed on this course.
- Boom at 10°: angle of attack about 35°. The leeward (red) telltale spins, the leech telltale curls in behind the sail, eddies peel off the back edge, and speed falls to 4.4 knots while the boat still heels hard.
Notice that both mistakes cost about the same amount of speed, but they feel very different. A luffing sail is quiet and lazy. A stalled sail heels the boat and makes it feel powerful, but the power is mostly sideways and wasted in drag. Beginners often over-sheet for exactly that reason: it feels fast.
A reliable trimming method
Trimmers use a simple routine called ease until it luffs:
- Slowly ease the sheet until the windward telltale just starts to lift. The sail is now at the edge of luffing.
- Sheet in a little, just until the windward telltale streams again.
- Check the leeward telltale. It should stream too. If it starts to lift, you have sheeted in too far.
This finds the smallest angle that still keeps the flow attached. It works on every point of sail except a run, and it adapts automatically when the wind shifts or the boat changes speed.
In the simulator, switch Auto-trim off, pick a course, and try it. The readout Best trim shows the boom angle the model considers fastest, so you can check how close you got. The chart Drive vs sail trim shows the whole picture: drive force at every boom angle, with your trim marked.
Steering to the telltales
Upwind, sailors usually set the sheet and then steer to the telltales instead of trimming. Turning the boat changes the apparent wind angle, and that changes the angle of attack just as moving the sheet does.
- Windward telltale lifting: you are pointing too close to the wind. Bear away a little.
- Leeward telltale lifting: you are too far off the wind for this trim. Head up a little.
Keeping both telltales streaming with small steering movements is the core skill of sailing upwind.
Telltales on the front sail
Most boats carry two sails: the mainsail behind the mast and a front sail (the jib or genoa) ahead of it. Telltales work the same way on both, but sailors usually watch the front sail’s luff telltales first when sailing upwind. The front sail meets the wind first, its telltales are easy to see from the cockpit, and it reacts quickly to steering.
A useful habit is to put telltales at three heights on the front sail: low, middle and high. If they all break at the same moment, the sail’s twist (how much the top opens compared with the bottom) is right. If the top ones lift first, the top of the sail is luffing early. Moving the sheet’s lead point forward usually fixes it. The simulator shows a single combined sail, so it does not model twist, but the reading rules above are the same.
Gusts, waves and flickering telltales
Real wind is never steady. Gusts change its strength and small shifts change its direction, so telltales flicker even on a well-trimmed sail. Do not chase every twitch; look for what they do most of the time.
Notice how a well-trimmed sail sits close to the stall: a strong gust can briefly lift the leeward telltale. That is normal. If the leeward telltale is lifting most of the time, ease a little.
Downwind: when telltales stop helping
With the wind well behind the beam, the sail works more like a parachute than a wing. The fastest setting is often deliberately stalled, so a spinning leeward telltale is no longer a warning sign. In the simulator, the best trim on a broad reach of 150° gives a stalled sail.
Downwind, sailors ease the sheet until the sail just starts to curl at the front, then pull it back in slightly. Leech telltales remain useful on a broad reach, where some airflow still passes along the sail.
Key takeaways
- Windward telltale misbehaving: too little angle. Sheet in or bear away.
- Leeward telltale misbehaving: too much angle. Ease or head up.
- Leech telltale hiding behind the sail: the back of the sail is stalled.
- Use ease until it luffs, then sheet in a little to find the best trim.
- Downwind, a stalled sail can be correct; trust speed over telltales.
Next: what to do when there is simply too much wind, in When to reef: heel, drag and speed.
The simulator uses a simplified physics model: airflow around the sail, stall and luffing, hull resistance, heel and round-ups. It shows the right trends for learning; it does not predict exact numbers for a real boat. Numbers quoted on this page come from the simulator's 40 ft monohull unless stated otherwise.