SAILING GUIDE / TELLTALES

Small threads. A deeper understanding.

Learn to see the air your sails work in. Explore where the threads belong, what they show and the small adjustments that make a difference.

01 / The basics

What is a telltale?

Telltales are light yarns or ribbons that reveal the air moving where they are attached. They help you notice when a small trim adjustment is needed.

A streaming thread follows the sail towards its trailing edge. A lifting, spinning or hanging thread needs context: its position, the breeze and the sail’s shape. Start by separating headsail pairs from mainsail leech ribbons.

Windward
The side facing the wind. This can be port or starboard.
Leeward
The side away from the wind. The sides exchange roles after a tack.
Luff
The sail’s leading edge, where air normally meets an upwind sail.
Leech
The sail’s trailing edge, where airflow leaves the sail.

02 / Placement

The right thread in the right place.

Choose a sail to compare. The drawings illustrate principles; sail size, construction and your sailmaker’s instructions determine exact placement.

Telltales on a headsailA headsail viewed from the side, with the luff on the left and the leech on the right. Three numbered pairs of threads sit just behind the luff at different heights. Each pair has a solid thread on the near side and a dashed thread seen through the cloth from the far side.

Three pairs behind the luff at different heights. One attachment on each side of the sail, slightly offset so both threads can be seen.

Solid: the side facing youDashed: the far side of the sail

Colors distinguish the two sides in this picture. Windward and leeward depend on where the wind comes from.

Positions are illustrative. Follow your sailmaker’s advice for your sail.

Read both sides near the luff.

Jib, genoa and staysail

01Place
Fit pairs behind the luff at several heights, visible from your working position. On a cutter, each headsail needs its own threads; do not simply copy the outer sail’s trim.
02Read
With wind forward of the beam, both threads streaming aft provide a useful starting point. A lifting windward thread and a stalled leeward thread convey different messages.
03Adjust
Hold your heading: if the windward thread lifts, trim in a little. If the leeward thread stalls or flutters, ease a little. Then compare the higher pairs.

This describes ordinary headsails. A specialist spinnaker staysail needs its own trim guide. An occasional lift is not always an error.

Sources: UK Sailmakers · Headsail telltalesUK Sailmakers · TelltalesNorth Sails · Steering upwindUK Sailmakers · Specialist spinnaker staysails

03 / Read the threads

Read the signal. Make the adjustment.

Try three sheet settings with the same course and wind. The two sides of a headsail can tell different stories at the same time.

Small threads make airflow visible. Compare three simple headsail settings and see how to adjust the sheet.

PRACTICE · UPWIND ON A FIXED COURSE

Same heading and wind in all three examples. Here we change only the headsail sheet.

Airflow toward the leech
Headsail viewed from the side: Both threads follow the sailA headsail viewed from the side, with the luff on the left and leech on the right. Solid telltale 1 is on the windward side. Dashed telltale 2 is on the leeward side, seen through the sail. Streaming aft Streaming aft12
Headsail viewed from the side

Both threads follow the sail

Threads on both sides point towards the sail’s trailing edge. This is a useful basic setting to start from.

1

WindwardStreaming aftSolid telltale · windward side

2

LeewardStreaming aftDashed telltale · leeward side, seen through the sail

What to adjustKeep the sheet setting and watch boat speed and balance.

The examples only change the practice illustration and are independent of the boat’s automatic basic trim.

01Headsail luff

Headsail threads sit in pairs near the luff, one on each side. Windward faces the wind; leeward faces away. They swap sides when the wind changes sides.

02Mainsail leech

The mainsail telltale sits on its trailing edge. It should generally flow out behind the sail. If it stays hidden behind the cloth, the top may be too closed: ease the sheet to open the leech and check again.

The boat’s threads illustrate basic trim. The examples apply with wind forward of the beam; they are not a universal rule in a wind shadow or deep downwind. Also read boat speed, heel and helm pressure.

04 / On the water

A calm routine on the water.

Choose a method, change one thing at a time and wait for the boat’s response before adjusting again.

  1. 01

    Choose your method

    Practise in steady breeze with filled sails and room to hold a course. Decide whether to trim to a heading or steer to an already trimmed headsail.

  2. 02

    Fixed heading: adjust the sheet

    Keep your direction and make one small sheet adjustment. Use the headsail guidance above. Do not change heading, lead and halyard at the same time.

  3. 03

    Fixed sheets: steer gently

    As a separate exercise with suitable basic trim: if the windward thread lifts, bear away slightly. If the leeward thread stalls, head up slightly. Leave the sheets unchanged for the comparison.

  4. 04

    Watch the boat respond

    Allow time for a response. Compare threads at several heights with speed, heel and helm feel before the next adjustment.

05 / Common mistakes

Common mistakes — and what to do next.

An unsettled thread does not always mean the sheet is wrong. Check the context before changing anything.

01

Confusing the sides

What you seeYou follow the same colour after every tack.

What to doFind windward and leeward from the wind first; the names do not belong to a fixed side of the boat.

02

Pulling harder for every signal

What you seeThe leeward thread is unsettled and the boat slows.

What to doAt a fixed heading, try easing slightly. More sheet tension can worsen already separated leeward flow.

03

Changing everything at once

What you seeYou cannot tell which adjustment helped.

What to doChoose one control, make a small change and compare before and after.

04

Chasing perfectly straight threads

What you seeYou react to every brief movement.

What to doLook for a repeated pattern. Gusts and waves change the flow; one instant is not the whole picture.

05

Only watching the lowest pair

What you seeOne part of the sail looks right, but the top behaves differently.

What to doCompare several heights. Lead position and twist may need attention using your sail’s tuning guide.

06

Forgetting a thread can stick

What you seeOne thread stays still while the others respond.

What to doCheck its freedom of movement and condition before making more trim changes.

06 / Installation

Give the threads room to move.

Start with your sailmaker’s guidance. Attach to a clean, dry surface and check that the threads move freely.

  1. Start with your sail

    Use existing positions or ask your sailmaker about placement and attachment. Choose lightweight yarns or ribbons and fittings intended for your sailcloth.

  2. Clean and dry

    Adhesive patches need a clean, dry surface. Follow the product instructions and sail-care guidance; do not improvise with stronger cleaners.

  3. Leave room to move

    Check that the thread can move without catching on a seam or reinforcement. On furling headsails, take luff reinforcements into account.

  4. Make the pair distinguishable

    Offset the attachment points slightly and choose visible contrast. Check your colour convention; red or green does not always mean windward or leeward.

  5. Check before interpreting

    Make sure the whole free end can move. A stuck, tangled or damaged thread needs attention before it can provide useful information.

Sources

Further reading from sailmakers. This guide summarises the principles for practice and learning.

TRIM COMPASS

Put that feeling into practice.

Move the wind and see basic sail trim and telltales together in our interactive guide.

Open the trim guide