Catamaran Sailing
Catamaran Pictures

On the Wire - Feature
Vol 1 - Issue 5 January 1997

Sailing by Telltales
How to read and use those little ribbons on your sails

By Kim Miller

Telltales do more than just enjoy rolling around in the breeze. Theymight like rolling around in the breeze on really lazy days, but mostlythey prefer streaming out with the wind running across their back. If thoseother boats are passing you by, take a look at your telltales. They shouldbe the first thing to tell when you are slowing down.

One of the surprising things about telltales is that they are so recent.Your local library probably has many books in its sailing section withno mention of telltales, and they will also be absent from the boats inthe photographs. Some sailers put what they called "tufts" ontheir sails, but they were an obscure minority. However, somewhere in thelast twenty years tufts have turned into telltales, and they have creptonto nearly everybody's sails and into most sailing books. Books and magazinesI have from about twenty years ago show not a telltale in sight and theyinclude some big name innovators such as Frank Bethwaite and Bob Miller(later known as Ben Lexcen).

What Do Those Things Really Do?

Telltales show the actual wind across the sail surface. That soundssimple, and simple is how the telltale likes it. However, the air flowon a sail can become complicated, and when it does the telltale will complainto you.

The wind you feel across your back when sailing is not the wind thesail feels. When the wind flows over the sail it has to make some compromises.

    1. There is a mast in the way that is not the wind's preferred shape.
    2. The sail is very thin, and the wind prefers a thicker section like anairplane wing without a concave lower surface.
    3. The angle of the sail to the wind sometimes makes the wind separatefrom the sail, or makes it build up against it.
    4. The sail is highly flexible and the wind prefers to push it into a lesspowerful shape.

In all these things the air flow will make its own compromises.

The mast means that there is a pocket of turbulence for the first fewinches of the sail. like a "wind shadow". The thin sail meansthat the wind has to follow a curved inner surface and can develop a thicklayer of still or slow air in there. The angle of attack might mean thatthe wind just can't bend around to the back of the sail quickly enough,and so it leaves a swirling bubble of air on the lee surface which robsthe sail of power. Or it might separate from the aft area of the sail andform a whirlwind on the lee side, also robbing the sail of power. Whenthe moving air hits the sail it slows down because of the friction. Thislayer of "slow" air can be several inches thick, and it can separatethe real wind from the sail, again robbing you of power and speed. Thewind can push the power producing surface of the sail inwards, causing,you guessed it, loss of power and speed.

The task of the telltale is to tell you what is happening with the airflow against the sail surface, and to allow you to make these compromisesbetween wind and sail as advantageous to you as possible.

Where Should They Be?

Do not put too many telltales on your sails. Although every telltalewill tell you something, you can only take effective notice of a certainamount of information at a time. As well as sail telltales, many sailorshave some form of telltale on their sidestays, masthead, and forestay separator.

On the mainsail the best sites for telltales is about twelve inchesback from the mast. Place a pair just above the forestay tang so it hasclear airflow unaffected by the jib. Place another pair halfway down thesail from the tang, or about the mid-position of the jib.

Another place for telltales is streaming from the leech. Attach themto the batten pockets at battens 2 & 3 (or 4) from the top. You willnot see too many small boats with leech telltales, but many larger yachtshave them. In a few more years this might be another change to small boatsailing coming from the racing world.

On the jib, place two pairs also. The top pair about one third downfrom the top, and the lower pair about one third from the bottom. Onceagain come in from the luff about twelve inches. Some people put two lowerpairs on the jib. The number two set is slightly higher than the firstand only six inches away from the luff. The second set is more sensitive(being closer to the luff) and so is used in light conditions, the lowerset (being further from the luff) is less sensitive and is used in heavyconditions.

Your telltales are probably already installed and flying from the samepoint on the sale for each side. A better idea is to have the starboardtelltale attached a couple of inches above the port. This is the same configurationas the number on your sail. By having them separated like this you canmore easily tell which is which in very bright conditions when the sunmakes it difficult to see.

Luffing and Stalling

There are two things which the sail might do to slow youdown.


Luffing is when you try to sail too high into the wind and the windcan no longer hold the sail out like a wing to induce an efficient forwardforce. The wind becomes undecided which way around the sail it prefersto move and is about to try both directions at once. Too far into a luffand the sail, especially the jib, flaps from one side to another.

The windward (inside) telltales will tell you when you are approachinga luff. When they start to point upwards or forwards you are either pointingtoo high or your jib is not sheeted in enough. To correct, sail off thewind a little or pull in the jib. A slight luff is not as serious as astall because the lee side of the sail produces the power. The inner sideeffects efficient air flow, but does not have the significance of the outersurface of the sail.


Stalling is invisible because the sail will still keep its airfoil shapebut it won't produce power. A stall is induced when you sail too far offthe wind, or have the traveller in too close to the centre on a reach.The sail's angle of attack is too great, and the wind can't stay attachedto the lee side of the sail. A pocket of turbulence results on the leeside, there is no real wind/sail contact, and the boat goes ever more slowly.

The leeward (outside) telltales will tell you when you are stalling.If they blow in circles or stream forward you are losing power. To correct,head higher into the wind, ease the main sheet, or run the traveller outfurther. You can understand this if you imagine that the leading edge ofthe sail wants to point almost directly into the wind. For most of itslife it will not be doing so, so it compromises on power output. When itis too far off the wind, (the wind is coming over the side of the boatand the sail is pointing too far along the "keel line") the compromiseis over, and you stop. It's kind of like an airplane stall, but you don'tfall so far.

Sailing By Telltales

    Going To Windward

When sailing to windward the apparent wind will not move forward much,so you can easily cleat the sheets and trim with the tiller. Telltalesshould be streaming, although the inside telltales can be "dancing"just a little. If your upper and lower jib telltales are behaving differentlyit shows that your clew plate setting or your jib block fore/aft adjustmentis tensioning the foot and leech differently.

The jib flow will effect the lower mainsail telltales because your jibis adjusted closer to the main, so take more notice of the upper telltales.All telltales should be streaming back, although the windward telltalecan be dancing just a little. Some people measure the "dancing rate"of the windward telltales going upwind. Any faster than a twirl every threeseconds or so and they correct, but this depends on wind strength. Telltalesare not used so much at higher windspeeds.

If the windward (inner) telltales start dancing too quickly, head alittle lower. If the leeward (outer) telltales stall, head up a littlehigher. Most people try to head up too high when going to windward andso they luff rather than stall.


Because the apparent wind can change quite quickly when reaching youwill need to watch the telltales closely. Be ready to play the jib andmainsheets in accord with the telltales rather than cleating them and trimmingwith the tiller. Whatever method you prefer, correct sail trim on the runaccording to The Two Simple Things.

    The Two Simple Things

      Correcting With Sail Trim: (Either with sheet ortraveller)
      If the outer telltale stalls let the sail out. If the inner telltale luffs,bring the sail in.
      Correcting With Tiller
      If the outer telltale stalls, push the tiller out. If the innertelltale luffs, pull the tiller in.

    Going Downwind

It was a great disillusionment for me when I found out that not onlydoes a sail boat not go its fastest directly downwind, it can even be itsworst aspect. Because catamarans can be so slow downwind it makes senseto get the best performance out of the sail as possible when running. Manycats perorm better by tacking downwind rather than straight running.

When tacking downwind the upper and lower telltales can tell you howeach part of the sail is behaving. Try to keep top and bottom at the optimum.Move the traveller in until the lower lee telltale begins to stall. Easeout the traveller until it streams backwards again and cleat the travellerin that position. Pull in the main sheet until the upper lee telltale beginsto stall, then let out the sheet until it streams backwards and cleat themain sheet in that position.

The lower part of the sail is controlled with the traveller, the upperpart of the sail with the sheet. Trim downwind according to the telltaleswith the tiller. If the telltales luff, head down a little, if they stallhead up a little.

But What About Those Leech Telltales?

Have you fastened some ribbons to your batten pockets yet? Give it atry and watch what is happening to the top portion of the sail. Whereasthe "standard" telltales are for steering, the leech telltalesare for indicating sail twist.

When the leech telltale disappears around the lee side of the main itmeans the sheet is too tight, the sail has too little twist, and an eddyis forming behind the mainsail. Ease the sheet a little.

When the lower telltales stall (depending on their position from thetop of the sail relative to the jib) they can also indicate that the jibis sheeted badly and causing the airflow to separate from the leeward aftsurface of the main.

Boundary Layer Separation

Take another look at the first illustration in this article. Thingsare never quite that perfect. Even on the best of days with the steadiestwind and the neatest of sail trim there is a compromise happening betweenthe wind and the sail. when the wind travels over the sail it slows downa little because of friction. This means that there is a stream of windagainst the sail which is slower than the real wind. This layer of windis called the boundary layer and can be several inches thick in places.

The boundary layer will always be there, but the thickness depends onthe wind speed. As the wind speed drops the boundary layer gets thicker,up to several inches. When it gets to the point where it can't keep contactbetween the wind and the sail, the moving air flow separates from the sailand the power is lost.

The telltales show how the wind is behaving close to the sail, whereyou can't see it or feel it. When the telltales indicate that the air isnot flowing smoothly across the sail, it is because of the compromise betweenwind and sail caused in part by the boundary layer. When the telltales are flowing smoothly, so is your boat.

Kim Miller

Back to Features