As a teenager learning to sail and compete, I had never seen or heard of ’tell-tails’. As far as I can tell, they first appeared in the 1970’s but have now become almost ‘standard supply’ for most sailmakers – at least on race sails. Prior to this, we learned to adjust our sails by ‘how they looked’ and through a ‘seat-of-the-pants feeling’ of how we sensed the boat was performing. But having lots of boats to race against was a major help in an efficient adjustment process.
Today, these little flying tails, most prominent down the luff of foresails, give us a visual way of seeing what the airflow is like at that point on the sail or whether it is stalling or luffing, so it’s worth paying attention to them. If you don’t have any, it’s a good idea to add a few, so let’s start with finding good material and then, some recommended positions.
Tell-Tail material & installation. You need something very light that still has wind resistance, yet something that does not absorb and get heavy with water. It also needs to be a dark color to be visible through the sail, as the air flow will be different each side. Often sails have a small round window to be able to see the leeward (downwind) tail and these really help. Personally, I like the old 1/4” plastic tape that was once used for audio recordings, but when it’s raining, it needs to be crumpled up a little, or it can stick flat to the sail. The other good option is a dark colored virgin wool that has enough natural oil on it to prevent absorbing water. If you’re using recording tape, you can attach it yourself with circles of waterproof strong-adhesive tape (like top quality duct tape) and bond the end of the recording tape to this about 2/3rd across a 25mm dia round patch of tape and then bond it to the sail after first cleaning and drying it. 100mm (4") length on each side normally works fine .. but can be a little longer on a tall sail to be seen easier.
If you’re using wool, it’s best to sew it through the sail. Cut the wool at twice the final length (so 220mm) and tie a single knot in the center of that length. Then thread one end through a needle to pull it through the sail material at the chosen location and then tie another single knot on the other side so that the wool will stay central. Work the single knot close to the sail before tightening it a little. Another approach is to make a single stitch back through the sail about 3mm forward of the previous hole and then back into the first hole, but check periodically to see it has not slipped with too much wool on one side.
Tell-Tail locations. There’s no doubt that one could cover sails with tell-tails and see a lot clearer how the air was flowing over the whole surface … but this would drive most of us ‘nuts’. Such a layout is really only required for prototype tests, as experience has shown we only need a few tell-tails at critical spots to be able to adjust the sails in an efficient manner. For the foresail, you need one at ¼, ½ and ¾ height up the luff on each side, located far enough back from the luff wire and seams so that its end does not get caught during its ‘gyrations’ while tacking .. so typically about 100mm back from the luff.
Personally, I’ve never found as much need for a tell-tail at the luff of a mainsail (unless there is no jib) so avoid them rather than find them entangled when reefing. But there’s another useful location that applies to both sails and that is on the extreme rear edge of the leech - about 70% of the way up on the jib and even in two locations on the mainsail ... say 40% and 75% up, but clear of any batten ends they could become entangled with. These will show how the air is leaving your sail – which should ideally be straight back in most cases.
Using the Tell-Tails to sail better. Perhaps the first thing to answer is, what are we aiming to do for which tell-tails might help us ? Let’s first consider sailing upwind - as downwind is another story.
First, we want to keep the airflow on both sides of the sail as ‘attached to the surface’ as much as possible. This is most important on the leeward (downwind) side where the greater distance is creating a slightly higher air speed, that in turn, lowers the pressure and creates lift – particularly true at the forward edge of the sail which has the most camber. (One reason taller sails are more efficient upwind). Wind tunnel tests have shown that when sailing upwind, about 75% of the total lift comes from the leeward side, so always give this side priority. I used to be concerned about the vertical seams of radial sails and even the roughness of some new synthetics sails with encapsulated fibers possibly disturbing that smooth flow, until I remembered the success of the dimpled golf ball ;) Those dimples cause small eddies that in turn suck and hold the passing air closer to its surface, so that the overall resistance through the air is less. Perhaps we will one day see special fabrics that hold the air flow closer to it in a similar way. But meanwhile, even small horizontal puckers and pleats in the luff may hold small vortexes that may be more beneficial than destructive ..... pleats that some even call ‘speed wrinkles’ for perhaps justified reason.
But the larger eddies at the leading and trailing edges are more of concern here.
If our sail is hauled in too tightly, or if we are sailing too far off the wind for the set of the sail, there’s a high possibility of stalling the flow, and then significant eddies on the back side of the sail will kill the drive and boat speed will die. At the rear end of the sail, we want the airflow to come straight off the sail like a jet to give us ‘a parting push’ as we move forward.
With a single mainsail and no jib, sailing to achieve this ‘sweet flow’ is actually harder than when sailing with a jib, even if a single sail is theoretically more efficient with its longer luff. It just requires a greater concentration as ‘the groove’ to efficiently sail in, is now narrower. By adding a jib, this helps to keep the air flow attached to the back of the main … at least when its leech is correctly aligned. The very worst scenario is when the leech of either (or both) sails is/are curling to windward – a ‘no-no’ to avoid, as this not only kills the jet flow but actually acts as a brake … like putting your rudder hard over …. and it can halve your speed or worse!
Sailing in very gusty conditions or in waves that are throwing you around, can rapidly change the direction that the air hits the front of the sail making a perfect ‘in line’ interface almost impossible. In such conditions we need to widen the interface angle to be more tolerant and easing off the Cunningham (luff downhaul) will generally help achieve that. Also, if you have the luxury of a wing mast, a slight increase in rotation can have the same effect as what is sometimes called a NASA Droop on part of an airplane wing nose … a physical addition to help guide the air ‘up, over and around’ the upper side of the wing, to avoid the deadly stall.
In calmer conditions, the ideal entry of the mast and sail is when it’s perfectly parallel to the apparent wind – which will obviously move more forward as you pick up more speed. And to get the best from the rig, we want to be sailing as close to the wind as practical but without actually luffing when the drive will disappear. That ideal spot is what we call ‘sailing in the groove’.
This air flow is taking place quite invisibly, even if we can clearly sense the effect on the boats performance. So here is where the tell-tails help … to give us a visual view of what’s generally happening with the air on both sides of the sail.
As already identified, the flow over the leeward side is the most critical …. so we need tell-tails there and they need to be kept flying straight back, because it they start swirling around they are showing you that this part of the sail is stalling or already stalled. On the windward side, it’s also possible to have all the tell-tails flying straight back, but may not be the ideal situation as you could be sailing in an almost impossibly narrow slot and we’ve already identified that it’s the leeward side that needs to be given priority. So just try coming a degree of two closer to the wind and you’ll generally see the top windward tell-tail start to point up a little and as this is typically the first indication of luffing .. it’s also an indication that you are now sailing on the safe side of that narrow slot …. away from the greater risk of stalling that would happen if you had steered slightly away, downwind. It’s fine to sail like this and it’s efficient. But if the top windward tell-tail is frequently flickering up, the jib leech is probably too loose and you’ll need to add a little more vertical tension by bringing the jib sheet lead slightly more forward, but don’t overdo this, as its better to sheet too far aft than too far forward (unless the wind is very light). Obviously, if the bottom tell-tail collapses first, you’ll need more foot tension, but this is less common.
The other important driver upwind is the way the airflow leaves the sail at the rear. Here is where a flat leech is very important and also demanding that you adjust the mainsheet (and kicker/vang if you have one), so that the leech is straight up & down with minimal twist, and also in a plane parallel to the boats centerline Having a tell-tail on the extreme edge of the leech of both sails, fairly high up, will give you a visual guide as to whether this part of the sail is working correctly for you. Such tell-tails should be streaming straight back, with an odd flicker being normal.
While checking below this ‘going away’ photo of my W17 Magic, note the straight leech that is parallel to the boats centerline and if you check the top of the mast, you will see an angle of mast rotation that is slightly higher than the luff of the sail – making sure that flow is directed to the leeward side of the sail for the most lift and least risk of stalling. The jib is also tightly cleated and the windward jib sheet is being used to pull the clew (after corner) slightly in towards the mast, helping to keep the flow smooth and fast over the back side of the main …. so adding to the lift that is part of your forward drive.
In stronger winds, it’s better to feather the sails to prevent excessive heeling than fuss over tail-tails. But in lighter winds, they should dictate your adjustments.
This photo shows high mainsail efficiency when coupled with a stiff wing-mast.
OFF the WIND
When reaching, just get the mid-height tails flying horizontal as the foresail will typically have too much twist to get all 3 tails to flow well. When easing off the mainsail for a reach, the mainsail will luff if too far out, or stall if too far in. This is where the mainsail tell-tail high on the leech is useful, as it will go a little crazy if you’re stalling, due to being hauled in too far in or if your leech has that dreaded hook to windward (see sketch above).
In all winds, the ideal longitudinal sail shape (camber) is more important than having the ultimate area, so in light to moderate winds it pays to slacken off the mainsail clew a little, but not before first slackening the tack, permitting the fullness to move back a little. Then adjust the clew outhaul to give a fairly full shape down low but as the sail will twist more when let out, the fullness needs to be less towards the top of the mainsail. Adjusting batten stiffness can help to achieve the ideal camber for each height, and adding a few strips of UNI carbon-fiber/epoxy is one easy way to do that.
There will be a little more on downwind sailing in Part 4.
Popular sayings that are worth remembering:
“When in doubt, let it out.”
Watch how the wind enters & leaves your sail plan ie:
“Trim the front of the jib and back of the mainsail,”
“First find speed - then point.”
“Highest tell-tails, tell tales first”
“We cannot direct the wind, but we can adjust our sails”
“Miss a shift – lose a gift”
“Tiller to the side of the fluttering tail”
(ie: pull tiller to windward to correct a windward tell-tail flutter)
(Your comments, personal tips and questions are always welcome ..
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