We begin with two telltales either side of the sail near the luff, and one on the leech. The "normal" situation is to have the telltales (red lines) streaming nicely, as illustrated in the diagram. The sail (in a thick light blue line) is sheeted at a moderate angle (alpha) to the apparent wind (green streamlines), we are beating but not too aggressively, and there is a good breeze ("high" Reynolds number, or Rn). The sail camber or draft is likewise modest, and the entry angle of the sail nicely matches the angle of the apparent wind. The sail is neither stalled nor luffed.
I have the Reynolds number marked down as "high"; in fact it isn't
really a high Rn as Reynolds numbers go, it is rather low. If we assume an IOM
No.1 mainsail with, say, 330mm characteristic chord in a breeze blowing at 3
m/sec or 6 knots, Rn is about 68,000, but because we have a good breeze it is
higher than the Rn we are going to come across in a moment.
Now let us increase the angle of attack of the sail by sheeting in, or by footing. Notice that the entry angle of the luff is no longer pointing nicely into the apparent wind. The sail coefficient of lift increases to its maximum and we are on the point of stalling the sail. As the angle of attack increased, a luff bubble (dark blue dashed line) formed and grew, because the entry angle has not matched the apparent wind direction. The diagram shows it has reached about half chord, and the leeward telltale has lifted and is twirling because of the turbulence in the bubble. Air flow has in fact reversed inside the bubble, and the boundary layer has separated from the sail at the luff. There are two things to note.
First, the air flow boundary layer has re-attached to the sail behind the bubble. We know this because the leech telltale is streaming nicely. If the breeze had been lighter, and Rn had been "low", then it is quite possible that the boundary layer would not re-attach, the leech telltale would twirl, and we would have a completely stalled sail. As it is, we have re-attachment of the boundary layer because of the strength of the breeze.
Second, whether we see our leeward telltale twirl or not rather depends upon where we placed it. If the luff telltales have been placed close to the luff, they will begin to lift sooner than if they had been placed a little back from the luff. At a rough guess, a sail stalls over a five- or ten-degree range of sheeting, starting at, say, alpha equal to 15 degrees with a modest little luff bubble, and ending at alpha at perhaps 25 degrees and a luff bubble pretty much covering the whole of the leeward side. It is not the case that we want to know the instant the luff bubble forms, because wind tunnel experiments tell us that a luff bubble is not necessarily a bad thing; in fact, it seems that the sail develops its maximum lift with a very substantial luff bubble. So we should position our telltales at, say, 40% of chord, perhaps only wanting to know about the bubble when it gets close to covering half of the sail. In fact, this is something of a personal preference. I would suggest that you actively experiment with your luff telltale placement at 10%, 20%, etc, of chord until you find the position you are most comfortable with. There are more comments on the "Air circulation" page, and on the Boundary layer page, where this placement should be closer to the luff for No.2 suit, and probably right on the luff for No.3..
What to do if the leeward telltale twirls? Well, I'm sure you know to either
ease the sheets a little, or to point up a little. As you do so, you now need to
watch the windward telltale, which we'll look at shortly.
First, let us back-track a little, and reconsider what happens as we increase the angle of attack by sheeting in, this time assuming the breeze is very light and hence we have a "low" Reynolds number. (Remember that our Rn is "low" only relative to the "high" Rn we had earlier with a good breeze. Strictly speaking, all our Rn's are low, and this one is very very low. If the breeze is, say, just 1 knot, we have a Rn of about 10,000 for our IOM No.1 rig.) The diagram now shows that, even though the entry angle of the luff is pointing reasonably into the apparent wind, and there is no, or only a very small, luff bubble, the boundary layer separates from the sail at the leech or trailing edge. We have the start of the sail stall, this time from the leech, and we see that it is the leech telltale that twirls and lifts, while the luff telltales are still streaming nicely.
The reason the boundary layer separates near the leech is that it has given up; it just doesn't have the energy to keep following the curve of the sail. This makes sense if the breeze is very light, of course, and so we know to flatten the sail for this very reason when the breeze is very light. Flattening the sail will also help with having the entry angle of the sail better pointing into the apparent wind, because we know we need to have eased sheets, and never to pinch, with a very light breeze anyway. If the breeze is stronger, however, two possibilities arise.
First, we have a badly shaped sail in the sense that it might have a kink in it that leads to flow separation. Second, we have too much draft or camber in our sail, so we need to think about tightening the outhaul.
With these comments, we can reconsider our leeward telltale that began to flick as we sheeted in, and can think instead about easing the outhaul and increasing sail draft instead of reacting by sheeting out. Increasing sail draft will also have the desirable effect of improving the entry angle of the sail into the apparent wind when we sheet in for close-hauled. There is a school of thought that says the sail should ideally stall both from the luff and from the leech, and that therefore both the leeward telltale and the leech telltale should begin to twirl and lift pretty much simultaneously.
Larry Robinson has offered me some words of wisdom:
What about the windward telltale? This will lift and twirl when we try and
pinch too much, or when we ease the sheet too much, because a windward bubble
will form as illustrated in the diagram. We have luffed up too much, have an
insufficient angle of attack, and have a low coefficient of lift. Notice that
the entry angle is mis-aligned with the apparent wind direction, such that the
sail shape is in danger of collapsing; the sail luff is ready to
"break". We must bear away, or tighten the sheet.
Our analysis so far has helped us decide on an appropriate sheeting angle for our sail, or on an appropriate heading to sail when close-hauled. It has also helped us adjust our sail draft, or, what amounts to the same thing, our sail entry angle depending upon wind strength. Now we'll see how a combination of upper and lower telltales can help adjust sail twist.
In the right-hand diagram of our sail, seen from the foot and gazing up, the relevant lower telltales (either or both leeward and leech telltales; leeward is coloured purple, windward and leech are red) are streaming, but the upper telltales have lifted. Ideally, the upper and lower telltales should be in sync, and so we know that the upper portion of the sail has stalled. It must be "eased", so more twist is required. For the IOM jib, tighten the topping lift; if it's the main, ease the kicking strap.
The reverse case is shown in the sail diagram on the left, where the relevant
upper telltales are streaming while the lower ones are lifted. We have excessive
twist. For the jib, ease the topping lift, and for the main, tighten the kicking
Adjusting the slot
Finally, it should be clear that we can adjust the slot between main and jib, or, what is the same thing, the relative sheeting angles of main and jib, using the telltales. Put simply, we want main and jib to stall together, or according to another school of thought, the jib to stall a fraction before the main. (No diagram here this time; but you now know enough to draw your own for this situation, don't you?)
Larry offers a caution on my generalisations:
You do remember how to see whether a sail or part of a sail stalls, don't you? Sail along in a steady breeze for several lengths, to let the airflow over the sails settle down. Then, leaving the sheeting angles as they were, bear away very slowly and watch for the stall. Remember that the airflow takes time to settle down. The effect is called "hysteresis", and if you are too quick to bear away once started on your course, you will get false readings, particularly if you've only just pointed up from a run or reach, for example.
©2021 Lester Gilbert