In the following enlightening WB-SAILS response to the article A Truer Tale of Telltales?, all explanatory interjections by drLaser appear in square brackets and in italicized font. Any long interjections by drLaser are in addition signed as "-drLaser". The reply received via electronic mail originally did not contain any emphasis (bold or italic typeface.) All such emphasis is added by the Webmaster to increase readability.
I'm sorry if I'm misleading, but questionable, yes, always... "It is not nature we observe, but nature exposed to our custom" - one man's fact may be fiction to the other.
After 25 years of sailmaking, research in the wind tunnel & a decade of aerodynamic modeling on the computer, in addition to a few Olympic games and some 20+ world championships, I do consider myself, too, what you call an expert in sail trim, sailmaking, and sail aerodynamics ...
I will try to give my response [to your A Truer Tale of Telltales? article] point for point:
1) Regarding my comment that "In the mainsail telltales are only needed in the leech",
No, the reason that we use the jib is to prevent separation at the LEECH of the main. This happens by suppressing the suction peak at the luff - more about this below. Thanks to the jib, the mainsail can bend more air & still maintain attached flow. We are mainly interested in the flow on the leeward side of the main, where the tellails would also be very difficult to observe (at least when the boat gets a little bigger).
In the jib overlap area, on the leeward side of the main, separation at the luff is never a problem (it never happens unless sheeting is grossly in error). When there is substantial separation on the windward side, the main will start to luff - easy enough to detect, and not serious (to boatspeed) at all.
In a fractional rigged boat it could be beneficial to have luff telltales above the hounds, where the main no longer enjoys the benefits of the jib. But on the leeward side you could not see them ... and as our windtunnel tests show, luff separation almost immeadiately triggers leech separation, thus the leech tell tale is sufficient, and the uppermost is the most important one.
3) You also note that "a jib or a genny has NO impact on the flow of air around the mainsail leech!"
The existence of a jib or a Jenny does have an impact... by affecting the flow in the front part of the main. True, the flow velocity at the leech of the main is the same with or without the jib - namely more or less the apparent wind speed. The trick lies in what Arvel Gentry says in your quote earlier:
"The primary effect of the jib is to cause reduced velocities over the forward-lee part of the main, rather than increase velocities. The slower velocities in turn give reduced pressure gradiants that help prevent separation and stall (on the main) rather than some higher speed 'revitalization'."Airflow cannot fight rising pressure (decelerating flow) for too long without separating. Closing the slot between the main & the jib kills the suction peak at the main luff. Thus, flow on the main surface needs to fight a less adverse pressure gradient (rising pressure), and has a better chance of remaining attached all the way to the LEECH. This is a very major benefit of the jib.
If you have a chance to go out sailing on a boat with a jenny, give it a try: Sheet the main on the centerline, and barber haul the jib out to the rail - your mainsail leech tell tales will probably stall. Now, try to move the jib lead towards the centerline, and see what happens (can't do that here in Finland anymore - it is freezing and all boats are ou of water till May or so). [E-mail was received November 27. -drLaser]
4) You suggest adjusting the main trim, not the genoa trim, in response to stalling mainsail leech telltales. True, but I am saying " stalling mainsail leech telltales may also indicate" [that your jib is sheeted in too loosely], assuming your main is properly trimmed...
5) Regarding my suggestion for installing just two pairs of steering telltales, this is the KISS theory again. In my sequence of Gentry tufts, I only have 2 sets of telltales. I don't know - to be honest, I've never tried "the Gentry tufts". The older I get, the less I seem to rely on telltales in general. I recently went sailing on a Soling, to do some masthead video work - it wasn't until some 3 hours of sailing I realized we had forgotten to put tell tales on the jib altogether...
The "Gentry tufts" are fine, but are they necessary? - at 1996 Savannah Olympics, for instance, no sailor or boat I could observe was using anything but simple 2-3 pairs of steering telltales (plus the leech tails). Arvel did not exactly invent tell tails - an engineer at McDonnel-Douglas, he borrowed the idea from the aeronautical industry & gliders, who were using tufts or telltails to detect turbulent or separated flow. He was the first to use them on a sailboat, though, in late 1960s. That's more than 25 years ago, and the tufts, as they were for him, have become telltales (or whatever we choose to call them). [The distinction between a "telltale" and a "tuft" is that (by dictionary definition) tufts are a "cluster" of telltales attached "closely together". -drLaser]
More important than the introduction of the telltales, Arvel Gentry was among the first ones to correctly explain "the slot effect", with the help of the analog plotter, as you say. Ironically, the analog plotter models ideal, inviscid flow, where separation does not exists. But if you are well informed and clever as Gentry was, you can draw many conclusions of real, viscous flow where separation exists, by studying inviscid flow.
Today, we have much more sofisticated tools to model real, viscous flow.
6) You also say that "Forward telltales will start working efficiently in high[ -er] winds, not in light air." I'm not sure I follow you here - in my opinion, you don't need the tell tales in high winds, rather you sail by the heel and the helm pressure. In high winds, leeward telltails will remain always attached and windward ones will always fly, because, as Gentry puts it, the stagnation streamline is permanently on the lee side. [Mikko's suggested two sets of steering telltales are for "high" versus "low" winds, too. So, Mikko's reference to "sailing by the heel and helm pressure" in "high" winds is not referring to wind conditions relevant for dicussing when you use which set of steering telltales. (For using steering telltales, "low" may be in the range 0-5 knots apparent, and "high" may mean 6-12 knots, with steering by helm pressure becoming the primary concern somewhat above 12 knots - depending on crew weight.) -drLaser]
7) Regarding the proper telltale material, you say the wool fuzz catches on the sail whenever you position a telltale near a seam. Then, just Dont! [The likelihhod of being able to position five Gentry tufts, on a window, away from any stitching, both on port and on starboard is low, unless the window is custom-made. -drLaser] On a sailboat, sails have a tendency to get wet. This is why I recommend yarn instead of Nylon for your driving tales. For bigger boats, woolen yarn is never too heavy - for dinghies, we used to use C-cassette tape, which is light & does not get wet, but nowadays everybody seems to favor the simple yarn.
8) From your description [of how you use the tufts to set the genoa sheet car -drLaser], I think I understand exactly what is going on. You look up to the leeward tuft along the luff all the way up (can you see it?) and realize it's stalling. To increase twist, you move the jib lead back, and the leeward tail flies.
Instead of moving the lead, you should EASE THE SHEET a touch. Actually, that's exactly what happens when you move the lead aft, the sheet is eased because it "gets straighter". You are adjusting the leech tension by moving the jib lead instead of adjusting the sheet - the end result is the same, but that is not the way to find the correct jib lead position per se. In the process, you can end up with a tight (flat) jib foot.
In the opposite case, if the upper windward tail is flying up, you move the lead forward, and that has the effect of TIGHTENING the sheet. The leech is closed, and the tell tale is fine, but you end up with probably a fuller jib foot than desired. You should simply tighten the sheet instead.
This is in essence why I would discourage the use of telltales for positioning the lead. Also, luffing up is no real measure: due to the triangular shape of sails, the loading decreases in the head of the sail at a much more rapid rate than at the foot, as the angle of attack is decreased (in more layman's terms, the sail always starts to backwind in the head first).
The opposite is true when you bear off - the narrow head stalls earlier than the foot. This makes the sail a "tip staller", a very dangerous type of wing for airplanes and that's why they are avoided in them. When sailing, tip stalling ends up in a head-dip for the out-hiking sailor at worst, while for the airplane it could be fatal (when one tip stalls earlier than the other, the plane rolls over & crashes). For a given sailshape, there is only one angle of attack (apparent wind angle), when the stagnation streamline will sit right on the forestay all the way from the foot to head - bear off or luff by only a few degrees, and the sail is no longer ideal (in that respect).
With this in mind, you understand that the triangular shape is not that
bad for the sails after all. In lighter winds, at larger apparent wind
angles, the top of the sail is heavily loaded - that means a high center
of effort and a larger heeling moment, but in [very] light winds
this is not an issue. As the wind builds up, you head up to decrease the
apparent windangle, and the sail automatically unloads in the head,
bringing CE down and helping you to keep the boat upright.