From: Subject: Telltales: One man's fact, other man's fiction Date: Tue, 25 Apr 2006 12:32:35 -0400 MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_NextPart_000_0025_01C66864.549A3B50"; type="text/html" X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 This is a multi-part message in MIME format. ------=_NextPart_000_0025_01C66864.549A3B50 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.arvelgentry.com/Telltales_One_man's_fact.htm Telltales: One man's fact, other man's = fiction
 
The "Gentry = tufts" are=20 fine,
but are they necessary?
At 1996 Savannah = Olympics,
no=20 sailor or boat
was using anything but
2-3 pairs = of
steering=20 telltales
plus the leech telltales.=20

Telltales: One man's fact,
other man's=20 fiction

by
Mikko=20 Brummer
WB-SAILS Ltd. =

In the following enlightening WB-SAILS response to the = article=20 A=20 Truer Tale of Telltales?, all explanatory interjections = by=20 drLaser appear in square brackets and in italicized font. = Any=20 long interjections by drLaser are in addition signed as = "-drLaser". The=20 reply received via electronic mail originally did not contain any = emphasis=20 (bold or italic typeface.) All such emphasis is added by the = Webmaster to=20 increase readability.=20

I'm sorry if I'm misleading, but questionable, yes, always... = "It is=20 not nature we observe, but nature exposed to our custom" - one = man's fact=20 may be fiction to the other.=20

After 25 years of sailmaking, research in the wind tunnel & = a=20 decade of aerodynamic modeling on the computer, in addition to a = few=20 Olympic games and some 20+ world championships, I do consider = myself, too,=20 what you call an expert in sail trim, sailmaking, and sail = aerodynamics=20 ...=20

I will try to give my response [to your A Truer=20 Tale of Telltales? article] point for point:=20

1) Regarding my comment that "In the = mainsail=20 telltales are only needed in the leech",=20

  • You point out: Then, how about Lasers?
    Got me on = this=20 one! Obviously, my article is about sloops - when speaking of a = cat=20 rigged boat, don't you refer simply to "the sail", not the = mainsail?=20

  • You state that "the way you trim all of your = mainsail=20 significantly affects the flow on the lee side of the genny, and = this is=20 the second reason why you should have additional = telltales on=20 your main forward of the leech."=20

    Yes, but I believe you can do with the leech tails to trim = your main=20 efficiently. I do not think you need mainsail luff telltales for = this=20 [for or a sloop rigged boat with a blade (100% jib), or a = sloop=20 rigged boat broad reaching (with a genny or a spinnaker) = -drLaser]=20 If you sheet the main in too tight, leech telltales disappear - = if you=20 sheet too loose, the sail will backwind.=20

    Sailing is a very complicated sport. The sailor has to deal = all the=20 time with heaps of information, more than anybody can handle, = trying to=20 filter out the essential. I think I mention somewhere in the = article=20 that what I present is the necessary, but sufficient amount of = telltales=20 you should have. I guess my reasoning is that more will just add = to the=20 confusion, taking your attention off more important matters such = as wind=20 shifts etc. The KISS theory - "Keep It Simple, Stupid."=20

    [Leech telltales will talk to you about stalling, but not = about=20 main camber, halyard/cunningham tension, outhaul tension = individually!=20 -drLaser]

2) Regarding = your=20 statement that "the goal behind using a jib or a genny is = partially=20 to prevent separation and stalling on the forward-leeward = part of=20 the mainsail":=20

No, the reason that we use the jib is to prevent = separation at=20 the LEECH of the main. This happens by suppressing the = suction=20 peak at the luff - more about this below. Thanks to the jib, the = mainsail=20 can bend more air & still maintain attached flow. We are = mainly=20 interested in the flow on the leeward side of the main, where the = tellails=20 would also be very difficult to observe (at least when the boat = gets a=20 little bigger).=20

In the jib overlap area, on the leeward side of the main, = separation at=20 the luff is never a problem (it never happens unless sheeting is = grossly=20 in error). When there is substantial separation on the windward = side, the=20 main will start to luff - easy enough to detect, and not serious = (to=20 boatspeed) at all.=20

In a fractional rigged boat it could be beneficial to have luff = telltales above the hounds, where the main no longer enjoys the = benefits=20 of the jib. But on the leeward side you could not see them ... and = as our=20 windtunnel tests show, luff separation almost immeadiately = triggers leech=20 separation, thus the leech tell tale is sufficient, and the = uppermost is=20 the most important one.=20

3) You also note that "a jib or = a genny=20 has NO impact on the flow of air around the mainsail = leech!"=20

The existence of a jib or a Jenny does have an impact... by = affecting=20 the flow in the front part of the main. True, the flow = velocity=20 at the leech of the main is the same with or without the jib - = namely more=20 or less the apparent wind speed. The trick lies in what Arvel = Gentry says=20 in your quote earlier:=20

"The primary effect of the jib is to cause reduced=20 velocities over the forward-lee part of the main, rather than = increase=20 velocities. The slower velocities in turn give reduced pressure=20 gradiants that help prevent separation and stall (on the main) = rather=20 than some higher speed 'revitalization'."
Airflow = cannot=20 fight rising pressure (decelerating flow) for too long without = separating.=20 Closing the slot between the main & the jib kills the suction = peak at=20 the main luff. Thus, flow on the main surface needs to fight a = less=20 adverse pressure gradient (rising pressure), and has a better = chance of=20 remaining attached all the way to the LEECH. This is a very major = benefit=20 of the jib.=20

If you have a chance to go out sailing on a boat with a jenny, = give it=20 a try: Sheet the main on the centerline, and barber haul the jib = out to=20 the rail - your mainsail leech tell tales will probably stall. = Now, try to=20 move the jib lead towards the centerline, and see what happens = (can't do=20 that here in Finland anymore - it is freezing and all boats are ou = of=20 water till May or so). [E-mail was received November 27.=20 -drLaser]=20

4) You suggest adjusting the = main trim,=20 not the genoa trim, in response to stalling mainsail = leech=20 telltales. True, but I am saying " stalling mainsail leech = telltales=20 may also indicate" [that your jib is sheeted in too=20 loosely], assuming your main is properly trimmed...=20

5) Regarding my suggestion for = installing=20 just two pairs of steering telltales, this is the KISS = theory=20 again. In my sequence of Gentry tufts, I only have 2 sets = of=20 telltales. I don't know - to be honest, I've never tried "the = Gentry=20 tufts". The older I get, the less I seem to rely on telltales in = general.=20 I recently went sailing on a Soling, to do some masthead video = work - it=20 wasn't until some 3 hours of sailing I realized we had forgotten = to put=20 tell tales on the jib altogether...=20

The "Gentry tufts" are fine, but are they necessary? - at 1996 = Savannah=20 Olympics, for instance, no sailor or boat I could observe was = using=20 anything but simple 2-3 pairs of steering telltales (plus the = leech=20 tails). Arvel did not exactly invent tell tails - an engineer at=20 McDonnel-Douglas, he borrowed the idea from the aeronautical = industry=20 & gliders, who were using tufts or telltails to detect = turbulent or=20 separated flow. He was the first to use them on a sailboat, = though, in=20 late 1960s. That's more than 25 years ago, and the tufts, as they = were for=20 him, have become telltales (or whatever we choose to call them). = [The=20 distinction between a "telltale" and a "tuft" is that (by = dictionary=20 definition) tufts are a "cluster" of telltales attached "closely=20 together". -drLaser]=20

More important than the introduction of the telltales, Arvel = Gentry was=20 among the first ones to correctly explain "the slot effect", with = the help=20 of the analog plotter, as you say. Ironically, the analog plotter = models=20 ideal, inviscid flow, where separation does not exists. But if you = are=20 well informed and clever as Gentry was, you can draw many = conclusions of=20 real, viscous flow where separation exists, by studying inviscid = flow.=20

Today, we have much more sofisticated tools to model real, = viscous=20 flow.=20

6) You also say that "Forward = telltales=20 will start working efficiently in high[ -er] winds, not = in light=20 air." I'm not sure I follow you here - in my opinion, you = don't=20 need the tell tales in high winds, rather you sail by the heel and = the=20 helm pressure. In high winds, leeward telltails will remain always = attached and windward ones will always fly, because, as Gentry = puts it,=20 the stagnation streamline is permanently on the lee side. = [Mikko's=20 suggested two sets of steering telltales are for "high" versus = "low"=20 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=20 dicussing when you use which set of steering telltales. (For using = steering telltales, "low" may be in the range 0-5 knots apparent, = and=20 "high" may mean 6-12 knots, with steering by helm pressure = becoming the=20 primary concern somewhat above 12 knots - depending on crew = weight.)=20 -drLaser]=20

7) Regarding the proper telltale = material, you say the wool fuzz catches on the sail = whenever you=20 position a telltale near a seam. Then, just Dont!  = [The=20 likelihhod of being able to position five Gentry tufts, on a = window, away=20 from any stitching, both on port and on starboard is low, unless = the=20 window is custom-made. -drLaser] On a sailboat, sails have a = tendency=20 to get wet. This is why I recommend yarn instead of Nylon for your = driving=20 tales. For bigger boats, woolen yarn is never too heavy - for = dinghies, we=20 used to use C-cassette tape, which is light & does not get = wet, but=20 nowadays everybody seems to favor the simple yarn.=20

8) From your description [of how you = use the=20 tufts to set the genoa sheet car -drLaser], = I think=20 I understand exactly what is going on. You look up to the leeward = tuft=20 along the luff all the way up (can you see it?) and realize it's = stalling.=20 To increase twist, you move the jib lead back, and the leeward = tail flies.=20

Instead of moving the lead, you should EASE THE SHEET a touch.=20 Actually, that's exactly what happens when you move the lead aft, = the=20 sheet is eased because it "gets straighter". You are adjusting the = leech=20 tension by moving the jib lead instead of adjusting the sheet - = the end=20 result is the same, but that is not the way to find the correct = jib lead=20 position per se. In the process, you can end up with a tight = (flat) jib=20 foot.=20

In the opposite case, if the upper windward tail is flying up, = you move=20 the lead forward, and that has the effect of TIGHTENING the sheet. = The=20 leech is closed, and the tell tale is fine, but you end up with = probably a=20 fuller jib foot than desired. You should simply tighten the sheet = instead.=20

This is in essence why I would discourage the use of telltales = for=20 positioning the lead. Also, luffing up is no real measure: due to = the=20 triangular shape of sails, the loading decreases in the head of = the sail=20 at a much more rapid rate than at the foot, as the angle of attack = is=20 decreased (in more layman's terms, the sail always starts to = backwind in=20 the head first).=20

The opposite is true when you bear off - the narrow head stalls = earlier=20 than the foot. This makes the sail a "tip staller", a very = dangerous type=20 of wing for airplanes and that's why they are avoided in them. = When=20 sailing, tip stalling ends up in a head-dip for the out-hiking = sailor at=20 worst, while for the airplane it could be fatal (when one tip = stalls=20 earlier than the other, the plane rolls over & crashes). For a = given=20 sailshape, there is only one angle of attack (apparent wind = angle), when=20 the stagnation streamline will sit right on the forestay all the = way from=20 the foot to head - bear off or luff by only a few degrees, and the = sail is=20 no longer ideal (in that respect).=20

With this in mind, you understand that the triangular shape is = not that=20 bad for the sails after all. In lighter winds, at larger apparent = wind=20 angles, the top of the sail is heavily loaded - that means a high = center=20 of effort and a larger heeling moment, but in [very] = light winds=20 this is not an issue. As the wind builds up, you head up to = decrease the=20 apparent windangle, and the sail automatically unloads in the = head,=20 bringing CE down and helping you to keep the boat upright.
 

   =

------=_NextPart_000_0025_01C66864.549A3B50 Content-Type: image/gif Content-Transfer-Encoding: base64 Content-Location: http://www.arvelgentry.com/Telltales/Telltales%20One%20man's%20fact,%20other%20man's%20fiction_files/{8B7A9D11-771A-11D3-AB9F-ACEB26279833}_988.gif R0lGODdh6gEeANcAAP////r6+vn5+ff5+fzz8/j4+PX39/fy8vT09PPz8/Ly8u/09Pru7v7q6vbv 7/Xs7Prn5/nj4/fn5/Xk5Pbj4/ji4vPk5PDw8O/v7+7w8O7u7u3t7ezs7Onp6efn5+bm5uPj4+Hh 4f////ff3/Xc3PXU1Ozd3ePb2+7V1fjQ0P/CwvPFxe3OzubLy+Dg4N7e3trf39zc3Nra2tnZ2djY 2NfX19bW1tPT09HR0dnPz93Fxc/Pz8rKysfNzcjIyMPDw/+7u+3AwO++vuu+vuO6uu22tuuzs+Wz s+Wysv+srOqtreuqquesrO2iouaoqOGhoeC9vd61td21tci5udepqeqcnOWenuKXl+SUlPeOjuWN jeOGht+dndadncObm92NjdOLi9uDg8eKir6+vru7u7e3t7Ozs6+vr6urq6yhoaalpaeWlr2Pj6WO jp2bm56NjeR/f+l4eOJ0dNt/f9p6etp2dt10dNd3d9Vzc8l6euBra9pra9VubtlnZ9RiYsNpac1k ZNpZWdReXtRVVc5cXMhZWc5YWM1SUspSUtJMTNNHR9JGRtNCQs1NTcpLS8dOTsZLS85HR8xERMdF Rbx9fbV0dKt7e7Zqaqdpaa1lZbZWVq5ZWcBPT7RKSrRERP80NNQ7O809Pcs+PsdAQM43N84zM8Y1 NcM0NNMrK80uLsMuLsonJ80iIsMjI/8AAP4AAPsAAPoAAPgAAPUAAPQAAPEAAO8AAO4AAOsAAOkA AOgAAOcAAOYAAOQAAOIAAMkdHc8cHMobG8UaGs4UFMYWFsIWFsUTE9MNDd0EBN8AAN4AANsAANkA ANgAANYAANMAANEAAMsLC8ULC80HB9AAAM4AAM0AAMwAAMkAAMcAAMYAAMQAAMMAAMIAAMEAALs9 Pb00NLc2Nq05Ob8qKr4mJrYnJ7weHr4XF7cUFLwICMAAAL4AAL0AALsAALkAALgICLgAALUAAAAA AAAAMwAAZgAAmQAAzAAA/wAzAAAzMwAzZgAzmQAzzAAz/wBmAABmMywAAAAA6gEeAAAI/gBFCBxI sKDBgwgTKlzIsKHDhxAjSpxIsaLFixgzatzIsaPHjyBDivwY547JO3KAjFzJsmXDkidTupxJ0yVM kzJr6twpkcWpU+3asWsXD56wU594Kl0K0SdQoUSNImVKtepBp0GHFj2a1KrXmQeKsmOnTVu2s9m6 jY1356tbnWHhjS2LNu3atm/ztow71yxatezY6h2csYEVcvC8YcN2jZpjx9cWs4MXJgnhyxsNI1bM +DFkyZQtYx5tUXPixY09R8Y2uTLp1wu7cGvH7dozZ8tyK0u2jNmzZ9jUQssCu7jDLt1o28atm7dv 4MKJG5+eELny27mX7e79Ozi74dSN/u9Ap07btWbLfPG6hesWe168lgHX1u7Uh/D4BY4vfz79+vbv xTdfffflF95+5qGnHnvu4QKffNjQZ5+BmEkQjze2NZNML7fYUossstQySy25JKOMM9dkww4fFE5n IYbPaMihhyCKSKKJKKrIYovFvZjhhh1+GOKIJZ6Y4oo8DjaKO9lA04wvuNhCSyyvvOKKK6/EQgst uZwIDTfuDJHka0s2+WSUU1Z5ZZZbdunMl2GOSVqZTkIpJZVWYqkll16CKaacXjGBDjfWNKMML7Wo eeWiWMJii5vYrDMJoJcJSqihiCrKaKOPnhjppJQOZmmhhyaa56avOAqppKFWNYc7/tpAs+Cpmy7q qInWaMNOq3q9GuustW56qzK57srrW77Kuh6twQ5b7LFKOQBIO9U8eYsszKIKCy26KAOcOs5A65W0 1FqLbbCLpsqtt9iAK65V5Fbry7XZMqput9+G+y5NQbjTjZO80AILuum+IgsuvjRzTTfx7EtVv/82 E/DABGNpMMIKM+wwUxADLHDFFh+c8MINb9xSEPB044zEtFipabO13LJMNeq0Y7JSKKvMssv12hrz zDXfzFPOKwfMc8Ww/EyzzUJPtIAGJqQwhBRUH4EE1VcjEcUN4mjHTDXVUIPNMr3MsmaVpybNiy/Q eFNy0xk9HfXUVV8tRdZbd63M/tdhj1322ZqqzbbbcGskt9RUS2E11nfn7TXYYpNttsWB17J2228X PhABDEwwQglCNLGFHoIk4ogop6hCjjnnEGOMMcUUQ4wwvwQjzVDsrMOON2MFBc866nDDjjOnarkL 26xp3hDnnoMuOummo646667DLjvttuOuO+9DtfN78MMXT8vx0CSv/ELMfx766KWfnvrqrb8e++y1 3y7U9r17D7zwxC9qPPLGghsDKECCFTjhC3oYBCNKcYpxODAc4PBEJzqxCUxY8BKXqEQlKEEJMTjB CXqI0DOU0Ytk4AIXu+BFL3BBC1vgIhaMikUtdrEMaGwDHuczyAALeMAELrCB/g+M4AQreMEMbrCD HwyhNkZYwhOmcIUtfGEMZ1jDG+awIDs0IAIVyEAHjgOCEqSgBTGBQQ1y0IMgFCEJTYhCFbLQhTD0 HxVtiMObMaCASPgCHyKRClaYoxzlCIcnNJEJS7RhDWtIgyLPcAYzlGEMPgBBBzSAgQsIJGfPMCEt QCbH+OQqc5q74wryuMc+/jGQgyzkIRO5yEY+MpKTrOQlU5ZJFnLySjL0pDZAWThRkpKPfgSkIAlp SEQqMg2MdCQkJUlJS4oAk5q8pStyuYxPGkQBJ2iBDk7gAB5NIAVGuAIc6rAHQ5SiFedARyA7Ucg3 rEENaDhDGcgASR/wIAYN/lEBMrxRqF60jJNZstwyrqGOdzjkAtncZjdb9M1wjrOc50znOtv5znjO s573zOc++/lPkAU0PgQ1aEMQqk1uehOc4iSnOdGpznCw0xLuhKc86ekDe+KTIfrkZzP82TN7UTOk BnEAEfJgil8YAxjfGAQg7gCGLhhhBSfIQF44RwEWONQOgVgEI1KRClSkAhKc2MQm3uAGN6hBDWaA JA5qIIMYgMCZEZnDOqzhDChtsmKpIlEvnJENdTxEqEQ1KlKVylSnQlWqb6GqVcWJVa1y1atgFStZ zYpWtbLVrXCFiFzpalePJi0Xe+3rX4da1KMmdalNfWpUpzrAxcKhsVvt/upXwzrWsp41rT5Ya1vf OpHN1tWWeP1saP2KkA8MQQlP0EIeSicKVJBjGMH4RiH08AUptAAFJuAATRCAAAiQgAQpWEE451C6 UIRCFKJQhB/qgIUgQMELZDgDGtBgBjL4AAYg8MAGNAIHdWSDGsvYhS0G9jLKyeIWvWiGNbpxjYgY F7nKZa5zoStd6loXu9qdCXe9C17xXoG8iTAvetXLXvfCV770tS9+9ctf/wJYwARmFtoOnOAFNxgi D07uchPR3OdGd7rVvW52t9vd74Z3vOU9b3rX2973xne+9b1vfvebkf7+N8ADthiqqkRjBTP4rw+Q gAVQwAIpsIETrDiG/jGIwYpVlAIUhMiDFqiwzQ4g4CIXuMADUiCEJVwhDH0gRCEiYQhHjGIVwPhF KSRBCDlUYQgtMIEDcgDJH5DhBz+oKQgU8JEGlIId2HjGMmiENk3FgsYn0oY6qnARB4R5zGU+c5rX 3OY3x3nOdb6zRfK85z7/OdCDLvShE73oRj860pOu9KUzHUlOe8TToBY1qUvtP1Q7Q9WstoirxUxm M6NZzWx2M5zlTOcT2BnPeuaznwEtaEIbGtGKZrSjIS1pSvvA0pjWtLM7Au1Qj/pDpS6etbGtkRdM 4Qtz+MMkUCGMYUgjGtKYhjFa8Q1IHAIRiDgEJDLuCEQ8AhIbR4TF/j3eCEiEQhWtKAQiCk0KVYgi FJNwxCD2sIUqFGEFI+AABgqQgB7Y0wc72AENZNABlmigHeqgxjN6sQsRzaLUsJgFLRDcDGosGA8j MTjCFc5wh0Nc4hQfecY3foiOfzzkI39EyU+e8pU7ouUvj/nMa37znO+85z8P+tCLvpKjJ33pTZ/F 09EW9akn2OrdwLpItJ7whTf84RGfeMUvPnaOexzkGE/72lGucpa7HOYyp7nNca5znvucB0AXOtGN jnSlM93pUJc61RGveJB4AAUtQAIXvoAHPxwiEopYhPCHH4nhG9/4iRAEH/BAhzBw4YNLuHkJLHCA ABiAAxvYgAeC/r4DHNyABi74wAc0QBNCkIUazlCGL3JRiy1tKWZra8YzruGNczChJrfP/e57//vg H7/4x3d8ybd8zfd8ThB9KzB91Xd92bd9Qed94Cd+5DcT5qcN6Kd+7Od+tAB/CTN/9Xd/NJF/usd7 vgd8AQiAATh8A8h8zgd90kd91od92sd9EBh+41d+55d+69d+7seB8kd/9lcTGWABEUABFUABSEgB EpCETEgBReiEEcAABCAAB+ECVvgCMYAD3vd9MiADHJABmVUTd0AW0OAMzJAMu9ALuZALuABayVB1 C8MORmAVQ1iER5iES9iESPiEERCFBIAQVugCWKiFN8CFXgiG/koxhrFihmiohmzohnCoFnNYFXVo hEyYh3rIh34IiFeYhVs4dIcYhjShiGV4hmm4hm1YQpEoh9TRha4oAzNgA7JoAy9AA26la1aBB+2g DdhQDTGiG8qwDM5ADdVgDfxUAUmSAa/YhbE4i7V4i1+hi7zoi+ihHcE4jMV4jMm4jLA4i7RoiyCA i1Uhjb34i9YojMRojNaAjHmhAB7QjDXQjN5IA/Q4dDHgViAwgYOBA+TgDdnAGNZADbaxGtuQDlhg HO4Ij/I4i/Voj/ioj3rBj/4IkAI5f4tRkAdZHAkpi/HojbLYkG31kJchkf94DQE5kBdpkAyRACHg AjPQjTMQ/gMyUAMfKQM2wFZd6IwxIIsvwI0x0JMy2YU0YAM2KYswyY0h4AEewAFMmWHF0QeSkA7e 0A3doA1T2Q2j0AdaUBEs6ZIwKZM0aQNDd5OuqJM86ZNA6YpDWZREGYsvuYxJuZRNOR1QKZVUaZVU mZVbSRFd+ZJuCZY1SZY5yZM7SYto2VZqSZSzeJRwqZRN6ZSwUZdXiZdYqZUNoQAf4AL36AI6hwEY kH0boAGg6ZmfmX2eaZqnmX2U9JmiCZqqqQCchgAKII5jUgFGoAS4mZtKYAIXgZmaGQOcSZqg2Zob IJyoiZqlGZqnSZzDCZsiIJu0mSS2qZu5yZsW4Zub2ZnJIEmcxlmcpdmdqxmarvmasTmboTKd1Lmb V7Se7NmeDhMQADs= ------=_NextPart_000_0025_01C66864.549A3B50 Content-Type: application/octet-stream Content-Transfer-Encoding: base64 Content-Location: http://www.arvelgentry.com/Telltales/Telltales%20One%20man's%20fact,%20other%20man's%20fiction_files/{8B7A9D0F-771A-11D3-AB9F-ACEB26279833}_987.dat R0lGODlhAQABAPAAAAAAAAAAACH5BAEAAAEALAAAAAABAAEAAAICTAEAOw== ------=_NextPart_000_0025_01C66864.549A3B50--