A Truer Tale of Telltales?

Some statements made by WB-SAILS in the "Telling Tales" article may be misleading, or questionable. Either my own experience, or what experts in sail trim, sailmaking, and aerodynamics write reveals that some of their suggestions should be taken with a grain of salt. Below, these questionable statements in the article are provided along with alternative views of the truth:

1) "In the mainsail telltales are only needed in the leech."

Nothing can be further from the truth! The WB-SAILS document is written for sloop rigged yachts where the genoa overlap may render this statement partially agreeable, but the fact of the matter is that the more airflow attachment you have to a mainsail, the more power you generate. So, it is important to determine whether you have attachment in forward parts of the mainsail, too.

If you consider a cat-rigged boat ,such as the single-handed dinghies like the Laser, or a sloop rigged boat with a blade (100% jib), or a sloop rigged boat broad reaching (with a genny or a spinnaker) the questionable nature of the statement is obvious.

Arvel Gentry, an aerodynamicist and sailor who "wrote the book" on how sails work, telltales, and sail trim would surely disagree with this. Arvel Gentry is the scientist to first figure out (correctly) how sails produce lift. So, I will mention his name often in the remainder of these comments. To find out more about him, just click on his name above. Here is what Gentry says about the "slot effect" topic (with my interjections in regular font) in a series of articles published in SAIL magazine in 1973:

   "In the case of a mainsail alone, the stagnation streamline comes in on the windward side of the sail, causing an upwash (forcing the airflow to turn towards the leeward side)... We get a very a high suction peak (large negative pressure) as the air tries the make the sharp turn around the mast to the lee side.... The pressure then starts to increase rapidly... The boundary layer will not withstand this steep increase in pressure, the flow will separate, and the mainsail airfoil will be in a stalled condition... The jib reduces the upwash on the main (gives the main a header). The primary effect of the jib is to cause reduced velocities over the forward-lee part of the main, rather than increases velocities. The slower velocities in turn give reduced pressure gradients that help prevent separation and stall (on the main) rather than some higher speed 'revitalization'."

So, as you see, the goal behind using a jib or a genny is partially to prevent separation and stalling on the forward-leeward part of the mainsail. And not putting telltales on the main forward of the leech is tantamount to saying "I don't care about either the main lift or the slot effect".

A further point is that mainsail trim affects the wind seen by the genoa! Arvel Gentry points out:

   "The flow about any lifting airfoil can be thought of as the addition of a circulation flow and a non-circulating flow... The direction of the circulation flow goes forward over the windward surface, around the luff and then toward the rear on the lee side of the airfoil... Note that the circulation flows for the jib/genoa and the main oppose and tend to cancel each other out in the slot between the jib and the main. This fact tells us that we will not get all the increased air speed in the slot that is claimed by the old ("Venturi Effect") theories... Some of the air that we would think might go through the slot is actually diverted by the combined circulation flows so that it goes on the leeward side of the genoa instead."

"The high velocities created by the mainsail in the region of the genoa leech in effect end up causing increased velocities and reduced pressures all along the lee side of the genoa... So, the main increases the upwash (a lifting wind shift) for the jib. This gives the genoa its great drive and also helps keep the lee-side genoa flow from separating. Proper mainsail trim is very important in getting the most out of the genoa."

So, the way you trim all of your mainsail significantly affects the flow on the lee side of the genny, and this is the second reason why you should have additional telltales on your main forward of the leech.

Where do you put them? The answer depends on whether you have a sloop or a cat-rigged boat.
 

For a sloop:

We need to talk about the vertical and horizontal positions of each telltale. Vertically, position three of them at about 1/4, 2/4, and 3/4 of the luff length. You need to make absolutely sure that they are (horizontally) in line with your leech telltales. Otherwise, the comparison of the conditions at the leech versus forward is much tougher, and also the time needed by the main-trimmer to check the telltale behavior becomes much longer. With unaligned leech and forward telltales, the trimmer can no longer rely on peripheral vision and must look at each telltale specifically. So, you either need to move your leech telltales, or position the forward telltales directly in line with them (taking into account the above vertical spacing suggestion).

What I mean by" horizontally in line" is not along the diagonal sail seams. It's not even along any "horizontal" draft lines that may exist on your mainsail. These draft lines are not actually truly horizontal -- by which I mean parallel to the horizon-- when your boat heels. At the dock, just put the main up, sheet it in tight, and heel the boat by hiking or by yanking on the jib halyard from abeam. You will see that a straight draft line will look like going down at the luff and going up at the leech! So, make sure your forward telltales will be truly at the same level as the leech telltales when your boat is sailing at its normal racing heel in medium winds.

To determine the horizontal positioning of these three forward telltales, now put your genny up at the dock, sheet it in tight, and mark with some tape the projection of the genoa leech on the mainsail at the aforementioned 1/4, 2/4, and 3/4 luff-length heights. The telltales should be about one foot further aft of the mainsail luff than these marks.

If you have a fractionally rigged boat, make sure that the top telltale is not any closer to the luff than three times the mast width along the for-and-aft axis. A more precise method of determining the most forward acceptable position is described below.

Finally, note that these telltale positions are appropriate for upwind sailing only when you have the slot effect. For reaches and runs, in which the main starts acting more or less like a cat-boat mainsail, the telltales should be even more foreward. But this topic will be treated later after reviewing some other comments made by WB-SAILS.
 

For a cat-boat:

For a boat without a genny, the forward mainsail telltales are located more forward as they become the "steering telltales". But before I discuss this, we need to consider the validity of other statements made by WB-SAILS.
 

2) "The stalling of the lower [mainsail leech] telltales may also indicate that your jib is sheeted too loosely, and causes the airflow to separate from the leeward of the main. Closing the gap by moving the jib lead forward or changing into a larger genoa will help."

The fact of the matter is that the existence of a jib or a genny has NO impact on the flow of air around the mainsail leech! Flow streamlines around mainsail/genny combinations determined by Analog Field Plotters in laboratory conditions reveal that at about two-thirds of the cord-length distance from the luff to the leech of the mainsail, you have exactly identical flow streamlines on the leeward side of the main either with a genny or without a genny. Arvel Gentry points out that:

   "At this point, we have about the same airspeeds when we have a jib and main as we had for the main alone... In fact, on the surface of the main itself, we have a slightly higher pressure (less negative) and therefore lower velocities when the jib is used than occurs without the jib... But in both cases, the distribution of pressure along the aft one-third of the lee of the main is very even... The boundary layer does not like rapid increases in pressure and it tends to separate under these conditions..." (So, there can be no separation at the leech of the main caused by the genoa trim because the genoa does not cause the pressures around the main leech to change rapidly!)

The proper trimming response to stalling mainsail leech telltales is to adjust the main trim, not the genoa trim. If the leech telltales are stalling and the luff telltales are not, you have a leech flow separation problem caused by the leech hooking too far to windward. The proper response is to tighten the outhaul first, and then the cunningham to let the leech fall off a bit. The idea here is that when you apply tension to any edge of the sail, the other edges of the sail will fall off some. In addition, the traveler may need to come down very slightly, too. But this will have drastic impacts on the general angle of attack of the main, and will affect not only the luff telltales on the mainsail, but also (via the "circulation flow") the genoa trim! For details, see the article titled "A Trim Primer for Main and Headsail Balance".

3) "The very luff of the sail nearly always has a separation bubble on one side or the other which is why the telltales shouldn't be installed too close to the luff.... Two pairs of steering telltales should be installed in the luff of the sail approximately ten to twenty inches behind the luff rope... In small boats the telltales should be nearer to the luff than in bigger boats."

The first sentence is absolutely correct, but what is "too close to the luff"? I interpret the range provided as "ten inches for small boats, twenty inches for large boats". Also, the figures shown in the WB-SAILS article depict the telltales quite aft of the luff.

The idea is simply that a telltale which is always within the separation bubble caused by the headstay and the luff tape for the genoa, or by the mast for the mainsail, is useless for steering or sail trim purposes. But how about a telltale that is so far forward that it is out of the separation bubble in 50% of the conditions -- say not in light winds but in higher winds. Such a telltale provides valuable information for correct steering in these higher wind conditions.

For instance, on a J-27, a telltale as far forward as only three inches from the genny luff is always out of any turbulence except in very light air (0 to 3 knots apparent.) On a Laser, the mainsail luff telltales are out of the separation bubble caused by the circular mast when they are as close as six inches from the aft edge of the sail sleeve as long as the apparent wind is above 4 knots. So, all the airflow information that you could gather by placing telltales this far forward on a J-27 or a Laser would be sacrificed if you followed the WB-SAILS advice.

Telltales are cheap! So, do some experimentation: cut eight or nine three-inch or four-inch nylon telltale strips for each side of each sail. Using circular "sticky-backs", place the genny telltales at your current "steering telltales" height, starting at only two or three inches behind the luff tape. Place the mainsail telltales starting at around six inches behind the luff. With one-half inches of each telltale inserted into one-inch sticky-backs, nine three-inch telltales will span a total length of 34.5 inches from the luff on the genny, and 37.5 inches from the luff on the main. (Read the fuller description in Section 4 below before you actually put the telltales on. Further details are provided there.)

Then, go out sailing in strong air (above 15 knots) for twenty minutes. Properly trim your sails for upwind work using your regular telltales, and then check how many of the furthest forward telltales on the genoa are always stalled on the leeward side. After you determine this, fall of ever so slightly (no more than one degree) and note that the most foreward short telltales will start stalling while the ones further aft are still drawing! Lower your genoa a bit and mark with a pencil or grease pen the sticky-backs of the telltales which were always in the separation bubble (if any). Then, get on a broad reach or reach on the same tack, with the genoa up again. Now do the same for the telltales on the mainsail. Repeat the whole thing on the other tack to compare results. The same number of stick-backs should have been marked on the two tacks.

After you come back from your sail, just take off the telltales you marked. If there are more than five shorty telltales (if you used four-inch telltales) or six shorty telltales (if you used three-inch telltales) remaining in a sequence, remove the extra ones starting from the leech end. Compare the position of your regular telltales with the most forward of the telltales remaining in each sequence. You will be surprised to see that the forward shorty is much more forward than your regular telltale. This is a measure of how much pointing information you were losing by the current horizontal positioning of your regular telltales...

Incidentally, the series of telltales that you just put on your sails are not even called telltales. They are called "Gentry tufts" after Arvel Gentry who first invented them. How to use these telltales? You need to wait for the next section to learn that...

4) "It's a good idea to have two sets of steering telltales. One set should be installed a bit higher and closer to the luff than the other. The telltales further away from the luff are more forgiving and you should steer according to them in rough seas and in heavy winds. The telltales closer to the luff are more sensitive and you should use them in smooth seas and lighter conditions."

The distinction between the smooth versus rough seas is correct. The distinction between lighter versus heavier winds is incorrect. Forward telltales will start working efficiently in higher winds, not in lighter air.

But that is beside the point. The main question is "why should we have two separate sets of steering telltales, one set for light winds, and one set for strong winds? What if the wind is "medium"? What if the wind is "medium-to-strong"? You get my point. The conclusion is that you need a sequence of telltales. You need the Gentry tufts. They work in all wind and chop conditions. It's just a matter of interpreting them correctly.

The fuller description of the Gentry tufts below relates to both items 3 and 4 above. The section is actually a copy of my response to a sailor who inquired on the WWW (Sailing World Instruction Hotline) about how to point well:

• The Gentry tufts should be placed parallel to the airflow. (I prefer placing them so that they are horizontal when the boat is at its optimal heel level when sailing upwind in moderate air.) Arvel Gentry suggests the series of tufts are placed starting only 2-3 inches aft of the genny luff. (Better yet, you can determine the starting position using the experiment noted above.) The free part of each telltale (outside of the self-sticking circle) is only three or four inches long. Furthermore, the telltales should be nylon rather than wool. As the lead tuft right at the luff of the genny, I prefer using a three inch long tuft. This minimizes the chances of the lead tuft fouling around the luff tape or wire. The tufts in a set are mounted only 1 inch apart. (After putting on the lead tuft at the luff, I position each self-stick circle 1/8 inch aft of the ribbon of the next tuft forward.) The aft-most ribbon will end up starting somewhere around the location of a "regular" telltale. Finally, make sure that neither the port nor the starboard set of tufts on the genoa are in line with a hank.

  The tuft system should be set up not too high (to give the driver a neck ache) nor too low (at least 1/4 of the luff-length up the tack). If you have a window on your headsail around this location, position your tufts across this window. (Ideally, you want a long window extending all the way to the luff rope.)

• Theory: The Gentry tufts are used to measure the size of the separation bubble on the leading edge of the headsail. When you sail at the verge of luffing, the "stagnation streamline" (the streamline that divides the windward and leeward side air streams) comes smoothly right into the luff, and all tufts on the leeward side will be attached. When you bear off a tiny bit, the stagnation streamline will come into the genny on the windward side, and the boundary layer on the leeward side will not be able to tolerate the slowing down of the air flow right at the luff, it will separate from the sail at the luff, but then reattach itself on the leeward side. This is when you will see the lead tuft on the leeward side lifting. If you continue bearing off more and more, you will see your 2nd, 3rd, 4th, and 5th tufts from the luff lifting one by one. By the time the 5th tuft (i.e., your regular telltale) lifts, you have footed off considerably. Regular telltales tell you that you are stalling after it is too late. The Gentry tufts system allows you to determine exactly how close to luffing versus stalling you are.
• Use: Just sail with the 1st tuft on the leeward side slightly agitated in strong winds. In smooth water, if you see the 2nd tuft lifting, you are already too late! You should have headed up earlier. In lighter winds, the 1st tuft may be in the separation bubble. In this case, sail with the the second tuft slightly agitated.

Arvel Gentry claims that his tuft system works on boats ranging in length from 23' to 63'. I have used it successfully on Lasers, J-24s and J-27s.

5) The telltales in the luff should be of woolen yarn glued onto the sail with a colorful sticky-back.

The problem with woolen yarn is that it's fuzzy no matter how often you coat them. So, the fuzz catches on the sail seams and stitching. Whenever you position a telltale in a place where some seam or another sticky-back is within its reach, it's much more advisable to use nylon telltales. Furthermore, nylon telltales are much lighter than woolen ones so they signal better in light winds while performing equally well in strong winds. The only disadvantage of nylon telltales is that if and when they get wet, they stick on the sail, and it takes them a longer time than woolen tales to dry and start flying again.

So, telltales that are low down on the genoa luff and leech can be woolen, but all other telltales on keelboats should by nylon (or 8 mm recording tape). As for dinghies that capsize quite often, the solution lies in fixing a few woolen ribbons as a backup system. One at the leech all the way up, and two in a sequence to be used as steering tufts (along the luff, right above your Gentry tufts) should suffice.

6) "Don't use the luff telltales when looking for the right position for the jib lead - the information you get may be more harmful than beneficial."

I don't get the logic behind this point at all. I have been putting one three-inch tuft three inches aft of the luff of the genoa high up along the luff (1/4 luff-length down from the head) on any boat I raced on. I use this tuft to set the genny sheet car position. It seems to be much more precise than just luffing up to see whether your genoa luffs high up or down low first.

The goal is locate the jib sheet car so that the stagnation streamlines at the top and the bottom of the genoa come into the sail at angles that minimize the separation bubble. So, I look at my Gentry tufts and start sailing so that the first one is occasionally agitated and the second one is drawing straight aft. The part of the genoa at the height of the Gentry tufts is then just shy of being at the verge of luffing, and it is at the verge of stalling. Then I look up to the leeward tuft along the luff all the way up. If it is stalling and twirling all the time, it means that the sail has does not have enough curvature at the top. It needs to twist off more. So, move the genoa fairlead aft a bit. If the sail is luffing up there, in which case the windward side tuft would be twirling, then I move the genoa car forward a bit. I think this is a bit more precise than luffing up. For further details, see "A Trim Primer for Main and Headsail Balance".

A word of caution is also necessary here: As noted above, both the main and the genny have "circulation flows" around them. That is, when you trim your main correctly, you alter the angle and velocity of the wind hitting the luff of the genny. So, the adjustment of the genoa car location is not a one-step process. Before the first race starts, you should sequentially adjust the genny first, then the main, then the genny again, and then the main again to find the correct genny car and mainsheet traveler car positions for the prevailing winds. For further details, see "A Trim Primer for Main and Headsail Balance".