There is an old sail trim adage, “trim the front of the jib and back of the mainsail,” or where the wind meets and leaves the sail plan. Telltales are a key tool helping you figure out what is going on at these critical junctures, and that is why they are placed just aft of the leading edge of the headsail and off the leech of the mainsail.
There are other places you can use telltales: off the leech of a non-overlapping headsail (jib) or sometimes in the middle of a mainsail in the upper sections, but these are less critical. For a sail to work, air has to flow down both sides. Telltales tell us about that flow. Without them we are flying blind. They tell us all sorts of useful things depending on point of sail, wind velocity, and whether we are trimming or driving. Let’s start at the front of the boat and work our way back.
Headsail trimmer reaching
The very first time we were handed a jib sheet we learned the golden rule of sail trim, “when in doubt, let it out.” Ease until the sail luffs and trim in just enough to stop. Telltales give us a more refined look at this concept. Ease until both telltales on both sides of the sail are streaming aft. If you have not eased far enough, they will just hang straight down. If you ease too far, the leeward telltale will flow but the inside (weather) telltale will lift up. For maximum power both should flow straight back. It is often difficult in low-light conditions to see the leeward telltales. Telltale windows are a must.
Easing the sail on a reach also allows the sail to open up (twist) from bottom to top. This effectively changes the angle of attack, and the more open top sections will indicate luffing (inside telltales lifting) well before the bottom. This is why we typically have at least three sets of telltales up the luff of a headsail. To get the top telltales to flow, move the lead position outboard and slightly forward to help the sheet pull down on the clew and close down the top sections of the sail.
This is where a second sheet led to the outboard rail comes into play. Depending on how far you have to ease, you may actually end up with tension on both, shifting the balance in puffs and lulls (inboard in the lulls, outboard in the puffs) as you try to maintain perfect trim.
A word of caution; you will never be able to get an eased sail to have all three sets of telltales doing the same thing on a reach. If you pull down hard enough to get the top telltales streaming aft, the bottom ones will be stalled. On a reach you have to compromise. Trim the middle of the sail perfectly. Get the middle telltales flying straight aft. Allow the top inside telltale to lift some, and have the bottom outside telltale start to hang straight down indicating stalled flow.
Ignore the telltales if you are overpowered. If the boat heels too far and wants to round up into the wind, all bets are off. You don’t need maximum power anymore, so you don’t necessarily want the telltales streaming straight aft. Ease the sail until the boat is upright and on her feet. Who cares if the sail luffs? You want it to luff to shed excess power.
Headsail trimmer upwind
Easing until there is flow over both sides works fine on a reach, but what happens when you want to go hard on the wind? Telltales still have a lot to tell you. First, sheeted in hard the sail should now finally luff evenly from bottom to top. As you head up, the inside telltales at the bottom, middle, and top of the sail should lift at about the same time. If the top inside is lifting well before the bottom, the lead might need to go forward. Be careful with this rule. Generally it is always best to have the top of the sail luff just a little ahead of the bottom. When in doubt, it is better to have the lead too far aft than too far forward. Letting the foot flatten out and the top twist off allows you to sheet the sail harder without stalling.
The game upwind is to try to sheet the sail as hard as possible without killing all boat speed. Keep in mind the golden rule, “speed first before you try to point.” Usually the reference to how hard you can trim is the sail’s distance off the spreader (or in the case of a jib how far inside the spreader tip), but telltales can provide a clue to being trimmed too hard. If both inside and outside telltales appear to want to break at the same time (which means the driver’s “groove” is very narrow), try easing an inch or so. Stable telltales mean a wider steering groove. The lighter the air, the more important this becomes.
In very light air, small changes in wind velocity mean big changes in apparent wind angle. This makes it very hard for the helmsperson to keep up. To minimize steering changes the trimmer needs to help, using the telltales just as if they were driving. In a puff the outside telltale will stall, and the sail will need to be eased to keep the flow. Ease and then trim gradually as the driver slowly comes back up to new apparent wind angle. In a lull the apparent wind will go forward, and the inside telltales will lift showing luff. Over-trim for a moment to keep the sail full, and then ease as the driver slides down to the new angle.