What's the deal with luff telltales?

I hear about them but I've never seen them in person and I'm not really sure what the theory is or where they should be placed and why.

Hmmm Jaime, the telltales are a very simple, effective, and inexpensive way of telling if you are using the wind efficiently. They are approx 1/2 inch wide by 8 inches, or so, strips of fast drying material (you can use string also but strips dry faster). They are generally attached (glued, sewn, etc.) approximately 1/2 to 2/3 of the way up, toward the top of both the main and jib, and they are placed on both sides of each sail. They will also be approx 1/2 to 2/3 of the way back from the front on the jib and about 1/2 way back on the main sail.

How do they work? As your boat moves forward in the wind, naturally the wind passes across your sails, usually on both sides. By looking at the telltales you can see if the wind that is flowing over the sails is moving as quickly and efficiently as it can. The preferred position of the telltales would be pointing straight back, as your boat moves forward. By adjusting the mainsheet and jibsheet (tightening or loosening up the main sail tension or traveler, same with jib) you can adjust how the wind is flowing across your sails. The most efficient use would be to have the telltales facing straight back on both the main and jib and on both sides of the sail. This will usually give you the fastest speed. You look up as you go and adjust as necessary.

If used correctly, you will certainly improve your sailing skills, your speed, and the probability of winning more races against your buddies.

Hope this helps. Sail Safe.
HD.

HD - Thanks for typing that out! I know what telltales are in general, but I hear about people putting them along the leech of the sail. What's the deal with those?

Thanks again!

Edit: D%OH! Sorry for the typo - I meant LEECH telltales... not luff telltales.

The ones on the leech of the sail can tell you if the sail is over-sheeted.

Ideally, you want the flow of air off the back of the sail going aft - the tell-tale streaming out behind the sail. That would indicate that the flow is attached (non-turbulent) over the entire sail surface. Attached flow = more power and less drag.

More often than not, though, flow separates from the aft edge of the sail, creating a vortex "bubble" of turbulent flow that causes the leech telltale to stream forward. Here's a diagram of a similar situation - just move the bubble aft and you can see how this "backward" flow works:



The telltales are put up high on the sail (3rd or 4th panel down) because that is the area that is most suceptible to over trimming with the main sheet.

Leach telltales, I'm sure there are a lot of opinions out there, for what it's worth here's my simplified explanation:

If leach telltales are flowing straight back it indicates that you have attached flow across the back side of the sail and that flow extends all the way to the leach. If you are sailing in conditions where you could use additional power, flowing leach telltales indicate that you can add power by changing to a fuller shape.

If the leach telltales are being sucked back behind the sail it indicates that flow separation (stall) has occured at some point before the the flow reaches the leach. More power and less drag will result if the sail is flattened.

Maximium power is derived when the leach tell tales are just on the virge of being sucked behind the sail and flowing. In overpowering conditions it is important to reduce drag and the sail should flattened to, or beyond, the point where the leach telltales flow straight back.

In short, I use luff telltales to drive by and leach telltales to tell me how to adjust sail shape.

What do you do if the telltail on the outside of the curve of the sail is streaming back and the one on the inside is flopping around and doing circles?

A great link was on the same topic going at the Hobie Cat Forums...

Tell Tails...

Great amount of information and some really good gif animations that describe them.

That is a good link, but it leaves some things unsaid (which might be just because I'm a n00b and should know better.)

Examples:

Okay, so can you guys translate that into actions on an H16?

Thanks again!

Basically, the traveller sets the coarse angle of attack for the entire sail. I use a tell-tale pretty much in the middle of the sail to set the traveller.

The mainsheet adjusts the amount of twist (the variation in angle of attack from the bottom of the sail to the top) in the sail. I use two tell-tales up high (2nd and 3rd panels down, top one is 2/3 aft, bottom is centered) to set twist.

Leech tell-tales can fine tune the twist and keep you from being oversheeted, especially in light to moderate air. They are also up high on the sail, since that's where the largest variation in twist occurs.