Friday, December 13, 2013


What's the story with those telltails anyway? Put simply, the telltails are the racers primary window into the inner workings of the engine. How so? Telltails don't say anything about the speed. What they do is they give you, the helmsman, and very precise indication of how well the engine is running. The engine being the airflow over the sails. Why is that better than a hud / instruments? Because - at a glance - the helmsman is able to determine if the windangle is right for the current trim. Upwind that translates into speed. The telltails will also - at a glance - indicate what course adjustment is needed to get to the optimum windangle. Looking at a hud, you only get a number. You'll need to compare that number to something else in order to make it usefull.
Jib with telltails and leechtails
Here's a picture of a jib with telltails and leechtails. Note the position and the colour coding. Starboard telltail is higher up to avoid confusing matters when colours can't be seen. Position? Right behind the headstay, so that the helmsman can see them and so that the telltails show the airflow over the front of the sail. Optimum speed is found when the airflow is good, and that means the telltails are horizontal on both sides. If the windward telltail flutters, you are too close to the wind. If the leeward telltail flutters, you are too far away from the wind.

The sail work as a wing. If there is one sail, it is pretty easy to imagine. Air flows over the sail and creates a pressure difference that pulls the boat forward. It's just like the wing on a plane, except the plane is pulled upwards. However, the pressure will not pull the boat forward in the sailing direction. That's where the keel comes into play. More on that in a later post.

Telltails show the airflow coming on to the sail. Leechtails show the air coming off the sail. If there are two sails, they work almost as a single wing. The main being a sort of trimflap for the jib. When there's a jib and a main, the end of the wing is effectively the leech of the main, so that's where the leechtails go;
Main with leechtails
The leech tails are important too, but not for the steering. Leechtails are more of a tool for the trimmer. The leechtails should point aft as if the sail continued some 10 inches in that direction. If they bend to the backside of the sail, the sail is stalling and the leech is too tight. If they flutter, the leech is too loose, and that's pretty much all you need to know in order to trim your sails and sail max speed. When the basic trim is in place, the leechtails take over and become the most important second by second "device" for the trimmer to make sure the boat is well balanced and runs at maximum speed. The helmsman stays with the telltails.

This is ofcourse a very short version of the truth. Of course there are other factors such as the fullness of the sails, the leech tension, the twist, halyard tension, headstay tension and what nuts. These trimfunctions are mostly about adjusting the sailshape to the current windstrength. Now, it should be pretty obvious why telltails are key to a good sail simulator. That's why we love the Flying Fizz. It not only behaves pretty much like a boat, it also has those telltails working almost like real ones.

Thanx to Mikko from WB sails for letting me use these pictures. Go there and read more great articles about saildesign.


  1. As far as i notice, the becky boat at Trw also has telltails!

    1. Yes, sort of... The Babysloop appears to have reefing lines trying to act like telltails.

    2. Rene's latest boat, the Clever, uses a leechtail.

    3. Yes, there's a leechtail on the Clever. It is probably the first working leechtail ever seen in SL. However, a leechtail cannot replace a telltail as they serve completely different purposes. Uhm, just like I've tried to describe in the telltail post above ;-)

  2. Nice info and last news from our virtual lovely building designs ;)
    Thanks Noodle!

  3. Have you closed look on the RM sails when pointing on irons?
    Try get backwards and look the luff reaction.

  4. Thanks for posting, Noodle! Read this with much interest! :)

  5. Hello Noodles! Nice write-up on tell-tales. I always watch them when I am sailing. (RL, of course!) Getting the most out of the sails is critical when sailing in light airs, which is what we do most of the time around here. In the winter months, while you are digging out snow, and bemoaning a frozen marina, we have slightly stiffer winds, generally around 10 knots, though in front of a front they can be 20 to 30 knots. Sail selection is important also. Crew position is also important on the smaller craft. But mostly, getting out and enjoying the sun, the wind, and the companionship of friends makes it all worth while. Keep posting.

  6. Thanx Jim. This lil sunseeker (or is it sunsucker) says it's nice to know that someone is actually out there while I'm freezing up here... (Over here ;-) Cheers


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