Telltale Tech Talk

Published on October 26th, 2015

However many boxes of clever electronics we load our boats down with to tell us where to go, how far away it is and how long it will take to get there, it’s a fact that no one has yet invented the instrument that will tell you how your sails are being trimmed.

Nothing is as accurate as a short length of wool to let you know how to trim your sails or how to steer to them. Telltales are the aforementioned lengths of wool, stuck to the sail, which show the airflow over the sail. But how do they work?

When the luff of your sail passes through the wind, the velocity of the wind decreases at the sail’s surface due to the friction it encounters. This is known as the ‘boundary layer’. If the sail is trimmed correctly, the boundary layer stays intact, the telltales stream aft comfortably and it’s performing as well as it can.

However, once the velocity of the boundary layer decreases too much, it separates from the sail, which results in even more of a decrease in speed and generated power – not what we’re looking for when trimming a sail.

If you are too close to the wind, or the sail is under-trimmed, the boundary layer slows and separates on the windward side of the sail, creating an eddy on that side of the sail which starts the telltale lifting. If you are too low or the sail is over-trimmed, the opposite happens – the boundary layer on the lee side of the sail slows, separates, causes an eddy and you lose a lot of power. The telltales will alert you to what’s going on by deviating from their normal, happy streaming.

That’s the theory, anyway, but what does it look like in practice out on the water – and what can you do to remedy the situation? Here’s how to read the sails… click here

MORE: While yarn or nylon telltales are the gold standard, there are also high tech options. What about teflon telltales for the rain, or for nights, a glow-in-the-dark system or telltales with a circuit sewn into fabric to offer an illuminating LED tickler? Click link above to get smarter.

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