On my water + land yachts previously I used to have the tell tails about 1/3 back from the luff but having a look at a couple of books tonight, I probably had them to far back.
We know that the 1/3 approx from the luff is about the max draft point and where the optimum shape should be so the airflow needs to be at its best at this point so from what I have read the tell tales should be froward of this 1/3 point, some of the figures quoted are 15-25% of the chord length (width) of the sail.
The number of tell tails should be minimum of upper, middle, and lower, but for very tall sails maybe an extra one or two.
Wool tufts on land yachts work fine as they don't get wet like on water yachts.
Sometimes you will see the tails higher on one side than the other its comes down the personal choice.

landyacht said...

try 25% of the chord length back , 4 telltales up the sail and also 3 on the trailing edge. that should be plenty

I don't see many on the trailing edge. What problems would these indicate, and what do you look for?

I guess I should have been more specific. I'm asking about the trailing edge ones. What should they look like when things are good, and what do they do when something's wrong? And what would the wrong thing be ??

Cheers Guys..job done! Sail is now complete (that's sail No 1, another 2 to go but less urgency).

Cheers Col