# Mainsail telltale placement



## malyea

I want to add 3 - 4 telltales to the main's leach...the guidance I see looks like they are attached at the very trailing edge so they stream aft of the sail -

- but I'm thinking I want to attach the telltales maybe say 10" forward of the trailing edge on each side of the sail rather than hanging over the trailing edge...

Who does what and why not the other way, whichever way it's done?

Thanks


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## Jeff_H

Mainsail telltales are installed on each of the battens (or just below the batten) on the aft edge of the sail. There is generally too much turbulence for them to do much good near the luff. and no point mounting them 10" forward of the leech. 

Jeff


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## paulk

Mounting them forward of the leech means you'll need twice as many, and, as Jeff has already mentioned, there's no point to putting them there. For mainsail performance you want to see flow OFF the leech, not near the leech.


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## RichH

Putting them AT the leach is OK; but, for better precision to compare the airstream velocity of each side then place them so that the aft END of the tales are at the extreme aft end of the leech section. In this position you will be able to pick up slight variations of flow between the windward and leeward side and also especially will show small airflow separations from the boundary layer (flow). With tales attached AT the leech exit and streaming from the AFT sections you wont be able to see the slight variations ... as the eyeball cant see if tales are a few degrees 'off' of coming 'straight back'. Of course you can put them trailing off the leech for a quick 'go/no-go' indicator; but, having a set so that the aft ends of the tales align with the aft end of the leech is so much MORE precise. If you are racing this will make a BIG difference in monitoring precise sail aerodynamic efficiency; as, sailboat racing is a game of constantly gaining 'inches' over your competitors.

This may be somewhat 'advanced' but a sail (or any foil) will not have maximum efficiency unless the airstream velocity is *exactly the same* as it leaves the leech on the windward side and windward side - known as the 'kutta condition'.

see: ArvelGentry.com --->magazine articles ----> Checking Trim on the Wind, Achieving Proper Balance, Sailing to Windward, Are You at Optimum Trim? These articles may be 30 years old but are probably the ONLY aerodynamically VALID articles ever written for the use of tell tales on SAILS.


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