The mast base should already have measurements on it... In any case, you measure from the joint between the extension and the base - which will probably be about 0.5cm shorter than the measurement to the pulleys. **

** which is nice as it allows for an extra half-cm for those big days.

If I am reading Mcmahon correctly we have not actually answered the question.
his question is about the luff length in relation to the mast + extension length.

Example:
My Pryde 11.6 says the recommended mast length is 596cm..
I have a 580 mast and an extension with 16cm extension on it.

Now - how far do I downhaul to ensure I get to the correct setting (as per the recommendations. I usually go block to block and it is good - so I assume that is what they mean by 596cm (block to block at the right mast + extension)

BUT I also have a Gaastra 9.8m sail. It rigs on a 520 + 8cm. I have the right mast and 8cm extension. But it does not downhaul block to block at those settings. If I did go block to block it would be WAY overdownhauled.

So what do I do for my Gaastra - my rule of thumb does not work at all. Also there is no marking on the Gaastra that tells me when to stop downhauling. If there were a marking on the luff that aligned with another on the base then it would be better.

So I dont have an answer - and dont believe the issue is a simple one at all. Manufacturers put the sail tack pulleys at different heights which means there is no consistant of getting a sail into the ballpark straight away.

BTW - thanks to those who helped me rig my sails and showed me how to rig for the visual clues rather then the numbers.

JB

no such thing as a stupid Q, just stupid people.

but in this case, its a very good Q.

A very good question!
The windsurfing sail manufacturers got together a few years ago and agreed on a standard method of measurement of luff length for windsurfing sails. I can't find the actual diagram I downloaded at the moment but the gist of it is from the tip of the sail luff pocket to the bottom of the tack pulley. When rigged perfectly by my standards this would mean the tack pulley and the mast base pulleys would be almost, or actually touching and a tape measure from the mast tip to the bottom edge of the tack pulley would give the same measurement as the luff rigging specification.
By the way, a recent issue of Boards (June 2008) tested RDM mast extensions and found that the measurements marked on almost all of them were wrong. Do not rely on the marked measurements without checking with a ruler or tape measure first.