I'm fairly new to the sport, and trying to understand how to rig a sail properly using some recently acquired second hand gear, in particular how to set the mast extension.
The sail is a 6.2m with instructions printed on the sail. It says for a 430 mast the mast extension must be set at 32cm. First problem - my mast extension has no markings to indicate length. I guessed that the extension length would be measured from the bottom of the mast to the end of the extension when inserted so I measured 32cm with a tape measure. I've down hauled the sail according to instructions (wrinkles on second section from top meet the dots) and yet there is a large gap between the bottom of the sail and the bottom of the mast extension. The photo shows the sail folded up, but with that down there is still a 10cm gap above the base of the mast extension.
Should I reduce the length of the mast extension to that the bottom of the sail meets the bottom of the mast extension when fully tensioned?
430 + 12 = 442
430 + 32 = 462 and 20cm too long
That's a very cryptic response. Care to elaborate?
." It says for a 430 mast the mast extension must be set at 32cm"
That's different to the writing in Black texta on the sail .. 440 to 442cm
Sounds like you have the right idea on down hauling the sail. Yes, in order to move the bottom of the sail to the bottom of the mast base, shorten up the extension perhaps on the 1st or second hole from the bottom, and down haul it again. The bottom of the sail should be near the bottom of the extension.
Another thought, does the top of your sail have webbing to adjust the height? If so, you could loosen the webbing some sending the sail down the mast, so it matches up with your mast extension setting, bringing the bottom of the sail to base of the mast.
." It says for a 430 mast the mast extension must be set at 32cm"
That's different to the writing in Black texta on the sail .. 440 to 442cm

These are the instructions I was following.
The short answer is yes, set the extension so that when the sail and extension pulleys are almost block to block when the sail is at the correct tension, and thats your extension for that sail.
If you sail somewhere well populated by other windsurfers dont be afraid to ask someone if they'd mind having a look and make sure the leech tension does actually look right etc.
One thing nobody has mentioned is no you don't measure to the bottom of the extension. You measure to where the sail pulleys will sit, and that will be about 2cm above the black plastic bit.
Assuming the sail instructions are correct (not the black texta) - You will be waayyyyy under downhauled if you measure how you did on that first pic.
Set the extension about 3 holes longer, then downhaul until the pulleys touch - thats the hard part though.
Firstly though you need to confirm if that luff is in fact 442 or 462 - depends on the age of the sail as basically old sails had long luff lengths and now its much different. Googling for specs required
Is that a one piece fiberglass mast ? If it is buy a second hand part, carbon two piece mast, constant curve.
I'm presuming its a modern North sail?
Otherwise the sail wont set properly and the mast WILL break, damaging the sail.
New sails require a huge amount of down haul force that your fiberglass mast wont like.
One observation - not sure if you were using it or it was just in the frame - but Be careful using a screwdriver for a down haul tool you may end up extra hole in your stomach if the rope snaps - consider a better implement that's not as lethal.
." It says for a 430 mast the mast extension must be set at 32cm"
That's different to the writing in Black texta on the sail .. 440 to 442cm

These are the instructions I was following.
Where is that photo from?
Which one is correct? There is a big difference between 462cm and 442cm!?
Why would the black texta writing be wrong?
@wind012 That photo is of the instructions printed (presumably at the factory) on the sail itself.
@Mark_australia the sail is a 6.2 North Sail 'e-type' and has luff "443 +- 1cm" printed on it.
@Imax1 it's a 2-piece fiberglass mast
Thanks everyone for the advice.
Looks like they've printed the wrong Fast Rigging Instructions on the sail - which means that 192 boom recommendation will be slightly out too - hence the Texta instructions.
This guide might be useful:
www.duotonesports.com/fileadmin/user_upload/dtw/downloads/NoCam%20Rigging%20Manual-LR-v2.pdf
Google or Youtube search: North Sails - Rigging a No Cam Sail
and watch other brand videos for rigging too - you might pick up other tips.https://old.north-windsurf.com/eng/nodes/display/pages/manuals-howto-s