All of my Aerotech sails have two clews. Trouble is I never know which one you are supposed to use when rigging up.
Does anyone know why there are two clews and which one you are supposed to use?
Stupid single page aerotech rigging guides say nothing about it.
There has been a lot of debate over whether this is science or marketing.
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=39171
Note too that NP have a single clew on its latest generation of race sails.
JB
Guess for sailing on Moreton Bay I will just set to high clew then and forget about it. There's no wind here. 
I reckon that the low one is going to give more twist and draft further aft - hence better for high wind or off the wind, top hole better for draft more forward, less twist and points better. Is that right?
My personal opinion is that it's a placebo, if you believe in it it will work, so doesn't matter which hole you use, as long as you believe it's the right one.
Obviously it's going to have an effect, but I think much less of an effect than changing the out haul downhaul a few millimetres.
I stand corrected.
I was reading the book upside down 
... and was standing on one foot at the tip of the mast looking down towards the boom with one eye shut and the other one squinting 
Here's an excerpt from one of the manufacturers rigging manuals:
The top grommet gives the sail a tighter leech for more power and better upwind ability
The lower grommet rigs the sail with a softer, looser leech for more control.
On the next pages it talks about boom heights:
Lower = more control in strong winds or in wave sailing conditions
Higher = early planing and better upwind performance
*Boom position will vary according to personal preference
I must have got my clew mixed up with my boom. Happens all the time and I am often forced to sail clew-less.
A thousand pardons
$0.02
Bottom makes duck gybes easier - less likely to get slapped across the face by foot of sail
bottom hole will give you 5 nots more wind when trying to get on a plane. top hole will give you 5 knots less and the sail will feel like a 5.0 instead of a 5.4 . I have a neil pryde alpha 5.4 with this set - up and i love the 2 clews. i am allways useing them both.
another tip too i learnt: if its real gusty and you are finding it hard to hold ya sail down in the gusts, say on a lake. switch to the bottom clew. it's amazing how much it helps
lol, bust out the red thumbs windtechno,stewie,konawindsurfing and all of the other user names. 
hi paddy mac,
well that may have been on the money for the dinghy's you were sailing.
i guess this is starting to get off topic and is certainly starting to go beyond my expertise.
from what i understand for yachting it is a lot more complicated. taking into account whether or not a gib is used, boat healing and centreboard area this will all alter the position of the draft for various angles of attack.
you could on a windsurfer have your draft further forward for upwind sailing if you increased your sheet angle and i guess the opposite could be said for sailing off the wind with draft further aft, you would need to sheet out more.
it mainly comes down to angle of attack. running off the wind you need the knuckle of the sail well forward otherwise the sail will stall. this is why speed sailors use lots of downhaul and no outhaul. that allows the sail to keep draft forward and lots of twist to maintain control.
opposite for going to windward, to much knuckle in the sail and you stall because of the angle of attack.
that said i'm prepared to be off the mark with this. one thing about this sport is we never stop learning.
ok, well i stand very much corrected then.
what you guys are saying is that reducing outhaul moves the draft back and increasing outhaul moves the draft forward
edit** i'm having trouble accepting this. does anyone have any theory to back it up. i thought the sail got back handy when you let the outhaul out because the sail was trying to pull sideways instead of forward.
i also thought that increasing outhaul lead to moving harness lines back to counteract the draft moving back.
edit part 2***
i copied this from wiki, we all know if it's written in wikki it must be true. 
A windsurfing sail is tensioned at two points: at the tack (by downhaul), and at the clew (by outhaul). There is a set of pulleys for downhauling at the tack and there's a grommet at the clew. Most shape is given to the sail by a very strong downhaul, bending the mast in the luff tube. The outhaul tension is relatively weak, mostly to provide leverage for controlling the sail's angle of attack.
The sail is tuned by adjusting the downhaul and the outhaul. Generally, the sail has to be trimmed more for stronger winds. More downhaul tension loosens the upper part of the leech, "spilling" the wind at the gusts and shifting the center of effort of the sail down. Releasing the downhaul tension shifts the center of effort up. More outhaul lowers the camber/draft, making the sail flatter and easier to control, but less powerful, and less outhaul brings more overall depth to the sail, more low-end power, shifts the center of effort upward and to the front, and may limit speed by increasing aerodynamic resistance.
It must be true then 
In an unloaded sail that definitely is the case - the draft moves forward when you release outhaul tension. I assume that my sail goes back handed when loaded because the glass battens are not very stiff and I tend to over sheet when going deep. I might try to stiffen the battens and see if it changes. hmmmm.... Maybe that's one of the secrets to going fast off the wind - dont over-sheet.
