Well,

All the things that Mary suggested I would NOT do. I'm sorry Mary but you just dispensed with alot of bad advice. Most of it will do exactly what you don't want. Robi should neither shave his battens nor put them in backwards. More flexible battens will only introduce more camber in his sail and probably have his leech hook. Putting a batten in backwards will create turbulance in the front part and have his leech hook really badly. It is all just bad advice.

Robi, I have a similar sail as you (made by redhead sails Australia), of course Matt uses the same sail as you have. Matt and I are walking different ways in order to get the behaviour we want. But there are a few basic thing you must take note off.

F16 sail quite fast and the apparent wind can lay very close to the direction you are sailing in. Don't expect a huge amount of twist nor very big gust responses. Both must be noticeable but especially upwind the difference between a correct twist profile and a wrong one can be as little as 4 to 6 inches at the top of the sail, that is 28 feet up. A top that has been completely weathervaned off while going upwind will only twist off by 10 inches.

Before you start working the trim controls and setting you must make sure that all battens have just enough tension on them to prevent creashes forming around the batten pockets in any and all winds. If you don't do this than the most normal reaction is to start pulling the downhaul till all the creases are out. However this can easily lead to a sail that is both too flat and that has a leech that falls away to far. The next reaction is to pull on the mainsheet to make the leech stand up again. End result is an additional flattening of the sail, probably too much. Currently this is the case with my Redhead sail. I hardly pull on the downhaul at all as that kills my sail. Other Redhead F16 sail users are reporting that they are using reasonable amounts of downhaul. You'll need to find out what works for you.

Also you must not overdo it on the outhaul (foot tensioning of the mainsail). Completely flat is slow and hurts pointing, too full quickly give excessive weatherhelm. About an inch of draft (at the max point) on the foot seems a good starting point. This should not give weatherhelm nor hurt pointing.

But assuming your battens are at the right tension and your foot is set right than the two major controls for your mainsail are ; mainsheet and downhaul. If your spreader rake and prebend are right, or at least close to it than you must remember the following two characteristics.

Mainsheet : When pulled will flatten the top (Most) and middle (less) of the mainsail and make the leech stand up.

Downhaul : when pulled will pull creases out, pull point of maximum draft in the sail towards the mast (forward), flatten the top and middle part of the sail and make the leech fall away

So both mainsheet tension and downhaul tension make the mainsail flatter in the upper and middle regions while having opposite reactions in the position of the leech.

It is important to note that you can get any fullness of the mainsail while keeping the leech postion (twist) constant but simultaniously pulling (slacking) on the mainsheet and downhaul. This also makes clear that there are always two ways to make the leech stand up more on a F16, being :

-1- pulling harder on mainsheet
-2- pulling less hard on the downhaul

Which one you should use dependents on the conditions and the preceding trim. Important to note here is that pulling harder on the mainsail is NOT always the best thing to do. In several circumstances keeping the mainsheet tension the same and releasing the downhaul a little bit is the better action.

Learn the sail with this complexity and availability of difference solutions. We sailing F16's, not Hobie or Prindle 16's.

By far the most effective tool to introduce more twist is the mainsheet. The effect of the downhaul is in my experience relatively small. I tend to look for the right twist first and than adjust downhaul and mainsheet proportionally to get the creases out and/or get to the right fullness of the sail for the given conditions.

Be careful not to walk into the trap of ever increasing both mainsheet tension and downhaul. The twist will remain the same when both are adjusted proportionally but you'll lack power and pointing because your sail will be trimmed to flat. When the boat is flighty and lifts rapidly in the gusts than make the main flatter by first pulling on the downhaul and when necessary also pull on the mainsheet if the leech falls away to far. Take note however that no amount of downhaul can have the top fall away when you have applied a serious amount of mainsheet. The downhaul is just too weak of an adjustment to do so; at least in my own experiences. Ergo, the mainsheet is the best and most potent control for the twist in the leech. This is also the reason why I wrote that it is easy to oversheet the mainsail. If you have done so than there is no better choice to get back at proper trim (twist) than releasing some mainsheet tension. Forget about shaving battens or putting them in the wrong way. The total effect of all of these falls well short of what can be done with the mainsheet alone.

But mainsheet has its limits as well. Because mast prebend can be wrong and than adjusting the mainsheet will not have the complete desired effect. When you have to much prebend than the middle of the sail will look flatter than the rest and its leach falls away taking the top (and squaretop with it). And the otherway around. Sadly several of us are still working out what the best prebend it. Especially with the new F16 mainsails. I'm sailing with 27 mm prebend at this time and I'm serious considering reducing this to 20 mm or even less. Some other is sailing with 50 mm of prebend which is twice as much as I have and this is ALOT. Glenn Ashby was sailing his Taipan with 15-20 mm prebend in 0-10 knots of wind, 25-35 mm in 10-15 knots and 35-40 mm in 15 knots and over. Phill Brander is at 30 mm prebend I believe. Mainsails can differ from one-another even when made by the same sailmaker; so this round of trial-and-error we all need to go through.

More later. Have to go to bed now