I've been experimenting with rigging my 3 batten wave sails on different flex masts. Here are my own findings which might be of interest..

First off a little something about the sail. The Hot sails KS3 is my all time favourite sail -I love them. It was a no brainer this year for me deciding which model to go for..  

The Firelight and QU4D are great sails, but the KS3..

You can rig them to have a massive wind range, they are super stable, and yet you can ditch the power instantly. This is thanks to the 'wonder' batten at the bottom. Well ok it's just a regular batten, but it works it's magic by 'S' bending. Sheet out and the S bend in the foil makes the sail totally neutral. Sheet in and it 'pops' back into shape and gives a deep knuckled powerful super stable foil to pull you through the turns.

Originally I was rigging the sail on the Hot rod masts and on my real old Simmers which I actually preferred. The Simmers were pretty stiff 'constant curve' but made the sail super stable. So when I was fortunate enough to get a deal with Unifiber masts I wasn't quite sure wether to go for their Flex top (recommended for the Hot sails) or the Constant curve, so I went for both!

I've now had quite a lot of sessions on the Unifiber gear and have done a fair amount of tuning and swapping. Below are some pics of the KS3's on the two mast types...

Constant curve

Flex top

Constant curve

Flex top

Constant curve

Flex top

Hopefully you can see the differences in the pics.

FLEXTOP:

Tighter leach in the head.

Flatter sail over all.

Less knuckled below the boom.

Less twist when light wind.

Direct handling due to tighter head.

Less locked in feel.

CONSTANT CURVE:

Fuller more knuckled foil at the bottom.

Much looser leach and floppy further down.

Almost 'slalom' in feel.

Super stable. Foil feels really locked in place.

Most of the top Pro sailors like Kauli and Morgan who use the KS3 appear to have the Flex top setting. I think this works really well in warm consistent winds and when 'nicely' powered up. I rig on the Flex top Unifiber when it's wobble and ride or when the wind is light and I'm barely planing.

Most of the time however I rig on the Constant curve. A lot of my sailing is in cross off where the wind blows over the land and can be gusty. Our 'cold' winds are more harsh and buffeting. A lot of the time I end up super stacked! For all of these I prefer the sail to be more stable and more tuneable. I had a day on the 4.9 at The Bluff, St Ives Bay in Cornwall where it was 40knot hail squalls every half hour followed by 10knot lulls. The sail handled fine in all of it, all be it I couldn't sheet in to bottom turn in the gusts! But was actually pretty comfortable.

I find the sail set with the tighter leach on the Flex top mast can feel quite hard work and 'heavy' when over powered, more downhaul doesn't really cure that as you flatten the sail and allow the centre of effort to move around more.

But in the lighter winds I prefer the quicker response and direct feeling of the tighter leached Flex top. 

Finally, a while back I asked Tom and Jeff at Hot sails for their thoughts on mast bend which I'm sure they won't mind me sharing..

"We were never full flex-top, more of a combi-curve, but we have come slightly closer to the pack in terms of mast bend. Almost any mast will work except hard tops.

Our specification (more towards flex-top)  on the KS3 gives a very progressive twist with the head quite open - this is the way Kauli likes it. Using a normal mast such as the Ezzy or Simmer makes the twist more even - this is the way Morgan likes it. A lot of guys are running less than suggested downhaul so there is not much twist to maximize low end and drive. "

[NB: I think the later Simmer masts are softer and more towards flex top than my super old bomb proof 80% ones..]