Twin Cam rig set up.

[mikerb]

This may go over a lot of old ground but hopefully some of the following may help someone else set up their sail. In my case it is a 7m Tush Lightning ( the latest model) but I suspect other freerace sails will be similar.
Somehow or other I broke the batten for the upper cam.........no idea how.........I just noticed it was broken when I rolled it out to rig it. It had broken half way along the tube section. Anyway I repaired it ( whilst I await a replacement) and since the sail is now fully "broken in" , whilst refitting it ,I thought I would play around with the set.
I applied a moderate amount of downhaul and then retensioned all battens just to the point that wrinkles were removed. Tush supply some cam spacers with a Lightning which I suspect are for masts with a smaller diameter than the standard tush carbon 75, so I did not use any.
I use a North XTR, so I then started adding downhaul one click at a time to note how the sail responded. As expected, the leech started to loosen, although the mast is bending both towards the leech at the top and away ( i.e. backwards). There came a point where additional downhaul loosened the leech further in the second panel but did not add any further looseness or twist down into the third panel. At that point the upper cam had insufficient tension against the mast, and the bottom cam had too much pressure against the mast.....i.e it would not rotate without a huge tug on the boom. Increasing amounts of downhaul also progressively reduced the amount of cam rotation/sail belly. One click off from this point and the sail rotated easilly on both cams and both cams had reasonable pressure against the mast (the lower cam still had more tension that the upper cam). One further click off and pressure of the 2 cams was about the same and rotation really sweet. I used outhaul ranging from neutral to about 1 or 2 cm positive but that made no difference to the observations. I reckon that this point is therefore the max downhaul.
I have noticed over a lot of time on the water with Tush Storms that too much downhaul compromises their stability and I think the Lightning ( despite having 2 cams) is similar. The cams needs good rotation to keep the foil stable.
I have been applying a little more downhaul than the "max" I found during this test....... and no outhaul. In fact any outhaul ( on that downhaul setting) appeared only to provide survival if hopelessly overpowered but destroyed the bottom end capability and also made the sail more difficult to "sit down" from a standing start. I have learnt from the 6.4 version which I have just recently bought, that quite a bit less downhaul is a far better set both at top and bottom end; and at that type of downhaul set, the effect of going from neutral to 1 or 2 cm positive outhaul merely determines the amount of power in the sail but does not alter its handling.
So the moral of the story is that increasing downhaul is not always the answer to being overpowered!

[quayfee]

HI Mike,

That's intersting. I have a Tush Storm 5.75 and had exactly the issue you describe when trying to depoweer it. Loads of downhaul and the drive was all over the place. SO the essence is that I should have left the downhaul and squeezed on a touch more outhaul to flatten it.

I'l try that next time I'm out with a decent blow.

Cheers

Keith

[basher]

mikerb,
I had the Lightning in the 7m size and always felt it set better on a mast that was softer than the C75 460. (Maybe the newer ones are different).
I guess a cam sail in that 7m size is at the cross over point between rotational and cam'd sail and will be used in very strong winds in a slalom or speed setting, but on a C75 it felt too 'rock solid' for me.
If the bottom batten is knuckling on the mast this can often be changed in several ways – one, by using slightly more extension which then allows the tack cringle to sit away from the mast more which in turn reduces rotation at the bottom batten. Two, by using a different extension, swapping for one where the pulleys sit aft of the mast more than for the XT you use.
A softer mast may also resolve the knuckling issue if you can't remove a spacer off the batten end.

Keith,
The rigging issues you are talking about are perhaps not as specific as for the cam sail that mikerB is talking about.

It is true of all sails that you add more downhaul to ditch power from the head by opening the leech – but with a rotational sail the secondary affect of more downhaul is to de-rotate the sail near the boom which effectively slackens the outhaul.
So that means less tension in lower cloth panels, allowing the centre of effort to move around a bit.

So what you must do is add outhaul tension if you increase downhaul tension. (This is less true for a cam sail, as the cams themselves ensure greater stability in that type of sail). Most rotational sails are always best set with some positive outhaul.


For a general setting you can indeed just set the downhaul so that the leech is open at the top batten and the vary power using outhaul.

But watch out that too little downhaul can make a sail powerful but prone to catapults. Too much downhaul can make a sail feel gutless.

That said, a good rotational sail set on the right mast should have quite a wind range, using a range of downhaul and outhaul settings.

The rule of thumb is that downhaul tension should dominate over outhaul tension.

[quayfee]

Thanks Bash.
That makes absolute sense.

I do remember on that particular session I didn't increase the outhaul with the with the downhaul. So the result you describe is exactly what I suffered. Looks like I might be trying it out on Sunday, forecast's increasing again )

Cheers

[mikerb]

Quayfee just to confirm what basher has said I always set my Storms as far as downhaul is concerned so that the battens either side of the boom still have about 50% rotation ( sit halfway around the mast).....or just a bit less for very strong winds. With the right amount of outhaul they typically set pretty flat...but still with rotation...so they have loads of spring to pump up onto the plane and can also be pretty fast. I also always have any adjustable head pulled as tight down to the top of the luff tube as possible to get the sail on the softest section of the mast at the top.

re the Lightning it does seem to me that a mast slightly softer in the lower section would enable a more "race style" set with a very loose leech but in fact my experimentation with set up proved effective. I used the set I reckoned would give the best performance today in conditions that were pretty variable. It tested both top end and bottom end grunt and took absolutely everything in its stride......it locked down beautifully, did not pull me out of shape, shrugged off gusts, and glided through the lulls. In the end the difference between the set I used to use and my new set is about 5mm downhaul less, and a very small amount of positive outhaul as opposed to neutral. PS Basher .....I had actually increased my extension setting by 2cm because the sail is mostly used on an Isonic 107 now which has a deeply recessed mast track. So that has probably had some impact as well.