I am running a Tiger main on an 18SX mast(comptip). If the lower telltales are flowing the upper half of the sail is stalled.
If the upper is flowing the lower inside telltale is flying up, outside flows. I can't get the entire sail trimmed properly. Also get small wrinkles at second batten unless downhaul is on pretty hard. Have been working on prebend, concerned about going much tighter on the diamonds( 500lbs now). This is a ST main. Any ideas on what to do?

This is your problem:
"I am running a Tiger main on an 18SX mast(comptip)"

They aren't made for each other, therefore you might consider that they won't perform optimally together.

I am obviously aware they are not made for each other. I'm looking for what element of tuning thats causing the problem. It may not be correctable, but am not sure. If a Tiger mast was set up without prebend would it exhibit a similar issue?
Plus, what a smart butt statement, "This is your problem". What do you know of modifying cats? The boat as is will outrun H16s and most TheMightyHobie18 upwind, I'm trying to stay with F18s and I20s.

The comptip is more flexible than the alu stick so I would guess the sail dont have enough luff curve in the upper section. Dont sound that way by your description though, sounds more like you have too much luff curve? You are certain the lower windward telltale is not acting up becouse there is a separation bubble behind the mast becouse of overotation? How far back is the lower windward telltale, 50%?

Photos of your different trims?

If your only problem is the lower windward telltale that is really good.

how much outhaul do you have?

Rolf, I hadn't thought about a bubble from the mast. I feel I am oversheeting the sail to get the windward low tell to fly therefore stalling the upper portion. I did have the mast rotated, rotation control angled toward shrouds. I tried adjusting the traveler out and sheeting hard and that somewhat worked but felt slower. The low tell-tale is about 50% back. It may be as simple as rotation, should the mast be rotated going to weather? When I had the upper flowing with the lower windward tell flying up the boat could out run 16s with ease in lightish air. If the low windward tell was flying back they would stay with me or pass.

Outhaul was on hard.

There's no provisions to put the proper prebend in a stock 18 mast so that the F18 sail fits right. The diamond wires, spreaders and attachment points are not designed for it. You may have to rig up a diamond wire/ spreader set-up like on the Tiger. The outcome is questionable due to the comptip.

I have considered changing the diamond system. I have the turnbuckle thats used on the Tiger and Fox. Its attaches to the front of the mast instead of the sides. The upper diamond attachment location is the same as a F18. I can't see how the spreaders are that much different. Comptip will always be a problem but the prebend in F18 masts is below the hounds and below the comptip. Measured at the spreaders.

Are you racing with this setup?

I personally wouldn’t make structural changes to my mast to get a telltale to flow correctly when you know the sail isn't correct for mast.

Yes, I don't race much but do from time to time. Was at the Outerbanks this weekend racing I-20s and 1 F18. Or should I say following?

Is there anyway to try a F18 mast on the boat.

If I could find a used one it would be rather easy to switch out. Finding a used F18 mast is not an easy task.

A few years back, Mitch Booth showed Dave and I how to tune our Tiger mast to match the luff curve of the main. He had us flip the boat on it's side, connect the main on the halyard hook, connect the downhaul, and mainsheet and sheet in hard as though you were going up wind. Note - the main is not in the luff track, it is outside the mast. Look at the luff curve of the sail, and compare it to the curve of the mast track. Adjust your diamonds and spreader rake accordingly to get them to match. That is your new "base setting". We found that we were carrying anywhere from 800 to 1,000 lbs of diamond tension to get some mainsails dialed in.

Mike,

So you basically had the main attached to the mast at the top end (hook) and bottom end (downhaul connection)?

The bolt rope was outside of the mast track, correct?

WOW! That is a lot of tension. How come you didn't go with more spreader rake?

Guess what I'm doing this weekend <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Wholy shnikies'

Correct, the bolt rope was outside of the mast track.

Increasing the spreader rake seemed to make the curve too severe. It's hard to describe, but increasing the rake with less diamond tension altered the shape of the curve. Less rake/more tension gave a more gradual curve, and more rake/less tension increased the rate of the curve.

800-1000 lbs? I am not sure my mast could take that load at the rotator bolt. Thats my main concern and why I haven't cranked tension up over 600lbs. It sounds like I need to do it anyway. Is there a way to induce the bend and then tighten the wires to hold it?

Thanks Mike.