Can anybody tell me what the difference is in behaviour of a catamaran sailing upwind in heavy weather in case I put on more diamond tension compared to more cunningham. What will give the best vmg?

Leo,

Diamond tension and spreader rake just gets you into the ball park.
Cunnigham gives you an opportunity to bat.
Then it's up to the skipper to hit the ball.

this is covered pretty well in the thread "mast pre bend"

They're the same but they're different.

You achieve the same result with both adjustments, however one is easily adjustable on the fly, the other is not.

If you pre-bend the mast a lot, you will have to pull the cunningham down quite far but it won't require the same tension (easier on the sails) to get the leach to open.

i dont agree with Will R...
while this is all related... its not all the same result or goal.

prebend is used to match your luff curve of your main... set for your sail.

downhaul is used as an on the fly adjustment to power or depower.

Nearly, there is more to it than that.....

Yes, prebend is used to match the mast bend to the luff curve to get max power, but you can also loose power by adding MORE prebend.

The diamond tension only (reallY) induces bend between the top and the bottom of the wires.

The downhaul streaches the sail (A little) ans then bends the mast along the WHOLE mast as the sail is attached at the top.

As others have said, prebend setting (and spreader rake) is the course tune for the mainsail as it is difficult to change (except on some A classes and F16's that run adjustable diamonds from the wire). You then use the outhaull, downhaul, mast rotation, mainsheet tension and traveller position to control the shape and power of the mainsail.

well stated scooby

if you're only goal is to match the mast to the sail with the diamonds, then why do you tension the diamonds on big air days and ease on light air days? Oh yeah, it's to bend or straighten the mast....... Trust me on this, you can get a sail MUCH flatter with less downhaul/cunningham by prebending the mast.

Especially with a boat like the I20, the increased diamond tension makes a big difference in flattening the sail when it's windy.

Yes, the mast bends a little differently but the overall effect is the same, open leach. Additionally, by tensioning the diamonds you get more stability in the mast and will keep the slot more open.

i agree with your above statement

what i didn't agree with was when you said "You achieve the same result with both adjustments"

I wouldn't want to start out with an incorrect pre-bend and expect to be able to simply use the downhaul to correct it.

After years with a square top I am (temporarily) back to using a pinhead main and find it difficult to control power and a bit hard to predict.

Do these arguments for diamond tension work equally for both square-top and pinhead sails?

DAVE - Your not allowed to ask questions about your boat until you have sailed it at least 5 times.

That being said..
even though i am not an expert (i only play 1 on TV), and event though i would think the cut would affect it, i would think (in general) these arguments are independent of the type of cut on the top of the sail.

Lastly... did you ever adjust the diamond wire tension or spreader rake on the mystere? PS it is set to 420lb of tension. I plan to re-evaluate the tension / rake with my new sail one of these days...

I have found with our Twister that a little less spreader sweep and high tension runs nice.
How have you found the new sails on the Mystere? I have tried other F18 mains with mixed results.

i have never used the stock sail (DAVE PARKER SHREDDED IT BEFORE I BOUGHT THE BOAT! WAY TO GO DAVE!!!) but know many who have. they did love the stock sail (esp downwind with its super roachy plan).

I use cut down tornado sails (as do all 6 of the local 5.5 and 6.0 mystere's here in Dunedin fl). i hear the tornado sail is much faster than the stock ones.

i have 2 tornado sails with different plans and they sail very differently. After a year (100 sails) on the USA 808 sail (used Charlie Ogletree / Johnny Lovell sail) it tore a few times and i had to get a new one.

i had a hard time getting used to the new sail. The second sail (USA 80) has much more draft and is great in heavy air (even though i would have thought it would be better in light air) and the one with less draft is a better all around general sail (responds pretty well in both high and low air).

You're not trying to get the prebend the same as your luff curve are you?
that would mean that you must never put on any downhaul or you would be trying to get the sail beyond flat, which is just folded i guess.
You just need enough prebend so that max downhaul gets the mast bend to match the luff curve.

no, i was speaking in general terms....

VMG,

Does that mean that you never change your diamond tension?