The M20 original (115 kg) is Uni rigged with a clean concept which means snail for the spi, internal halyard for the spi, carbon rig, cascading blocks and so on. This gives LOW windage and excellent upwind speed that no other M20 configuration can match! Most M20 are uni rigged, there are a couple of versions with jib, some with a jib bridle, stainless rigging, big fat main sheet block which means increased weight and increased windage!

The sloop is a "high lift" device and is good for downwind and reaching, but suffers on the upwind. The biggest gain if you added a jib to an A-class would be downwind, the upwind would probably be worse due to the increased windage and drag.

If you need more info on the sloop vs uni try searching in the forum.....

/hakan

My information is second hand but i think it's pretty reliable.

Marstorm's goal was to make the NEXT olympic catamaran. ie a boat that was a highly refined machine that the best sailors in the world could really use to sort out the best sailors at the Olympic and World level.

He wanted a boat where the team work in adjusting the rig and sails and gear changing going up wind would determine who got to A first...the goal was to make the boat as tactical as possible ... so the boat needed to tack like an A cat and give you total visiblity of competitors and wind ... That meant he went unirig. I think this point matches what Hakan just wrote with respect to the windage factor that marstrom considered.

If you dig up the results from Querbion where the possible replacements for the Tornado were evaluated... the Olympic test sailors thought he had succeeded in building this boat.

Now... the world has not really caught up to the class or the features of the class are not that importand and so the boat is now seems to be used in as many distance races as buoys races and so the faster version has the jib.

(the verbal argument of whether the jib addition adds or detracts is thrashed in another thread)

Personally, I think a sloop is easier to sail "in the groove" than a Uni. And when the wind comes up, you can spill the main while keeping the jib pulling, so you don't lose as much drive in a big gust as a Uni who has to dump a lot of mainsheet or make other adjustments to de-power, and as Matt said, the drive is down lower in the jib so you don't fight the righting moment battle as much when it's windy. A Uni will outpoint a sloop but a sloop will out run a uni, (go faster but lower) upwind. On a reach, there is no question the sloop is faster, same hulls, same crew, same sail area.

I find it much easier to watch the jib tell tales when driving a sloop than looking up at the main when sailing Uni, but I have more time on a sloop than a Uni so I really have not figured it out yet.

I agree... seems like with a Uni you are riding the edge more of optimal VMG and pinching. A slight header comes a long and if you aren't paying a attention you slow down immensely and have trouble finding the groove again. I struggled with that for a long time and still do. With a jib you have more of a zone to deal with so a small shift that would luff a uni will not be as harmful.

narrow groove...
so... for the olympics... EXACTLY the boat you would want to sort the men from the boys!

They have to stay in that groove AND find wind, change gears and race their competition. When they get it right... its real fast!

It's the old Bill Roberts adage... you don't sort out the best drivers in the world using your family sedan.... you need a formula 1 for them to demonstrate all of their skills and only in this class can you truly get a top guy to emerge based on skill and not luck.

The down hill is not run on the bunny slope either. (I am paraphrasing)

This has turned out to be a very interesting discussion as it brings up the question whether a sloop or uni is faster. I think we could all agree that round the cans a sloop is faster, whereas in distance racing the other way around.

Some of you say that a jib adds area for a lower center of pressure, which in turn gives less moment. This is a good thing. However, a jib also adds tip vortex, probably the single biggest drag factor of the entire sail. This in turn gives the main (the engine) dirty air. Shouldn't this be a part of the equation? Also, the infamous slot-effect, could some of you give a proper explanation of what it really is? And please don't be afraid to get theoritical, some of us here actually enjoy a challenge every now and then..:)

Oh no Jimi, we are not going down that path again! Which is faster, sloop or uni was discussed to death earlier this year. Search for the old thread, I think you will find what you want and more in it. Was quite nasty for a while. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

The slot effect wont stay down, no matter how many stakes you put trough its heart. Since the 70s the slot effect has been declared a myth. Arvel Gentry wrote some very clarifying articles on the subject.
http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf
http://www.arvelgentry.com/magaz/More_on_the_Slot_Effect.pdf

The jib adds tip vortex and form drag. We have very little scientific data on just how much. We also have very little scientific data on L/D ratios between unirigs and sloops in different sizes.

I am happy to sail both types, both uni and sloop. For distance racing I prefer a sloop, but the uni offers simplicity.

Will,

Running a jib adds a little to the boat set up. Personally I would always use one as I feel it makes the boat easier to sail.

I have some practical experience with this as well. We did put a jib on an A class before. The boat was maybe just a small amount faster up wind but much faster down, and it lost no pointing ability at all. The F16 runs both ways as well, and with or without the jib, the boat does not vary in its ability to point.

If you have a sail area total you can not exceed then go uni. If not then get the jib. If the rig and ballance points of the boat are designed to use either configuration, then you have the best, becuase you can run either way depending on your whims that day.

M

From the first article Rolf links below (Dang he's fat...I though I had found a cool article to post here <img src="http://www.catsailor.com/forums/images/graemlins/mad.gif" alt="" />):

"Thus we see that the primary effect of a jib is to cause
reduced velocities over the forward-lee part of the main,
rather than increased velocities. The slower velocities in
turn give reduced pressure gradients that help prevent
separation and stall rather than some higher speed
"revitalization."

Maybe we should be putting vortex generators on our sails?

Slot effect is indeed a myth in the way many people understand it, much like "maximum hull speed". But there is indeed an interaction between the jib and mainsail that is positive. Surely enough the mainsail is less powerful with a jib in front of it. But the mainsail allows the jib to operate very efficiently at surprisingly low angles of attack. As a result the loses in the mainsail are more then compensated by the gains made by the jib. Another effect of adding the jib is that it makes the mainsail less sensitive to stall. It is easier to keep both jib and mainsail in the groove then just the mainsail. This too is an performance enhancer.

For the remainder I too refer to other posts and threads on this subject. Otherwise book and documents from Arvil Gentry, Frank Bethwaite and Marchaj are a good read. Maybe you can get them at your library ?

The most convincing real life argument that I came across was sailing the F16's myself. I sail and race these both 1-up and 2-up = as a uni-rig and sloop. Interestingly enough the difference between either mode is to small to determine which mode it favoured. But the 2-up have added 60-80 kg to the total weight the boat needs to carry and transport at speed, something must have increased the saildrive therefor and the only difference between the two setups is the jib.

Pointing, speed, distribution of efficiency and other things may well be different between both setups but the end result is surprisingly similar.

I also confirm what Timbo and PTP are saying I too feel that the unis can easily fall of their groove and that the sloop power ahead mostly by speed and despite lower pointing.

Wouter