Hey All,

I probable outta just let this nagging question ride ( and go sailing),but I keep running into a pair of what I believe are persistant and fundamental myths among sailors regarding...,um, how sails work. The first is the way in which lift is created by sails (forget Bernoulli's equation !); I can live with this one though. The second has caused me pain , humilitation and often both : what do asymmetrical kites do for catamaran stability...?
I really would like to know how an asym. kite creates a lifting effect on the leeward hull of a catamaran (the best data I can find is definitive in showing the opposite! {Wouter, remember the Cherub research ?} ). We've had lengthy discussions on the F16 forum (kites,krashes and obese angels thread), all of which culminated with the frustratingly obtuse conclusion that the kite "calms the boat..." in wind and waves , if in combination with : a very flat main and kite and so long as one sails from the wire, feet on the transom, toes and undedicated fingers crossed for good luck...
I think hydrodynamic lift is created by a hull oriented so that the rig produces an upward force (really hard to keep this hull orientation in cats !). The extra sail area of the kite - while the hulls are oriented properly - provides a proportionatly greater intermittent hydrodynamic lift than sailing downwind w/out the kite (is this that false feeling of stability?). However, "intermittent" seems to me the unfortunate complication, because when hull conformation and wave frequency get out of sync, the kite isn't going to save a "pitch in", but will accelerate it, you'll
just swing in going seven knots faster than if you'd kept the kite doused...
As more cat classes begin sailing w/ kites, I wonder why smart N.A.'s keep moving hull volume and placements around in new designs, instead of innovating lift for the bow's ala a cheap (ha), simple, reliable, moveable T-foil rudder system ?
Cats are sailing much faster downhill with the asymmetrical spinnaker ; where are the compensatory innovations of design for what we'd all agree is a glaring weakness of catamarans - the nose dive if pushed hard...?

Best To You,


Paul

P.S.

just kidding about Bernoulli...

> Just kidding about Bernoulli...

But you are right! - I'm told that there are lots of incorrect explanations of the slot for example that invoke Bernoulli and that Arvel Gentry is the myth buster...

http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf

And an interesting thread on the same:-

http://www.boatdesign.net/forums/showthread.php?t=457&highlight=sailwave

I dont know either, but so far my pet theory is that the CE is lowered and moved forward with the kite up. Then you regulate CE further by stalling the main when you become overpowered.

I do not think there is an extra lift created by the hull, perhaps if you sailed high with the windward hull, but that's very risky in wild winds.


I think T-foils are coming, but the one-design classes have put their brakes on.

Looking forward to hearing what others think about this. Would be good to have a good explanation.

I'm told that the lift of an asymmetric will be noticeable if the spin pole is longer than 50% of the mast. I'm not sure if that's the total mast, or the spin hoist height (the latter would make a bit more sense.) My P-19 has a 15'4" pole, 27+' hoist, and lifts a lot. All the skiffs (12's, 14's, and 18's) seem to run tremendously long bowsprits, and typically sail downwind with a LOT of the front part of the boat out of the water.

Some time ago I've given up on providing explanations of what happens as I found out that these persistant myths are just that : "persistent".

Everybody putting in some time can look up the modern theories (although not all of them are, Arval Gentry did most of his work in the 50's and 60's) and educate him- or herself on the new insights.

But with respect to the kite, I'm willing to give you guys a hint.

Sure enough adding a spi will increase the pressure on the bows IF the remainder of the RIG is providing exactly the same drive as when sailing without a spi. And in most light to medium conditions we are sailing that way. That is the cherub example and also my personal experience as I sit further back in such conditions. However what happens when the wind picks up and a crew has to depower ? None of us depowers the spi, but depower the conventional "upwind" rig but making the mainsail flat or letting the traveller out. So in this situation, where the amount of maximum drive is limited by a FIXED dive resistance a larger portion of the drive is provided by the spi alone than say in light to medium wind conditions. How does this help calm down the boat ? Well that is easy. The spinnaker as a whole, by virtue of its inclined luff, lower centre of effort and other factors loads up the bows less per kg produced drive than the conventional "upwind" rig does. So when both setups produce the same drive; the conventional rig loads up the bows more than the rig with the spi. And that is what you feel. If you would power up the main fully again you would go both faster and pitchpole more violantly but because you are transferring drive from the mainsail to the kite, which is less bow pressing per kg drive, you calm the boat down while keeping up speed. Or if you really want to you can end up at the same controllability and higher speeds.

An important side effect in my opinion is the fact that a small amount of lee helm under spinnaker is very pleasent as it helps bearing away quickly when needed. Most conventional rigs however have an increased weatherhelm when broad reaching that is working again bearing away comfortably. This aggrevates the troubles when you encounter them. The spi therefor seems to pull the boat out of trouble once you encounter them while a conventional rig pushes you only in deeper.

I hope this helps

Wouter

Colin, recommending a boatdesign forum thread to anyone desiring a clear and correct explanation of anything is cruel and unusual punishment.

As for Gentry's journal article, it's good as a correct description of lift, but it looks like he screws up the correspondance between upwash and pointing ability. (Fig 35, p15) Anything that increases upwash (jib in the presence of a main) or increases downwash (main in the presence of a jib) decreases pointing ability. I wonder if he meant power to windward, because slats, slots and flaps on a wing will all create more lifting power, but at the expense of an increased downwash angle.

On an airplane wing the increase in downwash angle is partly what gives you more lift. Airplanes fly using Newtons Law of opposite and equal actions, etc. Slats, slots and flaps direct more air downward at a more vertical angle. Thus the vertical force componenet is increased. So the combination of downwash angle and amount of air forced downward is what lifts an airplane off the ground. These same principles work on sails.

Sorry for digressing a little.

Are you sure that you intepretate his writings correctly ? I'm familiar with his concept although via other sources than Gentry himself. These sources all express that adding the jib hurts pointing LESS than what would be expected from superpositioning the two single sails to form a double sail rig. In effect the maximum drive of the combination of the two sails is found at a smaller pointing angle than would be expected from just adding the characteristics of each sail individually. The explanation for this is than given via upwash and downwash principles. This is somewhat different from your interpretation.

Wouter

But you're still saying that a jib hurts pointing ability, just less than what is expected from less satisfactory lift theories. How does that contradict my interpretation?

On the topic of lift, I have the following link for those wanting to learn more - http://www.grc.nasa.gov/WWW/Wright/airplane/lift1.html

I'm certainly no expert in the field but I did fly VFR and learned to glide when I lived in Melbourne. The exact reasons as to how lift is generated are not known. However, the majority of lift is believed to be from the negative pressure on the upper surface of the wing as determined by factors like camber, chord length, angle of attack, etc, etc. The negative pressure on the upper surface of the wing "sucks" the wing (and the plane) upwards. This is how/why a plane lifts off the ground. The effect of compressed air under the wing and it's effects on lift/how a plane flies is less clearly understood.
Re: sailing, the "upper" surface of the airplane wing is, of course, the leeward side of your sail. This is why we must always keep the ticklers flying well on this side - maintain proper airflow needed to lower the pressure on the leeward side and thus generate lift. Withoutt the lift generated (ie turbulent flow on the leeward side = no lift = stalled wing), we're simply being pushed by the wind.

Now that's enough into a field that's too complicated for me to addresss further. Perhaps those of you better suited to explain this can give us all a quick refesher or correct the flagrant errors I've just stated.
Cheers
Ron

Ron,

Being a pilot myself and a mechanical engineer who endured hours of fluid dynamics I shold probably clarify my earlier post on lift.

Most people believe and are told (even many fluids books say the same) that Bernoulli's principle is what create lift on an airplane wing. Bernoulli's principle is only effective in closed areas like a pipe. It is true that the faster moving air over the "leeward" side of the wing is lower in pressure than the "windward" side, but this pressure difference cannot account for anywhere near enough force to lift a plane off the ground. The majority of the force that lifts a plane off the ground is due to Newton's 3rd law. When you force the air downward you need your opposite force to be upward. Flying in ground effect is a prime example of the downward flow of the air. So, it is a combination of both principles but Newton's law dominates.

I need to go to my home computer where I have a link that describes this in more detail than anyone here will care to read. I will post it over the weekend.

To back up my statements I have done experements in wind tunnels where I measured the pressure differential between the top and bottom of a section of wing from a C172 and calculated the lifting force on pressure alone. If that were the only lifting force a wing generated you will go off the end of the runway.

I think you ones more make a similar logical warp.

You are intepreting something in the way that is not correct.




That was not the point.

The point was directed at your statement that I repeat below :



In fact Gentry doesn't "screw" up, only in your intepretation of what he writes does that happen. Ergo, the error is to be found on your side of the story and not on Gentry's side.

Beside your statement



Shows you have not grasped the point in this respect either. We are not talking about "less satisfactory lift theories" we are talking about theories that are proven wrong (ventury effect) and the one that does explain what happens in full detail. You also seem to glance over the fact that the interaction between main and jib much limits the "hurting" of pointing. Yes, a sloop rig at its optimal pointing angle does sail lower than a uni-rig at its optimal upwind sailing angle, but sadly enough no-one was saying otherwise. Much more interesting is the "discovery" of how much more drive an added jib seems to give to a rig, disproproportionally to its size and how the difference between pointing angle is relatively small. Two things that are still not commenly understood by the larger public. Especially since many still believe that adding a jib hurts upwind performance while both the Gentry theory and practice show that adding a jib will enhance upwind performance.

But we can probably go at this for years.

Wouter

Hobienick,

I'm familiar with both theories however I always wondered how the downwash is created or excerting its influence on the wings if not buy the pressure difference over the wing surface. Something must cause the free airflow to arc downward.

No-one has ever been able to give me the answer on this question maybe you can ;

Wouter

Ron,

The link you poseted does an excellent job explaining what I tried to say. It also disagrees with your statements on lift

Wouter,
Look at the link that Ron posted. It does an excellent job. I will also try to explain it better.

I'm sure you have heard of the hand sticking out of a car window. You have probably done it. This creates a very crude wing. You know if you tip the forward part of your hand up your hand will be lifted into the air. The more you tip it the more you feel your hand being lifted, to a point (about 45 degrees). A tool we use in mechanics, vector diagrams, allows us to break down the direction forces act into components of X, Y, and Z. For the sake of this discussion we will use only two dimentions, horizontal and vertical. Once you understand it in 2 dimensions adding the third is easy.

On a piece of paper draw a line at an angle with the left end fo the line higher on the paper than the right. This is your wing. Now draw a horizontal arrow into the wing's underside. Have the point of the arrow in contact with the wing. This is your initial airflow. Now, just like light reflecting off a mirror, the airflow changes direction. Draw a new arrow starting at the first arrows point. The new arrow will angle downward. This is the new direction of the air.

To break this new direction of airflow down into vertical and horizontal components, draw a rectangle such that the arrow starts in the upper left corner of the rectangle and end in the lower right corner. The vertical sides of the rectangle represent the vertical component of the airflow and the horizontal sides of the rectangle represent the horizontal component of the air flow.

At this time think of these arrows as representing forces. You can see how changing the angle of the wing (angle of attack) will change the vertical and horizontal force components.

I want to say at this time I am keeping this explaination very simple and not using any math, just visualization. Chances are the horizontal component in your diagram is larger than the vertical which would not make sense in the real world. Again, this is just for visualization purposes only.

This vertical component is what keeps airplanes in the air.

I hope this very simple explaination helps. I will try to draw a diagram and post it over the weekend.

One of the reasons why I love sailing is the amount of mechanics involved in making a boat sail and truly understanding what happens when you make any adjustments to the boat.

Again, I hope I was clear enough on this. It is much easier to explain in person with a pen an paper.

I've found the article referenced below to be enlightening as to why the Bernoulli explanation is wrong. It's quite readable. I'm not an expert in this field, so I count vouch for its accuracy. Enjoy.

http://jef.raskincenter.org/published/coanda_effect.html

Wouter, in case you didn't notice, I said "it looks like he screws up." That is, I don't understand how he can say that a sloop rigged boat can have enhanced pointing ability and have an increased downwash angle. I have no problem accepting that a jib can "enhance upwind performance," and if that's what Gentry had said, I wouldn't have questioned a thing. However, "enhancing upwind performance" is more vague than "pointing ability." Perhaps it's just a matter of definition, but I fail to see how, as you say, "a sloop rig at its optimal pointing angle does sail lower than a uni-rig at its optimal upwind sailing angle" and yet have increased pointing ability.

hobienick: Don't worry about being to technical with Vouter.

He's like, king nerd amongst engineers.

I would like to say thank you, all, for geeking out with me. It's not often I get a chance to converse like this in a social setting. That said I also want to apologize if I am insulting anyone with trying to keep things simple. Since I am fairly new here I don't know everyone's background.

After reading the links on lift, those article do a much better job explaining everything about why Bernoulli make airplanes fly. Read those. Why try to reinvent the wheel?

Uh, I think I'm in the wrong chat room here.

Where's that chat room where we talk about which color sail is the coolest?

Ummm, that would be the Hobie forums

Mike. (running for cover...)