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How Sails Really Work


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How Sails Really Work The airflow diagrams in the sailing books are wrong By Arvel Gentry SAIL Magazine April 1973 Arvel Gentry is a research specialist in transonic supersonic mainsail when they are used separately 2 we must knowand hypersonic vehicle aerodynamics at the where the air flows when the two sails are used He is also a successful ocean racing skipper and an 3 we must know how the resulting December changes in1999 airflowamateur photographer affect the pressures on both sides of the sails and finally 4 we must know how the air very close to the surfaces of the So you think that you know how sails work the slot sails the boundary layer is affected by the changes ineffect backwinding stalling and all that stuff Youve airflow and the changes in surface these facts of sailing from books and from Until recently there has been no accurate way ofmagazine articles by the experts Well read on You are in obtaining all of this information Actual test a few surprises are difficult to make and when they are made it is difficult All the books give about the same explanation for how to separate the effects of what happens in the airflow awaythe main and jib work and about slot effect However CA from the sail surfaces from what happens in the in his book Sailing Theory and Practice states that layer air very close to the surface The only approach in thethe interaction between sails is still a controversial subject past was to use logical thinking and educatedand not fully understood As a research aerodynamicist I guesswork However the proper tools are now very interested in this subject and set about to solve this problem and to provide a clear a study to at last resolve this problem of the interaction effects In my research I made use of three important tools As the first step in understanding how the air flowsFirst the Analog Field Plotter a device for accurately around sails one must be able to draw streamlines the streamlines about any airfoil shape show the paths that the air takes The concept of aSecond a new sophisticated computer program that is streamline is very simple and we need only to look brieflycapable of calculating the pressures and air speeds on and at the accurately drawn flow about a single sail Fig1 toabout any airfoil combination And third a water channel get the basic idea The streamlines tell the direction of thewhere the flow patterns about airfoil shapes including airflow at different points in the flow field about an effects can be observed under controlled The airflow between two particular streamlines willconditions and photographed The results shown in this always stay between the two streamlines The stagnationand the articles that follow are therefore based on well streamline marked S in Figure 1 is the streamline thatproven aerodynamic analysis methods separates the airflow that goes on one side of the airfoil My research has revealed the astounding fact that all the lee or top side from the airflow that goes on the otherthe explanations in the sailing books on the interaction side the windward or bottom side The the jib and main are wrong In fact if the air really streamline leaving the trailing edge or leech of the airfoilwent like many of these explanations say then the divides the airflow coming off the top of the airfoil fromresulting effects on the sails would be exactly the opposite the air coming off the bottom The stagnation streamline isof what is claimed very important in understanding the flow about sails It will take a number of articles to present the complete Once a complete set of streamlines is determined weresults of the research that has led to these conclusions can make some very useful judgments as to how windHowever I like to think that each one of you will share speed and pressure vary in the flow field about the airfoilwith me a little of the excitement that I experienced when The relationship between speed and pressure is given byall the pieces of this puzzle started to fit together for the an equation called Bernoullis equation The Bernoullifirst time the puzzle of how sails really work how they equation shows how the pressure of the air and the speedinfluence each other and most important how to of the air are directly tied clearly these effects In this series I will try to Wherever airspeed increases in flowing around theavoid unnecessary mathematics and technical terms and sails the pressure goes down Where the airspeed slowsthough each article will deal with a particular aspect of the down the pressure will be found to increase Wind wellproblem a thorough understanding of it will depend out in front of the boat may be blowing at a certainupon the information contained in previous articles constant speed relative to the boat However when the To understand fully the interaction of a jib and a air gets closer to the boat its speed and direction begin tomainsail we must have correct information in a number of changeareas 1 we must know how the air flows about the jib and high speed low pressureRanger 23 Newsletter Sj low speed high pressure Sj Figure 1 Calculated flow stramlines about a jib If we look at the streamlines in Figure 1 we see thatsometimes they get closer together and at other times theyspread farther apart It is quite obvious that when twostreamlines get close together or close to the airfoil surfacethe air will have to speed up to get through the smaller areaand the air pressure will be lower Where the streamlinesget farther apart the air slows down and the air greater Now this is all quite simple but it is important to note Figure 2 Typical wrong sloteffect drawingthat before we can apply Bernoullis equation we mustfirst know how the air flows about the airfoil We mustknow where the streamlines go The sailing literature is This situation cannot exist in the real flow about a sailfull of these types of drawings Unfortunately they are just Instead the entire flow about the sails would adjust itselfthat drawings of where the particular author thinks the air so that the airspeed and pressures are the same on bothgoes sides just downstream of the leech The streamlines should Figure 2 is typical of the airflow diagrams used in the be equally spaced on both sides of the leech if they arebooks to explain the slot effect There are a number of equally spaced out in front of the sailthings wrong with this drawing but Ill just mention the Another important requirement is that the spacing ofmore obvious ones here First note that the stagnation streamlines right at the leech of the main must be the samestreamline for the mainsail Sm shows a slight amount of as the spacing of these streamlines out in front of the sailsupwash bending of the streamline leeward to meet the In other words the airspeed at the leech of the main mustsail The air knows that it is approaching the sail and it be about the same as the freestream speed I am assumingstarts to change direction even before it gets to the sail the sails are properly trimmed and have no flow However in Figure 2 the stagnation streamline drawn separation You will see the reasons for this leech recoveryfor the jib has no upwash at all Apparently the wind speed requirement in a later article and also why it doesknows that it is approaching the main but it doesnt know not apply to the jibabout the jib That cannot be and this is the crux of the Check some of the drawings in your own sailing booksproblem The streamlines for both the jib and the main See whether the streamlines at the leech are drawnmust show the proper effects of upwash This cannot be properly Also check the stagnation streamlines leading todetermined by guesswork both the jib and main for upwash None of the drawings I However thats not all that is wrong with Figure 2 have seen has both the upwash and leech at the streamlines marked A and B on each side of the drawn properly Since these erroneous streamline for the main Out in front of the sail drawings do exist it is easy to see why the venturithe A and B streamlines are the same distance from the explanation of the slot effect has persisted for so long thatstagnation streamline so the airspeed is the same in both is a wide stream of air seems to enter the slot between thetubes of air but by the time they reach the leech of the sails and simply speed up as the slot gets smallermain the lee streamline A is closer to the leech than is the Figure 3 shows a very accurately calculated set ofwindward streamline B We would therefore have high streamlines about a main and jib combination Contrast itspeed lowpressure air on the lee side of the leech with Figure 2 Note that the stagnation streamline for thestagnation streamline and air with a lower speed and jib Sj turns leeward as it approaches the luff and that ithigher pressure on the windward side has more of this upwash than does the stagnation 2 SJ SJ Sm Sm Figure 3 Streamlines about jib and main as calculated by potential flow program SJ Sm Figure 4 Water channel photograph of flow about jib and mainsail for the main Sm The stagnation streamlines how much air goes in the slot and most important of allfor the jib and main actually spread farther apart as they how much air is caused to flow on the lee side of both theapproach the space between the luff of the jib and the jib and mainmast In a later article we will see that the final airspeed in the This is also substantiated in the water channel photo slot near the leech of the jib is only about what it would beshown in Figure 4 and it is a very important point It if the jib were not even present and the flow on the mainmeans that the air that is going to go in the slot between the does not separate Exactly why the air behaves in thistwo sails actually slows down as it approaches the sails It manner and how it affects the boundary layer will beslows down and only starts to speed back up as it described in the coming the leech of the jib This means that the old explanation of the slot effect in Addendum to How Sails Really Workthe sailing books the venturi principle is actually wrongThe slot does not act as a giant venturi with the air This article appeared in SAIL Magazine in April 1973 Iapproaching the sails and then just speeding up in a high had thought a lot about how my SAIL series might bespeed jet of air in the space between the sails as Figure 2 viewed by the expert sailors These people were theerroneously indicates Instead the air first slows down authors of sailing magazine articles and books thatand then is speeded back up in the slot frequently said that It is important for every serious sailor Now this may at first seem like a trivial difference but it to understand the basics of how sails work Now here Iis a very significant factor The stagnation streamlines for was about to point out that much of what they were sayingthe main and jib show how the air approaches the sails was wrong 3I was well prepared to argue the technical aspects of my negative one I wonder how the Federal Tradefindings I had reviewed my work with several co Commission can allow the advertisements for paints orworkers including my boss the famous aerodynamicist bottom coatings which improve speed five to 10 percentAMO Smith However I was not prepared for the If any bottom treatment could consistently improve speedreaction that I got from Peter Barrett Barrett had by two percent over a smooth fair untreated bottom itcompeted in three Olympics and had won a silver in the would be on every racing sailboat in the world AlthoughFinn class and a gold with Lowell North in the Star He Marchajs book on the aerodynamics of sailing is far morewrote a monthly column called CARTE BLANCHE for detailed and accurate than Currys published in1925 IYacht Racing magazine and was quick to respond to my suspect that Currys may do more for 95 percent of thefirst article worlds yacht racers than Marchajs When I wrote an article 10 years ago viewing sails from a fluid momentumCARTE BLANCHE by Peter Barrett concept instead of a relationship I statedYacht Racing Magazine June 1973 near the end that the average racing skipper will probably not find many applications for sailing theory in its pureScience and Racing form I feel the same way today It is interesting to see how little the giant steps taken in The best sailors in general are not scientists Paulexpanding our reservoir of scientific knowledge have Elvstrom Bud Melges Rodney Pattisson these peopleaffected yacht racing Although aerodynamics has very attack sailboat racing physically and emotionally morenearly become a science 40 years ago it was an art and we than they do scientifically Although Pattissons FD rigs arehave specialists like the author of a recent article in SAIL as clean as any rigs in use on any boatmagazine Arvel Gentry who is a a research specialist in today it is a result of a commitment to detail carried to thetransonic supersonic and hypersonic vehicle ultimate and not any unique aerodynamic concepts All ofaerodynamics or like Jerry Milgram of MIT whose us know that windage and drag are bad even if we wereCascade sent the rule makers back to the drawing board lucky in escaping high school science Few of us practicethe sport of racing sailboats has in reality changed very our knowledge as thoroughly as Rodneylittle Another step in a long line of published material which Yacht design may have advanced certainly recent serves more to provide fireside conversation than to windesigns win over older ones but they are racing under the races appears as mentioned above as the first of a series inIOR a new rule and the frequently heard SAIL by Arvel Gentry I confess to being interestedthat new designs are faster than old ones and boats although skeptical when I read the editors comment thatwould have been outdesigned had we remained with the it is both disturbing and exciting to be told that all theCCA rule ignores the 10year reign of the Cal 40 or the fact books that describe how sails work are wrong Thethat the hottest boat in MORC may well be the Cal 25 a articles subtitle continued the theme the airflowdesign which must be nearly 10 years old So one must diagrams in the sailing books are wrong I waswonder whether designers are really able to create yachts disappointed but not surprised to find that the sum totalwhich get from here to there faster than their counterparts of the astounding fact in error in all the books is that theof a decade or two ago given a range of wind and sea stagnation streamline for both jib and main is rarely with enough upwash and that the authors of material on Also if one looks at small boats progress is the subject havent thought it necessary to discuss the The Snipe sails about as fast for its size and flow on the windward side of the jib before reaching theweight as any boat although Lasers beat Sunfish in light mast and particularly that the air isnt being accelerated inand medium air the reverse happens in a strong breeze this region Mr Gentry promises more but I am willing tofew if any modern designs can compete with either the state categorically that future articles will do little ifStar or Thistle in light air and 110s are able to give the much anything to improve directly the performance of either amore sophisticated Flying Dutchman fits in heavy air given class of sailboat or a reader In fact by implying that a Progress in sails and rigs is slow as well The adoption of major error in everyones thinking about air flow past sailsDacron and aluminum have been real advances in speed is about to be corrected and thus we will all of course beand ease of maintenance but improvements in the basic able to better utilize this air flow and race moreconcept of a complicated and sophisticated rig like a Star or successfully I believe that a disservice is being done theFlying Dutchman have been few in the past several years reader The primary reason for the perseverance of the status quo is Few successful racing skippers as mentioned above paythe relatively high level of development attained many years much attention to the scientific articles The skippers who doago despite limited knowledge and primitive research tools A worry about the technical aspects pouring over Marchaj onStar or Snipe of the late 1930s was a pretty sophisticated wind countless cold winter nights are rarely successful at winningbending machine races And those who make the mistake of believing that a Although I have been trained as an engineer thorough analytical study of sailing carried out by a in fluid mechanics I find my reaction to scientist in fluid mechanics must lead to better racingexciting claims in the field of air or water flow is usually a performance if only because our historical approach has so 4lacked in rigorous scientific approach are doomed to doing the improve directly the performance of either a given class ofworst of all on the race course The worst that is until experience sailboat or a reader In fact by implying that a major errorshows them that sailboats are already perfected well past the in everyones thinking about air flow past sails is about topoint of say an F11 jet airplane and that the major determinants be corrected and thus we will all of course be able toof success are not turbulators on the mast or slots in thespinnaker but good starts consistent tactics considering better utilize this air flow and race more successfully Ivariants in wind direction and speed a cool head and a well believe that a disservice is being done the readertrained crew In fact my articles do refute the old theories on how I am not suggesting that Mr Gentry is the successful two sails work together and the slot effect and they doscientist being an unsuccessful sailor pictured above I explain the slot effect in a correct and ironclad mannerenjoyed his article look forward to the coming ones and This it seems to me is no disservice to anyone particularlyam sure that he is a good racing sailor not because he has the readers of my articles If Mr Barrett will just wait for thediscovered that the air flow between jib and main slows rest of the articles he will see thisbefore it speeds up however He undoubtedly had Further I did not imply that Mr Barrett wouldnothing to do with the attempt to present his articles as a according to him be able to better utilize this air flow andmajor revelation contradicting the explanation of sailing to race more successfully Whether or not my articles willwindward given in all the books help a reader is not for Mr Barrett to decide Most sailors I know are interested in any new idea even if it is of aLetter to Arvel Gentry from SAIL Magazine May 301973 scientific nature My articles were prepared for those who are interested in learning the proper explanation for howDear Arvel sails work You may already have seen Peter Barretts latest column Mr Barrett also had the temerity to say that The bestin the June issue of Yacht Racing but if you havent Im sailors are in general not scientists Yes the best a copy are not scientists or plumbers or dentists or teachers for I have no real argument with any of his comments hes that matter They are people who regardless of theirentitled to his opinions except for the last few lines of the original occupation are somehow able to devote anpiece Whether Barrett wrote them or they were added by exceptionally large amount of time to the sport asthe boys in the editorial department is not the issue here compared to the average sailor Being originally involved What is important is that we consider the series of in science or engineering or any other profession has littlearticles you have written and we are publishing some of to do with itthe most important research that has ever been presented Mr Barrett also stated that I might be a good racing His comments it seems to me are uncalled for and in my sailor not because he has discovered that the air flowopinion attempt to backbite your findings I dont care between jib and main slows before it speeds upabout the veiled reference to SAIL but I care very much however Again he is wrong My findings have been ofabout how it relates to your own work The fact is it will be great help in understanding my sails and their trim toa major revelation to many of the readers and I think you improve boatspeed And in any case a correct explanationtoo feel very strongly about this At least that is what your of how sails work should be of more help to the averageown writing says sailor than old incorrect theories You might want to write them a letter correcting them Finally Mr Barrett stated that I undoubtedly hadon this point nothing to do with the attempt to present my articles as a Curiously in the same issue Ted Jones calls for more major revelation contradicting the explanation of sailing toresearch in general Confusion apparently reigns windward given in all the books Barrett is totallysupreme misinformed on this point also The editors if anything toned down my own enthusiasm for the subject Barrett isWith best wishes not only wrong hes certainly not entitled to make such aSAIL E Mason III My confidence and enthusiasm on the material in myAssociate Editor series is based on considerable research over the past two years and it is reinforced every time I pick up a new sailingYacht Racing September 1973 book or magazine article and find them so totallyArvel Gentry Comments inaccurate from an aerodynamic standpoint I suggest that Mr Barrett read the entire series and if heSirs then wishes to argue some technical aspect of my studies In his June Carte Blanche column Peter Barrett fine But he has no right to take a so what or sourcommented on my series of articles on sail aerodynamics grapes approach after having read only the firstin SAIL Magazine After reading only the introductory introductory article It will be the openminded expertsarticle Mr Barrett wrote that he was willing to state the serious students of sailing and time that will be that future articles will do little if anything to proper judge of my ideas not Mr Barrett alone 5Barretts Reply Yacht Racing September 1973 fundamentals without hurting his performance perhaps Mr Gentry objects to my comments on his series even assisting it But a complex sport like sailing with theexploding all the old myths about air flow past sails That is key factors so far removed from analytic study is besthis right as it is my right to object to headlines and editorial approached with a very pragmatic implying that at last the real the true the Mr Gentry I mean no offense Let us disagree words about sail aerodynamics are about to be tolerance and good humor perhaps I am mistaken I wishpresented when his series in my opinion adds very little if you much success in your sailing to the state of the art My credentials arent Peter but include a Masters degree in engineering Pewaukee and completion of all course work for a PhD insame specializing in fluid mechanics teaching collegelevel fluid mechanics and using an analog field to plot What Goes Around Comes as Mr Gentry did well over a decade ago I think our differences of opinion can be condensed to In October and November 1991 Sailing Worldtwo issues Magazine published two articles on The Aerodynamics of 1 Mr Gentry thinks that increasing the upwash in the Sails These were primarily extracted from Chapter 5 of thestreamlines approaching the jib and hence emphasizing book The Art and Science of Sails by Tom Whidden andthe fact that less air passes between the jib and main than Michael Levitt Chapter 5 A New View of Sailboatwould pass between headstay and mast if there were no Aerodynamics was based on material that I had furnishedsails and the corollary that the free air slows down as it to them back in 1989 for their bookprepares to pass between the jib and main to more closely The two Sailing World articles in 1991 prompted aconform to theoretical flow conditions is an important number of people to write in with their own ideas on sailpoint and one that previous writers have been in error in aerodynamics just as had happened back in 1973 whendiscussing I believe previous writers have considered the my original articles were published in SAIL Sailing Worldprecise amount of upwash so unimportant that they have decided to publish another article with answers to some ofsimply never even worried about drawing these the questions They had received 16 pages of questions andstreamlines exactly right from an analytic point of view postulations sent in by readers Within a couple of weeks 2 Mr Gentry feels that these slight changes in there arrived in the SW offices about 30 pages of streamlines should be of more help to the from Whidden Levitt and Gentry The Editors sailor than old incorrect theories I feel that the excerpts from both letters and responses into a kind ofaverage sailor and indeed any serious racing sailor will do dialogue The article What Goes Around Comes Around washimself far more harm than good by attempting to published in the April 1991 issue of Sailing the theoretical streamline flow past a sloop rig Tom Whidden was aware of my disagreement withas defined by advanced fluid mechanics It has been my Peter Barrett back in 1973 and mentioned it in some of hisobservation that the more a racing skipper becomes comments used in the What Goes Around Comes by purely theoretical concepts as evidenced by articlesails covered with telltales strain gauges in the rigging That was in 1973 Since then Gentrys theories havesmoke and cameras to visualize air flow and discussion translated themselves into a number of practicaloral and written of fluid mechanics applied to sailing applications and have improved the performance of boththe worse his racing results sailboats and sailors Speaking personally as a sailor As my Carte Blanche column in this issue suggests the sailmaker and author Ive found Gentrys of racing success are not an understanding immensely helpful in what I do In The Art and Science ofof the Kutta condition or circulation but good starts an Sails Michael Levitt and I worked hard to unite the latestempathy with the wind and sea especially the wind aerodynamic theories with the ageold practice of on steering and speed and instantaneous Back in 1973 Peter Barrett dismissed Gentrys decisions which usually turn out to be correct as possibly true but ineffectual Back then Michael with anything else works to the my coauthor was an editor at Yacht Racing and Barrett of the racing sailor vicepresident of North Sails where I am now president To draw analogies the Grand Prix driver who is a Last year Michael Levitt and I relied on Gentry to specializing in the Otto cycle the foundation for the aerodynamic theories discussed inor a PHD in vibrations is a rarity probably nonexistent our book excerpts from which were published in Sailingthe pro basketball star will find advanced study in body World Today the excerpts have provoked several muscle and bone structure etc a handicap including Peter Fenner citing Peter Barrett to argue the Sports which depend on pushing oneself to the limits same points in the pages of the same magazine Now thatsof human endurance such as distance running or are simple enough so that a serious competitorcan devote a great deal of attention to pure scientific 6


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