I`m sure this has been discussed before, but I have a few questions I`m hoping someone with some advanced knowledge on the subject can answer in a way that normal people can understand.
Foil design for cats should differ from the normal dinghy design due to higher speeds, I would imagine. I`ve always thought that higher-speed foils should have their max. thickness slightly further aft than the classic 1/3 aft of leading edge.
Recently fould this article on a skiff design for foils which confirms my theory http://www.activesportsboats.com/foil_design.htm, and looking at Hobie Tiger & Inter-20 rudders also leads me to believe this is correct - they seem to have concave surfaces toward the trailing edge.
Another question - I`ts widely accepted that high-aspect ratio has higher L/D ratio & is therefore more efficient, why then, does the Taipan have such short square daggerboards ? Even the Australian Mozzie sailors have gone for shorter daggerboards than allowed in their class rules, which puzzles me. It seems Australian design philosophy differs from the accepted norm in some cases, yet it seems to work for them.
My questions :
1. Does moving the max. thickness aft on daggerboards produce a foil with better upwind ability, as the article suggests.
2. would the Taipan & Mosquito have better upwind performance if fitted with high-aspect ratio daggerboards.
3. If you take a square daggerboard & make the tips elliptical, theoretically you reduce the drag & tip vortices, but you`d also reduce the area if you keep the length the same. Would the increased efficiency make up for the decrease in surface area and subsequent decrease in lift-producing surface ?

All those in the know, please answer !!

Steve,

Do a search on the F16 forum on the keyboards : Brobu, I-17 and read through the long thread where several of use dialed in his I-17.

In theory higher aspect (=long boards) boards have a more efficient lift to drag coefficient ratio but in real life there are a few extra considerations that limit effective application of these long boards. And all have to do with speed and not with depth of you local lake.

In some way long boards are like a more efficient driving system between the motor and wheels of a drag racer. Sounds pretty attractive but when realizing that drag racers need accelleration in stead of efficiency and that their accelleation is all but limited by the tracking of their slipping wheels it will be apparent that a more efficient drive system is all but USELESS in dragracers.

Something analogue is happening with boards and cats.

Wouter

Get a copy of "Theory of Wing Sections"


Amazon has it for a little as $8.95 here

It's required reading for naval architects.

The book is dated 1949 last modification 1957

This is an aerofoil book and not a surface piercing foils book.

surface piercing foils have progressed a long way since the 50's.

I'll bet my bottom dollar that this book only deals with high speed aerofoils and does not handle low speed aerofoils like sails and handgliders. let alone surface piercing foils which are daggerboard and rudders. Even just submerged foils like keels notice the proximity of the watersurface and display abnormal behaviour when compared to foils surrounded by one one medium with no transition bounderies.

It may be a good starting point for complete novices but Steve won't find the answers to his question in this book.

Wouter

The book is a classic reference on airfoil sections and includes a substantial amount of data for different sections. Just because the data is old, doesn't mean that it isn't useful, nor that the theory behind it is flawed.

It was (and still is) a required text in the Naval Architecture and Marine Engineering program at the University of Michigan - where I got my naval architecture degree.

The data is presented in terms of Reynolds numbers, which take into account both speed and viscosity. Doesn't matter what fluid the foil is in.

We were talking about foils in water, not foils in air. Besides, hang glider wings and sails are not foils. They are camber lines.

You have a better reference text, Wouter?

>>The book is a classic reference on airfoil sections and includes a substantial amount of data for different sections.

And it probably excellent in that. but it has been discovered since then that foils in proximity of surfaces and or transitions of phase will behave differently from classical Aerodynamics and their underlying idealisations of fluid. Not to mention foils that actually pierce through such surfaces

A few examples :

Why are airplane propellers different in shape than the screws on ships ?

Why have current board all a concave section near the trailing egde ? That is something that isn't part of classical Aerodynamics. Most used profile are not even NACA profiles anymore. Special profiles for boats have been developped.

What is ground effect ? And why have the russians designed a whole massive transport airplane around that phenomenon; an airplane that can carry the load of 3 or more Galaxy transport planes. Why is it intended to fly higher than its wingspan ?

What is venting and why will it never occure on an airplane wing ?

What is cavitation ?


>>Just because the data is old, doesn't mean that it isn't useful, nor that the theory behind it is flawed.

OF course but that was not the point. The point is that the data was developped under conditions that are not satisfied in real life maritime applications. The theory behind is not wrwong per se it is more that the developped theory is not applicable or insufficiently accurate for modern maritime uses. Sure a NACA foil will work just as a crocked plank will work as a rudder, but you won't win any first places with both of them nor will you find answers of weird phenomena that occure in real life sailing like venting rudders or cavitation.


>>It was (and still is) a required text in the Naval Architecture and Marine Engineering program at the University of Michigan - where I got my naval architecture degree.

It is may well be an excellent book for Aerodynamics 101, but does the material contained in the book answer any of Steves 3 questions ? That is after all the topic of this thread. I'm very sceptical about the last.


>>>The data is presented in terms of Reynolds numbers, which take into account both speed and viscosity. Doesn't matter what fluid the foil is in.


Like hell it does. Cavitation is encouterd in media in their non gas state. Airplane foils, he main interest of NACA testing do not really occupied themselfs with phenomenon like that. And as a naval architect you must know that. Reynolds numbers are cool but do not really do more than relate point the transition from viscous flow to turbulant flow and suggest how different parameters need to be chosen in model testing. They help in making data contained in graphs more accessible as well.


>>We were talking about foils in water, not foils in air.

My point exactly.

>>Besides, hang glider wings and sails are not foils. They are camber lines.

That is true. In that aspect they differ from foils. I was trying to point however that the theory of low speed foils (including wings) was developped after the book was written and that seriously different things are happening under these conditions. It is my understanding that NACA testing was mainly done from the viewpoint of establishing superior wing shapes for planes and control surfaces even blades for windmills. None of them were specically developped for use near surfaces or unders relatively large additional movements like rotations and up and down movements as encountered when sailing through waves. Rudders, keels and boards are in a pretty unsteady enviroment where the speed and direction of flow over the foil section is continiously changing with significant fluctuations. THis is completely different from steady state testing that has been performed on NACA profiles.

Sure there are many things that remain similar but again Steve has asked specific questions that point straight to contradiction in the basic steady state theory. And you, with all due respect, point him to a book that is most likely more of the classical stuff that didn't explain Steve observations in the first place.


>>You have a better reference text, Wouter?

I would start with Frank Bethwaites book on how performance sailing. I think Marchaj did some work on it as well and then I'm also at a loss. Bits and pieces are scattered over the net but I haven't encoutered a definitive book on it myself. But then again I moved to control system engineering and thus away from Nautical engineering. As a control engineer however you do encounter alot of situations where the practice doesn't do what the theory predicts it will. Than it is time to slap a control computer on the machine to put is all straight again. I must say that Frank and the Aussies as well as the russians tend to put more emphasis on real-life testing and only using theory as a start point. This has lead to some interesting developments. Sometimes people don't even know why it works but it does work nevertheless. OF course the Aussies discovered and pioneerd the prebend rigs. Do you know why they are better than the old rigs ? (of course this is not an unknown but it doubt wether it is part of 1950's curriculum)

Again, with all due respect to your person and the book itself. Those are not the things I disagree with; I have a problem with the application to this situation

Wouter

Thanks for replies, Matt, Wouter.

Matt, I have both "The Principles of Yach Design" & "the Elements of Boat Strength" sitting on my bookshelf, with the time to read neither. Hopefully I`ll get round to it some day soon as I have a genuine interest in the subject.
At the same time I`m trying to learn 3d Studio Max for architecture, just trying to keep up with technology in your own line of work these days is quite challenging.

So me asking the question is kinda like "guys, please give me a shorcut here, I don`t have time to study Naval Architecture for 4 years, just so I can figure out what the best solution is for daggerboard shapes."
Keen to build my own foils, but I`m not the type to just settle for what is "the done thing". Sure, NACA foil sections are tried & tested & operate over a broad range of conditions, but it seems that newer theories are challenging the old ones, have a look at wave-piercing hulls vs very rounded bouyant ones, a few years ago everyone would have laughed at the F18HT / A-class designs. Then there`s the unirig vs sloop debate, still open for discussion. No one theory is always going to win out over another, one may be better suited to a certain set of conditions than another.
When sailing against the Hobie Tiger, we are very close in speed upwind, but they can outpoint us by a large margin. Probably a combination of high-aspect rig, longer waterline & the Looooong daggerboards. Now, out of those three I can change the daggerboards up to a longer set with the least cost, I accept that it won`t be a major improvement as if I could change all 3, but I`m hoping for a gain of some sort. The best scenario would be for me to build a set of blades that are the same length as my current ones as they would still fit class rules, but gain in their efficiency as much as possible, hence the questions on how to "fine-tune" the design.
I`m well aware that higher aspect ratio foils / wings have a higher L/D ratio than low aspect, paragliding taught me that. It also taught me that flying a high-aspect ratio wing was far more prone to finding stall point much sooner than my old training glider, and was far more twitchy (thats what makes it more fun to fly .
I haven`t heard of any Hobie Tiger sailors pulling boards halfway up going upwind, even when it`s howling, maybe that problem is specific to a certain hull shape / sailplan, these factors always have to be considered.
Our class rules state max. daggerboard length = 1200mm, max. width = 300mm, giving quite a short square foil below the surface. Making the tip elliptical would increase efficiency, but substantially decrease the lift-producing surface area of the board. Where do you draw the line ?

So my questions are still open, and I`ve added another one !
Cheers
Steve

If you don't get your question answered here, try posting it on the sailboat section of http://boatdesign.net/forums/. There is a chap who frequents that forum called Tom Speer who seems quite knowledgable on foils and always gives a good answer.

If anyone has ever found any references to the best shape for a foil at the surface boundary I would like to see it. I have heard lots of people like Wouter say that traditional foils are not good, but have yet to find anyone to say what is good. Perhaps I will post that one my self!

Thanks Gareth, I`ll try that route. I guess my question could be re-phrased a bit :
If I want to go "high-aspect" on my daggerboards while remaining within class rules, I`d have to lose approx. 30% surface area as opposed to max. class specs, which gives a "square" board underwater. If I move the max. thickness back & have a slightly concave rear section toward the trailing edge as per the skiff type foil (NLF section), theoretically I should be able to point higher and also have a more efficient foil (elliptical tip). At what point would the increase in efficiency be nullified by the loss in lift-producing surface area ? Would it be less than 30%, in which case I`m better off with standard foils, or would the increased efficiency overcome the loss of area ? I guess I`d need to do a whole lot of mathematical calculations to figure it all out, only to find that in the real world it all works differently. And in order to do the calculations, I`d have to first become a Bill Roberts / Wouter / Matt Bounds type character (ie someone who knows these things through years of study or esperience )

Cheers
Steve

Steve,

Are the dimensions you gave the underwater area of the board, or the total length (including the part iniside the hull)?

What are the class dimensions of the underwater area of the board? Is there a thickness restriction? What about the shape of the trunks? Will the new boards have to fit through the old trunks?

The daggerboards need to be effective over a broad range of conditions, thus some compromise has to be reached on the foil section and the plan form (rake, tip, aspect ratio). You can't be optimum in all conditions.

Of course, you could just build a different set of boards for each combination of wind / waves

There has been discussion among A-cat sailors in Jacksonville about our canted hulls and AuscatV boards (short and square like Taipan 4.9 boards), versus BIM 2000 boards (52 inches, hollw training edge and narrow). Bim climbs to weather like an angel in light stuff: AuscatV does better footing in heavy air.
I have not seriously tried pulling the boards up in heavier air, but will now, by Yumpin' Yiminy.

The thing I learned was that you're better of building several different boards and reference test them in real life. Gives you more dependable answers and is both faster and not too expensive.

Theory can quickly loose you alot of time while travelling a dead end.

That is me saying it, knowing that I'm comfortable with theory.

Wouter

When you know the direction of sought potential but don't know by how much you need to alter somethign go for a half way solution. it will never be perfect but also never worse than before.

In case of ellipical wing ends. This is mathematically a sound way to go however in practical sense we don't see mnay of them. Even at the time of is invention greatest use (Spitfire airplane and era) the competition didn't copy it. The mustang I believe just had a trapezium like section at the end of its wing that could be regarded as a rough approximation of the elliptical wing.

One thing that strikes my most is that C-class fixed wings don't use elliptical tips (anymore). They all use a trapezium section on top of a rectangular section. I even believe that the the straight top is between 35 and 55 % of the foot of the sections.

So for your daggerboards I would not go with ellipical but with a trapezium section on the bottom 33 % of the wetted length with an end tip that is 50 % of the greatest width.

I suspect that this will get you close enough to lower drag while keeping sufficient area in the board when satisfying the class limits. I would also keep the trailing edge straight, you don't want a vertical vortex developping on your trailing egde. But it must be said that opinions differ about that. Some keep the leading egde straight and put the trapezium on the back.

Move your greatest thickness back as suggested by that monohull article. I know that several other board builders have done this as well. I seems to work for them. Somewhere between 30 % and 50 %. When able let the thickness reduce proportionally with the width of the board. Don't make the trapezium while maintain the full width of the board. That won't work well.


Make sure that your leading edge is a parabolic section. This is a compromise for good behaviour at both low and high speeds. A blunter parabola will give greatest steering force at low speed by ventilated earlier and give higher forces before venting at higher speeds. A sharp leading edge (a sharp parabola fairing) will have slightly less lift with a little more drag than the optimal parabola leading edge section when at low speed. At high speeds it will vent at 80 % of the maximum force attainable but flow will still reattach quickly when rotation is reduces. What ever you do do not use circular leading edges At has all the characteristics of a sharp leading edge but with the disadvantage that flow is reluctant to reattach. Bets compromise : 9-10 % thick section with with a small leading egde radius followed by a parabola over the forward 15 % of the cord. The higher the speed the finer the leadin edge must be. 18 foot skiffs mid wind range boards use radii of 0.5 %(source Bethwaite High Performance sailing)

Polish and buff your board and make them absolutely spotless and smooth the difference between smooth and buffed to a mirror can be 25 % addition force. Especially for rudderblade this is desired.

make the trailing edge sharper than the leading edge but you can get away with having almost a symmetrical crossection shape that is a little bit skewed towards the front Max thickness at 45 % or so.

Concave sections in the last 30% ; I don't know much about these. They seem to help prevent humming I'm told, but then again teh guy wasn't really an expert.

And buy Bethwaites book !

Good luck

Hi Matt, answer to your questions :
The boards will have to fit into square trunks (existing)with a shaped exit at the hull which currently fits the existing blade. I can re-shape that to a certain degree as my hulls are timber. Max thickness is 22mm, max length 1220mm, max width is 305mm but my trunks are 275mm front to back. My current boards are square, 270mm x 1020mm, so I could go longer although the Auzzies believe shorter is better, and puts less load on the boat & blades (they`re Obeche timber with 1 layer of glass-epoxy, very light & I stand on the tips to right the boat, so strong enough at current length.) I`d have to add a layer of cloth if I lengthened them.
Areas I`ve calculated are below hull only. If you look at the attached drawing, I tried to come up with a design that could be rotated aft to get the c/e further back so I could carry more aft mast rake to compensate for lee-helm under spinnaker. Last weekend I raked the mast forward so i`ts now dead vertical, fast upwind & downwind, balanced rudders, very light leehelm downwind, so it seems I won`t need that anymore. The shape was an attempt to get more "high aspect" within current class rules, but as I said I`d lose 30% area, which according to Bill means I`d have to up-size the rudders.
Looks like the best solution would be to get full-length boards with a rounded front edge to reduce tip vortices, maybe move max. thickness back a bit as per NLF section referred to in the Skiff article, and try it out. That way I`d gain surface area, not lose it. (current surface area = 0,159sqm, full length boards = 0,207sqm, new tapered design = 0,147sqm.)
Any suggestions ?

I will take a stab at answering some of your questions.

For the current trend in foil shapes look at the BIM XJ. The foil shape they call out, was the hot foil in articles 3-4 years ago.

I always think of the Elliptical tips as maintaining lift while reducing area (and drag). This is the theory. However they only really work in undisturbed fluid and where there are no nearby (~50 cm) boundaries. Boundaries can include the bottom or thermal layers.

There was some testing done in the 40-50's on tip shape. The flight testing of elliptical tips did not show the benefit promised by theory and wind tunnel. The tapered sections (which would include trapezoidal) worked about as well. I was shown a test report from North American Aviation. I remember thinking, all the wing shapes look to be within margin of test error. Tapered sections won because they promised some improvement, saved weight, and were almost as easy to build as a straight section (for rib and skin construction)

Elliptical and tapered sections are harder than straight sections for the homebuilder to build right. The best method I found was to use a CAD/CAM program to build a file for a CNC router to cut cores. This feel apart when the CNC router owner got a big contract. You could also build a plywood skin and rib foil to use as a plug for a mold.

In summary, in deep smooth water and design wind speeds, an elliptical high aspect ratio board will probably outrun a square low aspect board. under other conditions there will be little to no difference.

Finishing your hull and foils to a near perfect condition might have a bigger impact.

Thanks for replies, I checked in on boatdesign.net as per gareth`s advice, read some posts by Tom Speer and others on the best software for hull design, and based on this I`ve decided to buy foils, rather than build them !! .
Man, those guys take the subject to a level I don`t even WANT to try & understand !
There`s so much you need to take into consideration that I never even knew existed.
Wouter`s concept of build it & test it might very well be the best option, except that after you`ve built 3 sets of foils that don`t perform well, you could have bought one set that does. Think I`ll try it, anyway.

Cheers
Steve