g'day could i get some answers to some questions about centerboards

Discussion in 'Sailboats' started by nick125, Oct 1, 2007.

  1. nick125
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    nick125 Junior Member

    some may have seen my thread in sailing anarchy http://www.sailinganarchy.com/forums/index.php?showtopic=60287, story to date is ive been making a centerboard and rudder for my 125. (medium speed boat like a 420). the rudder (and subsequent rudder box) is nearly done so im starting to think about the center board now.

    firstly the board profile is set by class rules
    secondly max thickness and width where it exits the case is set so can only get a 6% foil although board tapers and about half way along it could be made to be 9%. is changing the foil % along the board a bad idea?
    thirdly does anyone have any good sections to recomend? (with some offsets?) as in better than naca 4 series.
    fourthly most of the stuff ive been reading is oldish (read brandsford ecks articles on the 505 site) has the ideas chaged much. He recomends a different section to the naca 4 seies althogh i cant make any sense of his offsets.


    thanks in anticipation

    Nick


    ps. if you havn't already gathered i'm not super educated. so please try to explain what your saying in realitively simple language. (but dont go too extreme ive got a bit of a base of information from what ive been reading) ie what laminar/ turbulent flow and some understanding of renyolds numbers.
     
  2. nick125
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    nick125 Junior Member

    come on some one out their must have a good idea? im probably going to start shaping in the next couple of days so some opinions would be good. if no one has any good sugestions im just going to make it so the foil is naca 0009 and for the top bits (which are to thin to get the desired thickness ratio) and just treat it as being the right thickness and just extend the trailing edge out to the designated width as fine as i can make it practically. (1-2mm).
     
  3. mmmboats
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    mmmboats Junior Member

    bloody hell where the heck is doug lord when u need him? its obvious your writing skills have bored the readers on this forum...
    anyway a very good sailor tells me "set up your boat the same as the best guys out there, and then get better then them"
    maybe it would be safer to do just this, look at what the best are using and copy that design
     
  4. bart streb
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    bart streb Bart

    "come on some one out their must have a good idea"

    I think us newbies have to earn our spot on the forum. I am building a lifting keel and posted, but it takes some time to get a response. I am going to build mine out of Plywood with a lead bulb. I am going to take some measurements on very similar boats, and fabricate. You should look at some of the leading boats and take some measurements if you can.
     
  5. nick125
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    nick125 Junior Member

    thanks. i do know what one of the best 125 boards are like (i helped build it) im trying to find something to give me the edge. the boat i've got isn't as quick hull shape wise as a lot of the newer glass boats so im trying to find something to try and over come that.

    as for your keel bart have a read of this thread http://www.sailinganarchy.com/forums/index.php?showtopic=60282 what pulpits saying is very good and is how we would tackle this at work
     
  6. bart streb
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    bart streb Bart

    Thanks, due to hull constraints, it looks like my blade will have to be 20-24" wide and 46" long (sorry, i am a yankee-too hard to convert to mm, and all my rulers measure in inches!!). 30" will hang out below the boat, which is 3' more than the old style keel right now. The bulb will be 200lbs roughly unless i can melt and pour the lead into the naca section I am looking at. Seems a decent sized keel for a 17'waterline on a 190 sq ft sail keelboat.
    The edge you will get is new sails and a smooth bottom. I would focus on that, and keep the foils very competetive for your class.
     
  7. messabout
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    messabout Senior Member

    I'm thinking you can google up some NASA sections with a bit of searching. The offsets for those sections are usually expressed in terms of percentage of chord (leading edge to trailing edge distance) That way you can size the section any way you like. The percentage numbers refer to the thickness of the half section. Thus a 6% section would show a maximum offset dimension of 3.0. The leading edge, or within the first 15% of the chord, is the most laborious to get right and that part is probably more important than any of the rest of the foil. Note that NASA section offsets often give dimensions at two and a half percent, 5 percent, 10 percent and 15 percent. so be fussy with that particular area.

    Too bad that the LE is the part of the foil that is most subject to damage. Smoothness in that area is worth careful attention. Farther aft you can get away with a little less perfection. Making the trailing edge knife edged may not be worth the trouble and the extra risk of damage. Except at very low speeds the flow will be separated well before it gets to the trailing edge. So knife edging has little to recommend it.

    Bart is right about new sails and smooth bottoms. Those two things make a lot of difference. The rest of the equation involves your skill and knowledge about setting the sails up.

    Your main and your mast must be agreeable to one another. The sail luff allowance (the curve in the luff of the sail) must be calculated to work with the bend that your mast exhibits. Some masts can be tailored to suit the sail by fiddling with the shrouds, spreaders, backstays, vang, down haul, and all that stuff. Have some serious discussion with your sailmaker about that subject. The jib luff is also cut with a curve, but it is a negative curve. The reason is to accomodate the sag of the luff wire. So you need to experiment with luff wire tension to control the curve.

    Tuning sessions with another boat is invaluable. The other boat sails as usual and you tinker with rigging, sail set, etc. to find the best combination for the particular wind strength that you are in. Make a lot of notes. This is a time consuming process but well worth it if you intend to go faster.

    You may very well discover that your boat is as fast or even faster than some of the newer ones who are beating you.

    All that is good advice but it can all go for naught if your tactics are less proficient than your competitors. The boards are the least of your worries.
     
  8. Guest625101138

    Guest625101138 Previous Member

    Nick
    Most of the following is in relation to the rudder and similar analysis can be applied to the centreboard.

    If you had no constraints then the most effective, lowest drag rudder would be a high aspect plan form using something like a NACA0020 section. Your class rules prevent this but you may be able to approach it.

    A fat rudder works over a wider range of angle. So you can reduce the planform area but still generate the required steering force. For example a NACA0020 will get a CL up to almost 1.7 while a NACA00a6 gets a peak CL of just under 0.6. So the rudder can be 1/3 the area to generate the same steering force. The maximium lift to drag ratio for the 20% is almost twice the L/D of the 6%.

    A high apsect rudder (planform area/chord) has lower induced drag.

    I think there would be merit in increasing the thickness ratio if the rules allow. Even if it is only over a portion of the rudder. I do not have the ability to analyse the 3-D flow patterns but at least part of the rudder is getting closer to optimum. And gut feel is that this would help.

    In terms of section shape I have found that the NACA 4 series are well known and it takes work to do better for slow speed applications.

    There is a very powerful and easy to use bit of software called JavaFoil that allows you to compare and customise foil sections:
    http://www.mh-aerotools.de/airfoils/javafoil.htm
    I have attached a screen dump of typical page from this. If you want to take the time to work through the analysis of a rudder I can help you drive JavaFoil.

    The profile shown is a modified NACA 4 series. It has a 6% thickness but I have added a more rounded nose and pushed the maximum chord further back. This will operate at slightly higher angle of attack, has the same drag at zero angle and the L/D maximum is higher than a standard NACA 0006.

    In practice I am not sure you would be able to detect the difference after you have done a lot of design work but you will gain understanding.

    Understanding this tool might help you identify a particular defficiency with the rudder and modify the design to overcome the problem but your room to play is slight.

    Foils are both the engine and control for sailing boats so having a way of easily working out the physics is very useful. Martin Hepperle has done all the hard work so mere mortals can benefit from his mathematical and computing expertise.

    Rick W.
     

    Attached Files:

  9. nick125
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    nick125 Junior Member

    Thanks for the replies. Firstly I know quite a lot about rigs. The way ive got my mast set up is a bit different to standard. basically there is 2 sleaves inside instead of 1 (the second is only about 700mm) and spreaders are 200mm lower and im sailing with less poke to try and get the top to do more in light winds and chop.

    as for boats hopefully ill be coming top 10 at this years nationals with the boat ive got the shape of its a bit odd but is closer to the good spectrum then the bad side (125s have quite large tollerances)

    Rick. thanks heaps for your post. the rudders already made so theres not much i can do now (ill try and take a photo of it sometime) (its 12%)

    ive been trying to play around with x foil but havnt really got anywhere (i cant even figure out how to load foils from the foils database ive got it saved in the same folder and everything)
    but im managing to get xflr5 working. but im struggling to understand the polar graphs and cant find any explanations on the web. whats desireable for cp graps? i take it you want as bigger difference between the windward and leward sides of the foil.


    so you think i should use your section over a naca 0006? and should i continue it down to the bottom (taper the foil) or where i can (half way down) bump up the section to a 9 percent?
     

  10. Guest625101138

    Guest625101138 Previous Member

    Nick
    I think you would find JavaFoil much easier to use than xfoil. It has the same underlying maths but better interfaces.

    Open the web site. Start the Applet. Go to the options tab and set density to 1000; viscosity to 0.0000013. You can also set the aspect ratio for your planform if you want.

    You can then just select foil you want on the geometry page and it will calculate and draw the profiles. It is very good software. You then go to the analyse page to get the data.

    The operation of the centreboard is somewhat different to the rudder. It is usually not as important that it will work over a wider range. About the only time I wished I had a better lifting keel in my yachting days was when I was trying to pinch up on a windward mark without making another tack. This is coasting with sails almost feathered and if you squeezed too much you could feel the point where the keel stalled.

    I doubt if there is much advantage in improving the operating range on a 125 but I have not done much dinghy sailing. You may in fact be able to reduce drag be making the leading edge finer although I have just played around with this and it is hard to better the NACA 4 digit Cd at low angles of attack.

    By understanding the physics you may be able to work through an issue with your chosen design once in operation. You may already have a view on a particular weakness.

    For example if you have a feeling that the boat has too much leeway in light air then you might want to increase the operating range by thickening the section as the cord reduces and increasing the radius of the nose. But you can probably achieve the same thing by sailing a bit faster and pointing lower.

    In summary I cannot think of any reason why the NACA0006 would not be a good choice and stay with that section for the entire depth.

    Rick W.
     
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