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70427Re: [bolger] Re: Micro with chine runners?

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  • Leigh
    Jul 23, 2014
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      Hi folk

      I'm going to chime in at this point as it's finally come around to my own area of expertise and a personal bugbear. 

      As a pilot I'm often confronted with pilots who still believe the old canard of the erroneous extrapolation of Bernoulli's Theory in which it's stated that faster airflow ( and lower pressure ) over the upper curved surface of a foil is what caused lift. This is false as the delta P is insignificant and easily disproven by anyone who has ever flown a cheap balsa glider,man aerobatic S300,or an F104 , all of which have symmetrical airfoils. Further more for an underwater foil to create " lift" under this theory , one would have to postulate differential pressure in the water flow around the foil. Water is not compressible so pressure variation are impossible. 

      So where does the lift come from? It's the deflected flow of the fluid medium that causes thrust in the opposite direction to the deflected flow. Wolfgang Langewiesche s book Stick and Rudder has the best explanation. 

      So what is the function of the foil?  A foil just modifies the drag characteristics of the wing/keel. Decide on the medium your foil will be in, the speed range you'll need, the loads the foil will experience and chose the profile that suits your needs. 

      So we can all drop the differential pressure discussion, especially for underwater foils, the ones in an incompressible medium. :). Foils just change drag characteristics. 

      Leigh Ross

      484-464-1575 (C)

      On Jul 23, 2014, at 0:46, "motorcyclejack2@... [bolger]" <bolger@yahoogroups.com> wrote:


      ---In bolger@yahoogroups.com, <c.ruzer@...> wrote :

      Take a look at the component force vectors of sail and keel, or foil, or hull. Keel/foil/hull may resist leeway ie oppose sail side/drag forces with near 100% drag. Heard of drogues, sea anchors, etc? The more immersed area the more drag... The drag near on its own may prevent, that is to say drastically slow, the boat going sideways. Now add some sail force drive vector and away she goes; and even slowly, with mostly pure drag reacting on the keel or foil and hull she may make good an acceptable course to windward. Good sails and good sail handling help!

      source info? that goes against any and everything i have ever read, heard, or seen about how sailboats work.

      first, drag is how sailboats run down wind. the sail, like your sea anchor or a parachute, has a bely...a 'container' in which to trap the fluid it is moving in. it's much like dipping a cup in a barrel to fill it. the wind (a fluid just like water) is trapped in the belly of the sail and so it can not easily escape and it builds up pressure as it collects in the belly. in a sail, this high pressure pushes the sail through the, comparatively, lower pressure on the other side of the sail. in a sea anchor or parachute, the high pressure greatly slows the anchor or chute's movement through the fluid.

      underwater foils, whether they be fin or full keels, and hulls do not have a belly shaped into them. in fact, most hulls and any foil shaped fin is just the opposite, presenting a convex surface to the fluid; not concave like your sea anchor (which is made to trap the water). this means that, while water may meet some drag resistance from  the keel being in the water, it is free to wash around the keel...even moreso a foil shaped fin or the curved side of a hull. this means that, as a creator of drag (to supposedly stop leeway), underwater foils are not very well designed. 

      drag works best when the force is perpendicular to the surface it is acting on. that's important to remember. so, let's examine a sail. when you are running down wind, the wind is pretty much perpendicular to the sail.  sailing close hauled, however, there is only enough wind behind the sail (drag) to basicaly keep it's foil shape. barely any drive is drag at that time. the boat is totally lift driven, close hauled.

      running is the slowest point of sailing. while a few points above a beam reach is the fastest point of saiing due to the combination of drag and lift, close hauled sailing is stil faster that running. this means that lift is a better driving force than drag.

      now, how does that apply to a keel or fin? well, if you sit in your saiboat, on flat water (just to simpify things), perpendicular to a 10kt wind with no sails up, the force of the wind on your hull will be trying to push the boat through the water with the force completely perpendicular to the keel. if you have ever been in such a situation, you will have noticed thatr the wind has no problem doing that at all. in fact, it will push the sailboat just as easily as it woud a row boat without a keel. 

      so, in a situation where drag is at it's greatest, leeway is at it's greatest.

      now, hoist sail and catch the wind. as the boat moves forward with greater and greater speed, leeway gets less. the faster the boat is moving the more effective the keel is at stopping leeway.

      but...wait! when the boat is moving forwards, the force of the water is no longer perpendicular to the keel. in fact, the kee is moving throught the water around 10 degrees away from being paralell to the water moving across it. in other words, drag resisting sideways motion is now very low...but leeway is also very low.

      so, let's think about this. while drag production (in a direction to resist leeway) is highest, leeway is greatest. yet, while drag production is lowest, leeway is least.

      if your theory is right, a sailboat should be rock steady while it is not sailing and have all kinds of leeway when it is. but it doesn't. that's because the keel/fin works like the sail, only underwater and it creates lift in the opposite direction of the wind. the two forces pinch the boat forwards almost like shooting a watermelon seed from between your fingers. that's lift not drag. the keel can be a lot smaller because water is a lot thicker than air....around 4% of sail area is a good approximation, depending on keel type.

      an iceberg has tons of mass under the water creating all kinds of drag, but if you stuck a big sail on one it would never sail up wind; only down wind. that's because the underwater surface of an iceberg isn't shaped to create lift. if it was, you could sail one up wind.

      i am not going to argue the point further. believe what you will. plenty of actual info out there on how sailboats work for anyone who wants to know.

      as far as chine runers, as used on paradox...they work very well. i have seen tons of videos of paradoxes sailing close hauled; even one in storm conditions chugging along close hauled where most traditional sailboats would have trouble pointing. there is no doubt they work. i doubt they work better that the micro's keel and switching to them would create a ballast placement issue.

      adding a small set of well placed and well designed chine runners to the existing set up might help but, it might not. you can only do so much in light airs. and they would create drag ( against forward motion ) to slow the boat because they would add the the wetted surface. 

      probably better just to not pinch her in light airs or to feather, if you must squeeze every point to the wind that you can.

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