I call your bluff. The naked mast drag. A myth?

Discussion in 'Sailboats' started by pbmaise, Nov 17, 2013.

  1. Zulu40
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    Zulu40 Junior Member

    Now that I re-read the source I see the argument comes from the Kamm back
    This form of aerodynamic manipulation was made popular in the 80's predominantly in European cars. This because there are ideal body lengths to particular profile thicknesses, and these are difficult to achieve for practical roadable cars. The part solution Kamm observed was to simply cut off the trailing edge of the body where the flow has gone turbulent anyway, which reduces skin friction of the now missing part of the car, and hence the drag quotient OA.

    This is ok but generally we are only looking at drag in cars expressed as Cd or CdO, because lift has the detrimental effect of lifting the car from the road surface, limiting tyre adhesion and aspects of safe control. Safe to say its application in foils is more than limited where the chord of that facsimile wing can be achieved, ie the sail.

    Upthread form there some discussion centers around the use of the teardrop profile spar.
    I think if used correctly the teardrop spar would need to be rotated, so that when changing from say Stb to Port tack the mast would be rotated to produce a fairer curve on the suction side of the sail. This because in situations of some angle of attack the suction side produces more force than is lost on the pressure side.

    To explain further, not to do this creates a pocket of air midway on the profile all the way up the mast. This pocket of air will try to stick to the desired airflow at the intersect of the mast/sail creating eddies across the sail, and that cohesive effect will reduce the power of the suction side destroying much lift. When we rotate a teardrop mast that pocket of air is eliminated, and the flow across the sail is more laminar for a greater distance across the sail so increasing lift. As turbulent drag sourced at the mast earlier is also reduced, the quotient Lift/Drag is improved on both sides of the equation. And yes, you can dump lift an increase drag in an instant simply by counter rotating the mast.

    Finally, I'd have trouble with the idea that an ideal shape has little production for the first 20% of the chord. In the illustration below the pressure product is mapped all around an ideal foil. One can see that it produces lift even in a forward direction, this because lift is generated perpendicular to the direction of the flow and the skin angle is indicated thus.

    Bring attention to the lower part of the illustration, drag on the other hand creates a countermanding vector which pulls the overall lift vector back. It proves the greater success at producing force by the suction side rather than the pressure side, and the importance of the overall Lift/Drag consideration.

    Edit: To add. If we were to eliminate the bottom skin of this foil we have perhaps an ideally formed sail, now if in your minds eye you can reproduce the pressure strength on new vectors continuing from those on the pressure side to the lower side of the sail (those lines would be then, straight), you can perhaps see how this will inevitably produce drag on a less competitive vector thus corrupting our OA Lift/Drag to some lesser amount.

    [​IMG]

    Image courtesy Aerodynamics of Model Aircraft http://www.mh-aerotools.de/airfoils/index.htm
     
    Last edited: Jan 23, 2016
  2. brian eiland
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    brian eiland Senior Member

    Drag Reduction of Bare Mast

    So is there a somewhat simple method to reduce the drag of a bare mast as found in my aftmast rig?....perhaps some sort of splitter plate arrangement incorporated into the mainsail slot of the mast (existing alum mast), or glued onto a carbon mast section,...etc??

    Looking for ideas....
     
  3. brian eiland
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    brian eiland Senior Member

    Thought I might repeat this posting. :idea::?:
     
  4. brian eiland
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    brian eiland Senior Member

    Mikko, you have firmly convinced me of the positive effects of a mast at the leading edge of a mainsail. And both you, Tom Speer, etc, have convinced me of the extra drag problems of a bare mast. So I'm looking at improving the situation of the bare mast, even while many CRUISING sailors might not be so concerned. Myself, I want to go upwind as best I can.

    One portion of your posting that I would like to have you expand upon, would be that sentence I underlined?
     
  5. brian eiland
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    brian eiland Senior Member

    I think you have hit the nail on its head.
     
  6. brian eiland
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    brian eiland Senior Member

    I like this idea as well,...sometimes just more understandable.



    Mikko, I found an interesting article by Arvel Gentry that supports your contentions about the effectiveness of the mast at the leading edge of the mainsail:

    http://www.academia.edu/1172567/Studies_of_Mast_Section_Aerodynamics
     
  7. UpOnStands
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    UpOnStands Senior Member

    Avel Gentry low drag mast?

    Arvel Gentry developed the G_LD mast for the Star series. One key design goal was lower drag.
    Anyone know of its application to 10-11m masts?
     

    Attached Files:

  8. Mikko Brummer
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    Mikko Brummer Senior Member

    The Gold Star mast was not exactly a success... partly I think because Arvel used a 2D model for the simulation (at the time 3D would have been out of computational reach), and flow of the mast sail combination is highly 3D. But partly also because I think the mechanical properties of the mast are more important than the arerodynamical.

    I've designed a mast profile myself for the Star, 470 and Finn, and those masts were sailed by top Olympic athletes - in none of the cases, could we find a measurable performance difference to the existing class standard mast. All these mast were designed and measured to have similar bending characteristics with the class standard. 3D simulations on mast-sail combinations reveal that what you gain in the mast you tend to loose on the sail. A particularly good mast (more drive from the mast) will disturb the sail more (less drive from the sail), so it tends to be a ┬▒zero game.

    In this respect, even a round mast is not nearly as bad as its reputation. One proof is the 470, which used to have a circular (Z-Spar) mast profile, that performed absolutely equal to the standard (pearshaped) Superspar or the sophisticated, truncated ellipse profile Proctor Beta.
     

  9. UpOnStands
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    UpOnStands Senior Member

    many thanks for the info.
    A check on the nets seemed to show that no one was using the G LD profile.
    Bummer.
    My plan is for an all jib bi-plane aft unstayed mast configuration with Hoyt booms so there would be nothing behind the masts. I worry about vortex eddies developing on the masts and was hoping that the G LD profile would lower drag and suppress harmonics at anchor.
     
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