Sail shape with rotating mast

Discussion in 'Multihulls' started by waynemarlow, Jun 15, 2012.

  1. waynemarlow
    Joined: Nov 2006
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    waynemarlow Senior Member

    To date despite extensive research on the "tintanet" I have yet come across a researched foil shape for rotating masts and soft sails combo at the sort or wind speeds a beach cat encounters.

    With the wing masts we have ( A Class ) we can use rotation to get camber depth and foil shape for a wide spread of wind speed and using both seam shape and luff shape, have a reasonable stab at making a reasonable foil shape.

    But what is that shape, I have looked at a number of different sail manufacturers and they range from very flat almost board flat sails relying on the wing mast to put in camber ( but they end up having a terrible exit from the mast although to the air path that may not matter ) to quite deeply cambered relying on the mast to be almost central.

    Any comments from experiance anyone or links to good foil shapes ?
     
  2. Gary Baigent
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    Gary Baigent Senior Member

    Well, I rotate mine probably more than some; that is to have a sweet curve from wing mast leeward shape to the luff and flat curve of the mainsail. Imo, many wing mast to mainsails are set with a scallop between the above connections, not rotated enough ... and that I believe is losing power and pointing ability. It is a sort of carryover from conventional fixed masts where there is this draggy, turbulent dead area between mast and sail ... and that is what people are used to. The easiest way to find the correct angle is to do a test varying the positions mast rotation positions. When it is right, boat will feel unhindered and be sailing fast, the acceleration is quite obvious. Do this on varying points of sail and mark the spanner sheeting positions.
    I remember when Blake and Quilter stepped from their champion Steinlager maxi monohull to early ORMA trimaran Steinlager 2 (I went for a sail with them, took photographs) - but that large chord wing mast only rotated 45 degrees either side and again, and although the boat was very fast, even with a too baggy main (imo), there was more potential to be gained from a correctly setup mast/main.
    Vodafone is a rocket ship but to me, the same thing applies; wing mast is not rotated enough; the owner and crew will probably say BS - again just imo.
     

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  3. HydroNick
    Joined: Apr 2010
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    HydroNick Nick S

    That's an interesting comment Gary, it looked to me like the AC 45s in SF in October were doing exactly what you suggest: From the windward side the masts looked to be over-rotated to windward compared with their wings (sail) producing a noticeable windward side kink between the mast and the wing. Again, to my eye, from the leeward side it looked like the competitors were aiming to produce as fair a curve as possible.
     
  4. tspeer
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    tspeer Senior Member

    The AC45 wing works well with roughly 20 - 30 deg of flap deflection. There's a definite kink, but it doesn't have to do with achieving a smooth contour on the lee side. The flap leading edge is well to weather of the lee side.

    The attached figure shows the flowfield around the AC45 top section with the flap deflected 25 deg, at an angle of attack of -5 deg relative to the main element chord. The grid is fitted to the streamlines, so it illustrates the flow around the wing and through the slot. The gap between the grid and the section itself is the boundary layer displacement thickness.

    With regard to trimming rotating masts, I used to think that trimming for a smooth lee contour was the best way, but now I'm more inclined to trim to put the stagnation point near the leading edge. This can result in a kink in the lee side. The tradeoff is a large separation bubble on the windward side and a pressure spike on the windward leading edge, vs a small separation bubble on each side and no pressure spike. The latter generally has less drag.
     

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  5. HydroNick
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    HydroNick Nick S

    A bit belated; but, thanks Tom
     
  6. hump101
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    hump101 Senior Member

    On the F40, mast rotation is a really important way to depower the rig (that is, removing mast rotation depowers significantly), so conversely it is also critical to apply it for max power.
     
  7. nzclipper
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    nzclipper Junior Member

    Steinlager 1 was the tri 2 was the maxi?
     

  8. Gary Baigent
    Joined: Jul 2005
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    Gary Baigent Senior Member

    Correct - and I was wrong. S2 was/is the maxi. Thanks for pointing it out.
     
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