Swept forward keel

Discussion in 'Hydrodynamics and Aerodynamics' started by valefrisbee, Dec 12, 2011.

  1. valefrisbee
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    valefrisbee Junior Member

    Anybody knows about studies or thesis on swept forward keels?
  2. gonzo
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    gonzo Senior Member

    I've only seen them in model sailboats, the kind that actually sail, in the 70's. There were claims of better speed.
  3. valefrisbee
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    valefrisbee Junior Member

  4. Alik
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    Alik Senior Member

    Yes, this link is our 8m design Pluto800. The keel works but stiffness was the issue and keels was prone to twisting. One should use carbon for fin for such arrangement.

    We also have such keel on 6.3m boat (see picture).

    Later they modified into almost straight keels on both boats. Besides designed more than 10 years ago, both of them are very competitive racers in their area.

    Research of these keels is presented in Larsson's book, there is a graph showing optimum sweep.

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  5. michael pierzga
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    michael pierzga Senior Member

    Search Seahorse magazine for design comment on the Australian designer John Swarbrick...Z Keel...VO60...TOKIO.

    Several articles were run
  6. Earl Boebert
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    Earl Boebert Senior Member

    Er, actually the idea was (and, in the case of my boats, is) used on free-sailing boats to move the CLR aft so that the rig is at or behind the LCF. This tends to reduce the tendency of LOA-restricted classes to dive on the run. Having the CLR well aft also greatly improves tracking on the run. This is particularly important for vane-steered models, as the relative wind on the vane is near zero when the boat is going dead down wind. This reduces the ability of the vane to make minor adjustments of course and promotes "hunting" for the proper course, which means you're sailing a longer distance than the other guy. I should add that not all model yachtsmen agree with this analysis.

    FYI: Vane boats are match raced round robin style, one beat and one run against each competitor. 3 points for winning the beat, two for winning the run. The attached picture shows two vane steered 36 inch restricted class boats sailing at Central Park, New York City. This was taken during the UK/USA Challenge Cup regatta this Fall. The US team won, for the first time in six tries.



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  7. jehardiman
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    jehardiman Senior Member

    You need to watch very carefully here because there are lots of studies, each looking at some different effect of sweep and taper. It is important to keep in mind that Sweep is defined by the 1/4 cord line, not the foil shape. Almost all modern foils have forward sweep (-4 to -7 degrees) and sufficiant taper (40-60%) to avoid root stagnation, flutter, and structural issues but the leading edge will not angle forward. Any foil in which the LE angles forward is not optimumly loaded and is designed for other conditions. See Figure 134 and section 14, chapter 9 (volume III) of PNA and both Heorner and Marchaj devote considerable space to sweep and taper interactions.
  8. DCockey
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    DCockey Senior Member

    As jehardiman mentioned sweep angle in the aerodynamic/hydrodynamic community is conventionally measured at the 1/4 chord line. But I've seen some writings on boat design which use the leading edge angle which can be confusing.

    Too much forward sweep with insufficient bending and torsional stiffness can lead to divergence where the foil twists and bends a little which causes a moment which leads to a larger twist and moment which causes a ..... Not a good situation.
  9. Mild Bill
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    Mild Bill Well, not entirely mild.

    I haven't seen Larsson's book yet, but I noticed that the fin keel on the Pluto800 has about 19 degrees forward sweep. According to Fig. 11 at the very end of NACA TN-1491, that sweep angle on a wing with constant chord would result in approximately elliptical lift distribution at all angles of attack (short of the onset of stalling), which would help minimize induced drag.

    Was minimizing the induced drag a major factor in selecting the angle of forward sweep, or is it just coincidental that other considerations dictated a similar angle?
  10. Alik
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    Alik Senior Member

    Another factor was arrangement and comfortable entrance to cabin - the keel trunk is in cockpit while the CG and CLR are in proper positions.

  11. Tanton
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    Tanton Senior Member

    Forward sweep.

    Placing the bulb in location. Moving the ce of keel aft, for balance between the hull and the rig. Making the keel removable from the cockpit.
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