Forward Swept Daggerboards?

Discussion in 'Hydrodynamics and Aerodynamics' started by Iridian, Jun 22, 2021.

  1. Iridian
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    Iridian Junior Member

    In looking at the wiki page on Forward Swept wings, I noticed that one of the primary issues with forward swept wings (yaw instability) isn't relevant for a boat.. In addition, aeroelasticity induced tip divergence shouldn't be a downside on an upside, so long as it is relatively minor.

    Forward-swept wing - Wikipedia

    The primary benefits as I see them for forward swept dagger boards would be:

    1. Inward Spanwise Flow would reduce wingtip vortices on the daggerboards.
    2. Forward swept daggerboards would pop up in any collision with a gradually ascending seafloor. While it wouldn't help in a sea wall or rock situation, any gradual ascent should not cause binding in the daggerboard trunk. (this probably isn't a huge benefit regardless, as groundings shouldn't be happening on a regular basis).
    3. While partially flying a hull, there would be reduced air dragged down the foil blade.

    Material strength and sea weed seem to be the primary downsides, though with modern composite built daggerboards, I imagine the majority of the strength is there for collisions rather than typical use. Kelp cutters and the ability to slide the daggerboards up to clear them should be sufficient to manage the sea grass.

    I'm not sure that keel stalling at the tip vs the root makes much difference for a daggerboard.

    Has anyone tested a forward swept daggerboard?

    This thread suggests that a forward sweep of 20% is optimal..
    Forward swept keels

    Some reference reading:
    Daggerboard vertical angle.
    Forward Raked Daggerboards ?
  2. hoytedow
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    hoytedow Carbon Based Life Form

    Forward swept daggerboards are great for collection trash at sea.
  3. JRD
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    JRD Senior Member

    Agree with Hoyt. For me the disruption to stop sailing to pull the board up to clear weed or trash would outweigh any hydrodynamic benefits.

    I think side loadings due to weight of ballast in a knock over or crew standing on a small boat centreboard during capsize recovery would be the primary failure mode, the rake in either direction is just adding to length.
    hoytedow likes this.
  4. jehardiman
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    jehardiman Senior Member

    Iridian; be very specific in you terms here; as sweep is not the same a rake. Sweep is the angle that the quarter cord line makes to the flow; with zero sweep being perpendicular. In forward sweep (generally considered a negative angle) the tip leads the root, and vice versa in aft sweep (generally considered a positive angle). Rake on the other hand, is the angle that the leading edge of some object makes with the body in the projected view (again aft being considered positive); masts are raked, funnels are raked, prop blades are raked. However, leading edge rake aft is only tenuously tied to sweep. Think of a typical small boat Bermuda/Marconi rig, the mast is raked slightly aft, but the sweep of the quarter cord points along the sail is forward.
    I think if you do an inspection, almost all modern keels and dagger boards have forward sweep; I know all of those I've designed do. With -4 to -7 degrees of sweep and 40%-60% taper, the leading edge will not have a forward rake.
    See this thread;
    Swept forward keel
  5. revintage
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    revintage Senior Member

    This makes me believe Iridian is referring to smaller catamarans:
    "3. While partially flying a hull, there would be reduced air dragged down the foil blade."

    On a typical modern cat the daggerboards(image is a Nacra F18) has very small taper, still with a light aft sweep when perpendicular.

    Iridian, I guess you by forward swept, mean where the upper part of the daggerboard with parallell, straight leading and trailing edges have a negative angle?

    About 3, it should be favourable from a ventilation perspective. Question is if it will noticeable at the speed you sail a beachcat?

  6. tlouth7
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    tlouth7 Senior Member

    Moths use forward sweep specifically to reduce ventilation when flying, I see no reason that wouldn't also apply to a cat. But do you keep the windward board down? If not then ventilation wouldn't be relevant.
  7. Doug Halsey
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    Doug Halsey Senior Member

    And if you do keep the windward foil down, ventilation might not be a bad thing. It tends to reduce all the forces (drag, as well as lift).
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  8. Iridian
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    Iridian Junior Member

    @jehardiman I'm specifically referring to rake, rather than sweep, though I think you could get some of the same performance benefits, specifically on spanwise flow from the sweep.

    The rake specifically should give those ventilation and kickup improvements, though @Doug Halsey makes a good point that ventilation on the windward hull daggerboard might actually be a good thing.

    Here's a couple of examples I mocked up with Forward Rake, Forward Sweep (No Rake) and both.

    Attached Files:

  9. Waterwitch
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    Waterwitch Senior Member

  10. redreuben
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    redreuben redreuben

    My understanding is forward sweep helps surface piercing foils avoid ventilation.
    For example leeboards or dagger boards between the hulls, that is foils that don’t have a hull above them acting as an end plate.
    revintage likes this.

  11. Paul Scott
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    Paul Scott Senior Member

    FWIW, Amati has a forward angled keel, and after 20+ years I love it. Good when backing up too. Has great windward bite in chop, and doesn’t seem to mess with the helm downwind. It does gather kelp, but it usually washes off, and can be fairly easily ‘helped off’ because it’s near the surface rather than down by the bulb. We hit a rock more than a decade ago, and the wise heads concluded there was less damage than a swept back or vertical would sustain. It seems smoother than ‘normal keels’

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