atriculated shoal draft keels

Discussion in 'Boat Design' started by sandy daugherty, Jul 18, 2011.

  1. sandy daugherty
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    sandy daugherty Senior Member

    A designer of note is proposing flaps on a luxury catamaran's shoal draft keels.
    I would like to hear of any past attempts, summaries of failures, recommendations/comments about leading edge and trailing edge devices, and some WAGs about loads. And what foil section should be used. I notice that most commercially produced LAR keels are pretty fat (deep chords) and wonder if this isn't just a way to simplify production in a boat whose performance has taken a back seat to cost and accomodations.

    And if that isn't enough, I wonder whether end plates could work. Has anyone read any tank tests?
     
    Last edited: Jul 18, 2011
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  2. Tad
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    Tad Boat Designer

    I'll guess you are talking about trim tabs? It always helps to add a link to what you are asking about..........

    Intrepid sported a trim tab on her keel in 1967, after that I think all the 12's had them and they were popular on IOR boats through the 70's and 80's, until keels became very short in the cord and much deeper......
     
  3. sandy daugherty
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    sandy daugherty Senior Member

    Please note the question refers to a pair of shoal draft keels with very low aspect ratios. I am thinking of modifying the keels on my PDQ 36 with moveable leading and trailing edge surfaces. The construction would be similar to conventional rudders. Is there a foil section that is suited to this idea? Is there a way to estimate the loads on controls?
     
  4. Tad
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    Tad Boat Designer

    See The Design of Sailing Yachts by Pierre Gutelle, he has quite a bit on trim tabs and articulating rudders......

    The control loads will be the same as any rudder, speed^ * area * lift coefficient * center of pressure to control axis length.......
     
  5. sandy daugherty
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    sandy daugherty Senior Member

    Thanks for that suggestion, Tad. I just received Gutelle's book and have begun studying it. I think it will answer most of my questions, and has already confirmed some of my guesses. There is less drag from a thinner keel*, and assymetrical keels induce yaw in the wrong direction, going from a weather helm to a lea helm, charging a drag penalty from the rudder.

    *So the fact that all production shoal draft cats have thick sections suggest that the builder chose to go with easier construction and/or to make them beefy enough to support the boat on the dry, rather than to up the windward performance.
     
    Last edited: Aug 1, 2011
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    I doubt if trim tabs or flaps would do much on a low aspect ratio (LAR) keel other than creating some additional turbulence. I assume you are trying to create adjustable lift to control leeway. With an efficient keel it may be worth reducing leeway to reduce hull drag, but a LAR is not much more efficient than the hull as a leeway control device so the effect if any will be hard to detect.

    End plates reduce the turbulence thrown off the end of a fin keel in much the same way as wing tips are used on modern aircraft, reducing the energy needed to create the turbulence and therefore reducing drag, but again the effect is far more significant on a high aspect ratio foil. A plate on a LAR keel will be so long that the skin drag will counter any improvement IMHO.
     
  7. sandy daugherty
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    sandy daugherty Senior Member

    But they are all a LAR keeled cat has, Terry. Given they are the choice of the absolute majority of builders and buyers, any inprovement would be a blessing, and possibly marketable.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    You must have noticed the resemblance between a deep fin keel and the wing of a plane. The tiplets - whatever they are called - are only seen on planes with high aspect ratio wings. That's because they do not work well on lower aspect ratios where other measures like elliptical wings are more effective. The hydrodynamics of keels is similar to the airodynamics of wings apart from different densities and Reynolds numbers.

    I feel your pain, but as far as I know there is nothing to be done, at least with end plates. I am not particularly familiar with CAT design but I would be willing to bet every reasonably recent performance CAT design has retractable, high aspect daggerboards, and of course rounded hulls with minimum wetted surface area.

    The convenience and safety of shallow draft and the likelyhood that CAT owners might forget to raise the DBs probably explains the preference for LAR keels, plus, I doubt not, reduced cost and simplicity. Having one DB catch a rock would, I imagine, be more dramatic on a CAT than a monohull, especially in a marina.

    Without a doubt any improvement would be marketable, so I encourage you to research further and persist, especially as my knowledge is partial at best; I know more about aircraft wings than keels. One option might be to add a daggerboard, or perhaps a leeboard could be tried as an experimental measure to evaluate any improvement.
     
  9. sandy daugherty
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    sandy daugherty Senior Member

    Re-read your response and see if it doesn't sound a bit condescending.
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    I did and I didn't, even after another re-read, but If I have offended you I apologise. I didn't appreaciate your level of skill; I checked your profile as I usually do before replying to someone I don't know but there is no hint of formal boat design training.
     
  11. sandy daugherty
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    sandy daugherty Senior Member

    Yes, I am offended. You have expressed your disapproval of the suggestion; I hope you won't need to repeat it.
     
  12. timothy22
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    timothy22 Junior Member

    This idea is being proposed by Chris White

    for his new Atlantic 47 cat. If the A47 fin is even remotely similar to the one on the A48, this will be an extremely low aspect ratio fin. The movable section wil have to be something ike 25% of the total fin area, I suspect. From his website, I gather he thinks the A48 did not need much help from the daggerboard, so it saves weight and complexity to replace it with a big trim tab. The trim tab idea was used by S&S on a few CCA rule racers, and my experience is limited to one sail as a kid and is is my recollection that very little deflection was necessary to acheive the desired effect.
    http://www.chriswhitedesigns.com/atlantic_cats/a47/
     
  13. DCockey
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    DCockey Senior Member

    What benefits are you seeking?
     
  14. philSweet
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    philSweet Senior Member

    D-

    Just guessing, but perhaps the increase in induced drag from the tabbed keel can be offset by the reduction in induced drag from the hull due to reduced leeway. Since the fin has to be dragged around 100 percent of the time whether its needed or not, it tends to be undersized for close hauled work. It may be most useful at lower speeds and should probably be designed for low Fr.

    Another approach might be to install a small waterjet on the inboard side of each hull to blow on the keel. This would simultaneously add thrust and improve keel lift. Very low power requirement to use in "add a knot" mode.

    Do we have a plot of residual hull resistance (as a function of speed and leeway) for something like the boat in question, I only have a couple plots for full keel monohulls.
     
    Last edited: Aug 6, 2011

  15. DCockey
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    DCockey Senior Member

    I assume by induced drag you are including any increase in drag due to leeway, even if not associated directly with lift production.

    Effectively cambering the keel with movable leading edge and trailing edge devices can decrease or eliminate the leeway angle. Whether drag is reduced would depend on:
    - How much drag is generated due to leeway by the hull and the interaction of the hull
    and keel.
    - The change in lift distribution and it's corresponding effect on how much lift the rudder would need to produced to balance the boat. This could be positive or negative.
    - Added drag due to the movable geometry. Practical considerations will cause the shape with the movable surfaces deployed to be non-optimal. Also there would likely be gaps and/or pumps associated with them.

    My guess is any increase in speed due to such a system would likely be only measurable in direct comparison with another similar boat without such a system, ie a racing type situation. The benefits are likely to primarially phsychological.
     
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