Vacanti keel question - nose diving

Discussion in 'Boat Design' started by Olsonist, Mar 23, 2011.

  1. Olsonist
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    Olsonist Junior Member

    Planform is fixed.

    You can run this question forward as in what is the best foil for this particular application, regime, mode, ...? That's the design question.

    Or you can run the question backwards as in given these available foil choices what are the features, tradeoffs and liabilities of each? For lack of a better term, that's the shopping question.

    I guess I'm trying to identify what feature might be causing this submarining so as to avoid that choice.
     
  2. jehardiman
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    jehardiman Senior Member

    As Alik and MikeJohns alluded to, it is a function of span, aspect ratio, and tip/bulb design and possibly section than any one feature. When running downwind, there is a moment couple between the driving force and the drag (which it is important to realize are identically equal in magnitude and opposite in direction) which forces the bow down against the longitudinal trimming moment about the center of flotation. This is not new or just a function of a fin keel, square rigged ships often reefed royals/topsails or scandalized the fore/mainmast to prevent driving the head under when sailing downwind.

    Normally for a fin keeler, the center of drag is relatively close to the bottom of the hull. If you play with the "foil", not necessairly the section, that causes an increase in seperation between driving force and drag, then the bow down moment increases. Bad thing is, most canoe hull fin keel racing boats like an Express rapidly lose trimming moment and wetted surface as they trim bow down due to water plane changes so the problem snowballs on you. This is why you pull up the centerboard and move everyone aft on most racing boats. Additionally, the vessel will roll and has the wave orbital to contend with, both which will greatly alter the AoA of the keel, adds drag low, and renders the slection of a drag-bucket section more or less moot.
     
  3. Alik
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    Alik Senior Member

    The question of bow diving is not simple. It involves hydrostatics issues (longitudinal stability), but given running trim, and also effect of boat's wave system on longitudinal stability. I am not aware of any systematic research of effect of Fn on longitudinal stability (but there is reasearch of Fn effect on transverse stability done by Necahev).
     
  4. Olsonist
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    Olsonist Junior Member

    The point from the Vacanti article is that change of keel section alone corrected that problem so he's saying it was important enough of a variable by itself to get right. However, Vacanti doesn't give enough information to replicate or analyze the problem. What was the original section? What was it replaced by?

    I take it that if bow diving downwind is complex; so optimizing a keel section for upwind L/D would make sense; downwind the section choice might be less sensitive (up to a point). The other tradeoff would be stalling after tacks while accelerating. Garrett's Symmetry of Sailing gives a nice table of these sorts of tradeoffs on p. 146.

    I'm leaning strongly towards something like 63A012. In particular the A means the tail section is straight. I can't imagine how'd I'd fair a keel with a concavity. CNC yes, but fair, no.

    Some similar classes spec their keel sections. The Santana 35 uses the 63A012 section. Soverei 33 uses the 0012. The J120 specs the offsets but doesn't name the section. I'm still tracking down Carl Schumacher's spec.
     
  5. Olsonist
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    Olsonist Junior Member

    I should add that Vacanti doesn't say anything about the planform either. My fin keel might be more or less susceptible than a high aspect bulb keel.
     
  6. Alik
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    Alik Senior Member

    This is what we call marketing.
     
  7. Tim B
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    Tim B Senior Member

    I agree with Alik.

    There's all sorts of factors that may contribute, but I find it very suspicious that a change to the foil section (perhaps reducing Cd by 5%) would make that sort of difference. Although the keel will have some bow-down pitch moment associated, the rig is going to be a much larger contributor. You can also play with the CG location to alleviate this, of course, though it is less effective except in very small boats.

    Tim B.
     
  8. Gilbert
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    Gilbert Senior Member

    Why not ask Dave Vacanti what he meant or to expound on his rather offhand comment?
    I am confident he is still alive if he is still in business.
    I was a casual acquaintance of his about the time he started his software business and he was always friendly and personable.
    And it goes without saying that no contributor to this forum would engage in self promotion or 'marketing'.
     
  9. sorenfdk
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    sorenfdk Yacht Designer

    You can't go wrong with the 63A012. If there is such a thing as a classic keel section, this surely must be it!
    But remember that if a laminar section like this is not kept clean, a 0012 section might be a better choice!
     
  10. Tim B
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    Tim B Senior Member

    I'm going to start taking offense at people who think that a laminar section is a good bet for a keel. Work out the operating Cl at several points of sail, and look at the resultant Cd curve. Now apply a percentage course correction to that data, and see if a laminar section is really beneficial.

    You will see high Cls upwind, as the keel has to produce a lot of sideforce. Downwind the Cl will drop to near 0.

    The NACA laminar sections were designed for relatively specific, constant-loading or high speed applications. Yachts do not operate at low Cl on the keel, or at constant loading, so please do a little more research.

    Tim B.
     
  11. Paul B

    Paul B Previous Member

    That paper does some comparisons. However, it does not include the most successful section for a boat of your type in the comparison. It also does not include the best section for "low aspect" fin keels.

    The sections I am referring to are not 63A012.

    The article also shows a bulb keel where the bulb is not a very good shape and also an "elliptical" rudder planform that no decent design office would have ever used.


    I would really like to hear from Mr. Vacanti about this change of section that prevented nose diving when sailing offwind.
     
  12. sorenfdk
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    sorenfdk Yacht Designer

    Then I'm afraid you'll have to take offense at a lot of designers - some of them even successful ones with huge research budgets behind them!:)
     
  13. sorenfdk
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    sorenfdk Yacht Designer

    Well, that's a great help!

    Me too!
     
  14. Tim B
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    Tim B Senior Member

    I did say "a good bet". If you can justify using a laminar section, then that's great. These days, the analysis isn't that hard, so there should be no excuses for arbitrarily picking a section!

    Tim B.
     

  15. DCockey
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    DCockey Senior Member

    I recently looked at and felt the keels of several daysailers which promise traditional looks with good performance at not insignificant prices. It was interesting to see and feel the leading edges of the keels. There were lumps and bumps and in one case a relatively sharp leading edge. Also most had rolled-on bottom paint which wasn't quite a standard "rough" surface also wouldn't do anything to promote laminar flow.
     
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