Why no aerodynamic control/stabilizer surfaces on high speed cats?

Discussion in 'Boat Design' started by RatliffFranklin, Jan 24, 2007.

  1. NADreamcatcher
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    NADreamcatcher Junior Member

    hull slippage

    To all,

    I stand corrected on the issue of slippage in a turn. Even when a hull hooks in a turn there is a certain amount of slippage...Even when it pitches it's occupants through the side of the hull!

    NAD
     
  2. charmc
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    charmc Senior Member

    I'm not sure I understand the question but, if I do, the answer is an elaboration on my previous post.

    Water is 784 times more dense than air. For a powerboat, aerodynamics is largely ignored, as few commercial or pleasure craft operate at speeds where it has any impact. One exception is docking manuevers, in which wind forces can play a big role. That happens because in docking the engine(s) are at idle speed, generating just a few hp, and rudder force is very low because speed is just a few mph. Even a moderate speed wind acting on the boat will affect manuevering, since it is interacting with very small hydrodynamic forces.

    At racing speeds, the force of the apparent wind is much greater, since it is a function of boat speeds around 150 mph or higher. The propulsive and steering forces are of course much greater, and are still acting against water which is 784 times more dense than air, but 95-99% of the boat is completely out of the water, flying in the air. The predominant forces are still hydrodynamic, but, as in docking, aerodynamic forces are significant when compared to hydrodynamic forces, and have an effect on manuevering the boat at those high speeds.

    That's a wordy and somewhat sloppy explanation, but I think the point is clear that, whenever aerodynamic forces exceed some fraction of the hydrodynamic forces acting on a boat, they will affect manuevering.
     
  3. NADreamcatcher
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    NADreamcatcher Junior Member

    Formula's?


    Charlie,

    Beautiful response for a layman's understanding. If you or anyone would care to dig into this a little further, let me ask the same question from another perspective, but first let me add my two cents on what an aerodynamic surface is and is not...at least what I think anyway. Please beer with my thought process on this one.

    Any object moved through the median of air is an aerodynamic surface.
    Any object moved through water is a hydrodynamic surface.
    Any object moved across the surface of water is a surfacing hydrodynamic surface.
    Any object moving through all three at the same velocity is a boat.

    Please correct any false assumptions I may have there.

    So comparing apples and oranges they are both fruit, but they are different.

    Necesity is the mother of most invention?

    Ok we have cause for invention here...we have offshore cat's breaking 150mph...blowing over...nose diving...and barrel rolling. We need to make them go faster without as much risk, hence the proposed aerodynamic controls...ok that works for me.

    Since any object moved through the air is an aerodynamic surface, then a 150mph cat is an aerodynamic surface. Apples and Oranges right? The same but different. Ok I'm way off from where I was going with this one.

    Back to relative density. If water is 800x denser than air? How much denser is it @ 400mph? Does the density change or just the resistance felt? Is there a formula for computation if different?

    Seriously Iam trying to learn and if anyone would care to dig in here please do so.

    NAD
     
  4. Dan Ellison
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    Dan Ellison Junior Member

    Offshore boats have ran 200 plus but they do not race at those speeds, unlimited hydro's do. You would never see a offshore cat approach a turn at200 and throw it sideways. Offshore cat design has not changed that much in yrs. most of the gains in speed are do to light weight materials, drive and propeller technology. As a hull builder I look at all types of race boats. Even sailboat technology can be of use.
     
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  5. Dan Ellison
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    Dan Ellison Junior Member

    Its 800 times denser as a solid, if your going through it not on top of it.
     
  6. RatliffFranklin

    RatliffFranklin Previous Member

    Dan - Have you been following the restoration of Bluebird to running condition?

    It can be found at http://www.bluebirdproject.com/

    Click on the Diary button.
     
  7. charmc
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    charmc Senior Member

    Density doesn't change, but resistance force (drag) does.

    Here's one article explaining the effects of drag: http://en.wikipedia.org/wiki/Drag_(physics)
     
  8. NADreamcatcher
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    NADreamcatcher Junior Member

    lost in the design stage!

    Charlie,

    Thanks for the link...I got lost into aero stability formula's and theories. All that pitch, roll, yaw stuff that seem's so relevant to high speed on the water.

    I understand the resistance (drag coefficient/Reynolds number) increasing with speed. I know there are program's to support certain hydro dynamic (hull design) scenario's, whether they are empirical, physical law or a combination, planning, non-planning etc.

    What I think Iam after is a way to calculate the force of impact of an object against the waters surface giving a way to understand those forces. I would think it would be the same way of calculating say the lift of a supercavitating surface at a given angle and speed. Or is it all a measure of resistance/drag?

    Much like say wind chill is calculated...

    Sorry Franklin I realize I am off your topic here a little, but it all seems so relevant somehow. At least to me anyway.


    NAD
     
  9. c-cat
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    c-cat Junior Member

    In most offshore racing organizations aero-controls are outlawed. It would be comforting though to have gyro-controlled canards inbetween the sponsons, with perhaps some tune-ability, so one would be able to fly a kitty on the wiskers.
     
  10. NADreamcatcher
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    NADreamcatcher Junior Member

    flyin a kitty!

    In 1980 something I designed and fabricated a canard wing for the front of my OPC tunnel using a homemade hot wire, foam and composites. Mounted it between the pickels and used foot operated controls...it worked well but unfortunately APBA said sorry that's not legal it can only be a fixed wing. For a time you would see some hulls with a wing of some sort up front (fixed)...hulls got better and they disappeared again. There is no question they would be useful! There has been debate on the ability of some form of a Gyro being capable of sensing hull pitch accurately and quickly enough to be the key sensor in canard contol. It has to be able to sense pitch and or roll only and never be affected by shock loads. I would think hull speed, trapped air pressure, pitch, trim, CG, and possibly others would need to be taken into consideration for an accurate auto control of a canard. At 200, 300, 400 mph across the waters surface it would seem more advantagious for sensors to know what the air is doing in front of the craft in time for proper adjustment to be made. Things like water surface conditions, air movement and so on. In many ways they are compensated for now by the driver's instincts. I for one do not believe that an automatic control is the answer, but it would sure take some of the work out of the task of "Flying on the Whiskers" as you said!

    NAD
     
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  11. afrhydro
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    afrhydro Senior Member

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  12. NADreamcatcher
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    NADreamcatcher Junior Member

    Vertical tail

    Jim Noone on Coniston Flying the way it's supposed to be.

    Looks like he believes in a vertical tail! But I don't think there are counter rotating props under there.


    NAD
     

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  13. afrhydro
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    afrhydro Senior Member

    i had a problem with my stabilizers
    they would steer the boat in a cross wind real bad
    i had to cut them off
     

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  14. NADreamcatcher
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    NADreamcatcher Junior Member

    Yea don't think a vertical surface is to good around 70ish...probably more beneficial into the triple digits.

    That's a nice lookin boat! What kinda speeds she run?


    NAD
     

  15. afrhydro
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    afrhydro Senior Member

    still working that out
    i am upping the motor to a v-6 now
    i got to confirm it again i made some changes here and there
    67 so far with the 115
    i put a clever on and i loose speed cause I'm still to deep on the motor no lw pick yet
    the v-6 should get me close to triple digits or just over
    im all stock i have no money for fancy go fast parts
    i just build the boats its my hobby and at the same time its also my carrier

    cake and eat it to so to speak
    i speak with the miss madison team and a few others from time to time i had them right at 80 degrees same as theres
     
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