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Thread: 'Face cup'

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    Team Member Smokin' Joe's Avatar
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    Default 'Face cup'

    'Face cup' is is a terrible phrase because misleading. There's camber and there's cup. Cup is
    a large camber change over a very short chord length at the trailing edge. Slowly varying camber at
    the trailing edge will not have the effect of cup. Why? You have to understand something
    about (hydro- or air-) foil theory. A prop blade is a hydrofoil. An airplane wing is an
    airfoil. Cupping the trailing edge is like the effect of dropping the aileron on a wing, the lift
    on the wing is increased. You can't get the same increase in lift by smoothly cambering the
    trailing edge of the wing. The leading edge of an airfoil is completely different than the trailing
    edge. At the leading edge, the angle of attack produces the main component of lift.
    Camber in the foil section also produces lift. With a surface-piercing
    prop, the leading edge must be very thin to reduce drag because each blade enters the water every revolution.
    Now, an eddy is created any time there's flow past a sharp leading edge. Just move a paddle in the
    water and you'll see what I mean. An eddy shed by an edge is nothing but drag, and that's
    what we want to reduce. You can only reduce that by reducing the angle of attack. The
    paddle's a rather flat hydrofoil, you can feel the side force (the 'lift') as you move it at nonzero angle of attack.
    You can get rid of both the eddy and the side force by moving the paddle blade parallel to the direction
    of travel (zero angle of attack). If you put camber in the blade of a paddle or prop then you'll recover
    the lift. So 'face cup' is not cup, it's gently-varying camber. If you camber the leading edge
    right (with a pitch gauge, and being able to calculate the camber you need) then you can reduce the
    leading edge drag and make up for the lost pitch with camber. There's a myth that anything that increases top
    speed reduces acceleration, and vv. Correctly cambered, a prop will show an increase in both top speed and
    acceleration. That's what I've known how to do since 1978. Tracy Hawkins and Mike Schubert
    are running props made like that in F1Sport. Roddy Foreman ran one at Kankakee 2011 in SST 60 (until he broke a piston skirt). Hans is running one
    in SST45 now as well. All the factors mentioned above affect the location of 'center of pressure', the average position
    on the blade where the side force ('lift') is focused. A center of pressure too near a very thin leading edge
    poses an obvious danger, but center of pressure location affects performance as well. PTR, Progress through research,
    is Alex Hledin's motto at PTR. It's a damned good motto, we practice ptr too. And it does make a difference to understand physics!

    'Dr. Joe'
    mccauleyandson.com

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    BRF Team Europe Member Per's Avatar
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    Thank you very much! What a great article. Not many people want to share their trade secrets that way. Hats off...

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    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by Per View Post
    Thank you very much! What a great article. Not many people want to share their trade secrets that way. Hats off...
    I know only one (by accident Swedish) marine engineer who'd know what to
    do, more or less. Most prop shops, even with pitch gauge will still not know what
    to do. I.e., I'm pretty sure my speed secret remains largely in my head since I didn't say
    how to calculate! The marine engineer told me that the surface drive people figured it out independently.

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    Sam Cullis Mark75H's Avatar
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    Excellent
    Since 1925, about 150 different racing outboards have been made.


  5. #5
    BRF Team Europe Member Per's Avatar
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    Quote Originally Posted by Smokin' Joe View Post
    I know only one (by accident Swedish) marine engineer who'd know what to
    do, more or less. Most prop shops, even with pitch gauge will still not know what
    to do. I.e., I'm pretty sure my speed secret remains largely in my head since I didn't say
    how to calculate! The marine engineer told me that the surface drive people figured it out independently.
    Is his surname perhaps Bäckmo?
    I have followed your discussion on "angle of attack" on a different forum...
    I'm sure not many can produce a good race prop regardless of what tools and knowledge they have in possesion.

  6. #6
    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by Per View Post
    Is his surname perhaps Bäckmo?
    I have followed your discussion on "angle of attack" on a different forum...
    I'm sure not many can produce a good race prop regardless of what tools and knowledge they have in possesion.
    Bodo himself, as you know from the other forum.

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    On your website, it says that diameter is established through experiment. Theory for wind turbines says that power is proportional to rotor area ,ie. square of the diameter. Is largest workable diameter desirable?

    When I ran ASH in the 70's I was always trying to run the bigest diameter wheel I could. At the time we had the propshaft about an inch above the bottom and were limited by the ability to get on plane.

    Thanks:
    John

  8. #8
    Team Member Smokin' Joe's Avatar
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    Default surface piercing props

    Quote Originally Posted by rumleyfips View Post
    On your website, it says that diameter is established through experiment. Theory for wind turbines says that power is proportional to rotor area ,ie. square of the diameter. Is largest workable diameter desirable?

    When I ran ASH in the 70's I was always trying to run the bigest diameter wheel I could. At the time we had the propshaft about an inch above the bottom and were limited by the ability to get on plane.

    Thanks:
    John
    John,

    There's no book or paper that tells you how to calculate, at least none based
    on sound theory. You can take a look at Gerr's book. 'Large area' comes
    from the simplest approximation and we don't know where it breaks down. It
    assumes 1-phase flow. Large area is not simply dia.^2, there is asymmetry.
    Large dia. and a small chord makes the flow more 2D, large chord and smaller
    dia. makes the flow more 3D. You want the flow to be more like 2D to reduce the
    drag caused by the tip vortex. In EP 12x23-12-25 was the working solution, in
    SE/SST60 12.5x23-25 is the solution. No one can calculate that difference. 13"
    dia will not work on short courses in SSTY60, 11.5-12" dia also will not work.

    My success (SST45, SST60, EP, F1Sport) is largely in drag reduction. It's a trick to
    build a prop that will set a kilo record and also out accelerate all the others
    off the beach. Drag reduction.

    Joe

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    Great info on props!!!!

    Soooo....how do you measure a wheel with a pitch gage or computer..ie..prop scan?

  10. #10
    Team Member Smokin' Joe's Avatar
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    Rundquist pitch gauges, rh and lh. I once had a scan made on an SST60 prop at
    Baumann prop to compare with my Rundquist measurements.

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