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Differential of propellers for air and water

  1. Jan 16, 2009 #1
    Is there a way to determine the diameter (or other dimension) differential between a propeller traveling an object through water as opposed to one traveling through air, the hp of the driving motor being equal in both cases?
     
  2. jcsd
  3. Jan 22, 2009 #2

    Danger

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    Welcome to PF, Jordy.
    I'm sure that such a formula exists, but I have no idea what it is. Note, however, that all marine propellors are short and wide-bladed. Most aircraft ones are longer and narrower. Heavy lifters, such as the C130, tend to have 'paddle-blade' props for lower speed usage. Those are still not of a marine profile. There is a reason for that.
     
  4. Jan 22, 2009 #3
    This doesn't make any sense. Propellers that are used for liquids and for gases have very different designs other than diameter. A 1m diameter marine type propeller might not even have measurable performance working in a gas.
     
  5. Jan 22, 2009 #4

    Ranger Mike

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    from a practica l stand point..marine probs gotta be stout and stubby..lot mre to run into in the sea/river than in the air..submerged logs and my fav the manatee...
    you can't have a lot of prop sticking out in the water w/o hazard

    also a low reving boat engine..diesel in big fleet ships revs a lot lower speeds ..like under 800 rpms...vs P51 mustang with turbocharge rolls royce merlin..1450 hp @ 3000 rpm
     
  6. Jan 24, 2009 #5
    I have noticed that "Boomer" sub propellers have designed somehwere between those of aircraft propellers and boat props. There is obviously some reason and advantage for this.
    What I am looking for is the most advantagious shape and size (in my case small diameter is better) to use as a low rpm driving prop in water.
     
  7. Jan 25, 2009 #6

    Ranger Mike

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    sub props designed for QUIET...sewer pipe sailors get real itchy ifin they got a noisy ride..for obvious reasons...
     
  8. Jan 25, 2009 #7

    Danger

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    Quite right. And those boomer props are still pretty huge when compared to an aircraft one. They also have a 'spiral' profile which you will never find on an aeroplane.
     
  9. Jan 25, 2009 #8

    Ranger Mike

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    Danger..do they not also reduce thermal signature of wake...??
    was not satellite wake recognition a trend the DOD was researching a few years back?
     
  10. Jan 25, 2009 #9

    Danger

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    No clue about that, Mike, but it sort of makes sense. I never thought of a thermal signature from the prop, since they turn pretty slowly. I always thought of the design as being a compromise between speed and anti-cavitation (efficiency/noise factor).
     
  11. Jan 25, 2009 #10

    Ranger Mike

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    thanks Danger...maybe i'm wrong on the thermal..but remembered briefing on satellite spy in the sky and some kind of signiture connected with prop wake....thermal would be too faint with billions of gallons of H2O around it..maybe cavitation wake...but wonder what other properties are generated by prop wake??? or maybe there is thermal signature...good food for thought as it is 15 Degree F outside...
     
  12. Jan 25, 2009 #11

    Danger

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    Where the hell do you live, man?! I'll grab my swimsuit and come join you.
     
  13. Jan 25, 2009 #12

    Ranger Mike

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    A submarine disappears from sight when it dives, but it can still leave a trail of sound, heat, sonar reflections and magnetic anomalies.

    A vessel travelling on the surface or under water gives rise to detectable local disturbancesin the Earth’s magnetic field.

    Wake effects - a hydrodynamic phenomenon When a submarine travels on the surface or at periscope depth, variations in the pressure field, vortices and the wake are detectable. When submerged just below the surface, the submarine can generate detectable surface waves and water velocity changes. The temperature of the surface water will be altered by the wake, and this can also be detected by IR sensors or radar.

    work was done on wave pattern signature and pressure distribution generated by the prop but stealth is main design critiria.. Ref the WAKE -subs have a plate system that directed the wake generated back between other reflector plates substantially attenuating the peak amplitude, integrity and outward momentum of the remaining wake.

    ELF signature Galvanic currents flowing in the hull and in the water around the hull generate underwater electrical potentials. Under certain conditions, this can cause extremely low frequency (ELF) electrical fields to be radiated into the water. Detection of the ELF signature can be prevented by short circuiting the electrical current by earthing.

    The Bar is Open and the smoking lamp is lit!...Daytona on in a few weeks..my Formula Car engine getting a rebuild and life is good in West OH-10..that is O H - Ten for the U of M types..
     
  14. Jan 25, 2009 #13

    Danger

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    That sounds great... except for the little thing about me not being allowed across the border. Damned criminal record. :grumpy:
    I actually had to wear a jacket over my T-shirt today when I went for a beer and smokes shopping trip. It was only -21C, but I'm getting pretty old and feel it more these days.
    I would wish you good luck for your race, but I don't believe in luck. Instead, I'll tell you what I tell people on my pool team... do the best that you can under the circumstances, and have fun doing it. That tends to beat out the guys who are there only to win.
     
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