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Propeller operation...this one's for you Dave.

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  • Propeller operation...this one's for you Dave.

    Dave,

    I more than understand EVERY situation is different, and other than the old rule of thumb regarding screw vs motor size as when to add reduction gear or stay direct drive, there is no definitive answer without complex equations for fluid dynamics. Well beyond the average knuckle dragger such as myself.

    So I'm asking a few questions here...chop away!!

    On my DeBoer 688, I was going to run her with that same 12V 10A Redmond RV Blower motor I used in the 1:48 DeBoer SKIPJACK. Runs about 8k RPM no load at WOT. The SHARK has direct drive swinging a 3-3/4 OD 7 bladed screw purchased many years ago from good old Al Matava (I miss Al), I think made by the UK's "Prop Shop", I have NO idea what the pitch is and I probably don't want to know lol!!

    The screw on SHARK is close to the diameter of the motor, hence direct drive works good enough. If you remember, I had posted previous numbers of static in water testing, and I know that I can simply run all day at 50% throttle drawing about an amp. Above that the curve shows amp draw increases exponentially. If I do go with this motor, I do plan on a 4:1 Gear reduction to swing that "big ass" prop.

    I did do a current test on your 1:48 scale 688 Prop years ago. I only remember she jumped to 7A when started and settled to ~5A WOT as I held the motor compartment under.

    So since YOU designed that prop, and know people who/have run her, any idea what would a good RPM range be? I'm sure something more than 10 RPM but less than 10K, in the water. I'm kind of hoping what you think the RPM should run at, in the wet, can help affirm my choice or aid in choosing an alternate motor, say brushless where one can determine. no load RPM at WOT for a given voltage and also spec the Power rating.

    Is there a way to correlate (rule of thumb again) between no load RPM and and in water on a 7 bladed prop, will its RPM drop 10, 25, 50%??

    Say I have a brushless motor, 630KV at 12.8 volts WOT, Max rated at 528W, (which I do have laying around lol). So WOT, no load, derated 20% for losses gives me ~6400 RPM, add 4:1 reduction brings it to ~1600 RPM BEFORE I hit the water. NO idea how much it drops at that point.

    But if I can assume 30%, WOT in the water I'm at 1100 or so RPM, dropping to 550 at 1/2 throttle. See where I'm going with this?

    Knowing this boat, it's performance, having yourself designed an fabricated the prop, is an RPM range 0-1100 reasonable? Are we close?

    I'm sure I don't want to purchase a 20k rpm screamer suited for a pump jet. I'd prefer torque and acceleration the lower KV brushless offer if I go that route.

    All of this is more of a thought experiment than anything else, things I muse about in the late hours of the evening while others watch "The Tonight Show". Is looking for a "base" RPM a good approach other than say, I'll just use this "big ass" motor with an ESC rated at 30A?

    Regardless. I am going to run with 4:1 reduction just because I like even numbers and I'd say greater reduction is better in this case and the choice between the Redmond brushed or the Quantum brushless may come down to fit and ease of build.

    Thoughts, comments, bodily deconstruction?

    v/r "Sub" Ed

    Silent Service "Cold War" Veteran (The good years!)
    NEVER underestimate the power of a Sailor who served aboard a submarine.
    USS ULYSSES S GRANT-USS SHARK-USS NAUTILUS-USS KEY WEST-USS BLUEBACK-USS PATRICK HENRY-K432-U25-SSRN SEAVIEW-PROTEUS-NAUTILUS

  • #2
    I've never measured RPM, in or out of water. The big number is current draw under load. Always shoot for less than 50% of the stall current at high-throttle. Empirical study is the way to work out the required gear ratio (if reduction is needed) for a specific motor-propeller combination. If full throttle produces a current draw about half of the motor's stall current, then you're good. If not, alter the gear ratio.

    Sorry, for the short pithy answer, Ed. I just don't know enough math to work things out theoretically. If I can't see it or feel it, I don't understand it. I should have paid attention in school!

    David

    "... well, that takes care of Jorgenson's theory!"

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    • #3
      Understood Dave!



      Again I would think lower/slower would be better regardless, I don't want to spin this thing like a rotating neutron star.

      Now that I'm playing with this new fangled 2.4Ghz tech and admit to being our guinea pig here with it, I'm going to install an RPM sensor aboard and get telemetry for it. I had planned to do that anyway going fwd with this TX. If I have it, I like to use it. Hopefully get some data for us gorillas to "rule of thumb" ourselves to happiness. I can look down and see I'm at 12.8V, drawing .9A and the prop is spinning at 256RPM all at 50% throttle.

      Thanks!!
      v/r "Sub" Ed

      Silent Service "Cold War" Veteran (The good years!)
      NEVER underestimate the power of a Sailor who served aboard a submarine.
      USS ULYSSES S GRANT-USS SHARK-USS NAUTILUS-USS KEY WEST-USS BLUEBACK-USS PATRICK HENRY-K432-U25-SSRN SEAVIEW-PROTEUS-NAUTILUS

      Comment


      • #4
        Originally posted by QuarterMaster View Post
        Understood Dave!



        Again I would think lower/slower would be better regardless, I don't want to spin this thing like a rotating neutron star.

        Now that I'm playing with this new fangled 2.4Ghz tech and admit to being our guinea pig here with it, I'm going to install an RPM sensor aboard and get telemetry for it. I had planned to do that anyway going fwd with this TX. If I have it, I like to use it. Hopefully get some data for us gorillas to "rule of thumb" ourselves to happiness. I can look down and see I'm at 12.8V, drawing .9A and the prop is spinning at 256RPM all at 50% throttle.

        Thanks!!
        Absolutely, the more data, the better the understanding of the overall situation. Looking forward to your findings, Ed. Good stuff, sir.

        David
        "... well, that takes care of Jorgenson's theory!"

        Comment

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