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Coaxial propellers?

Discussion in 'Technical Discussion' started by Blue Ghost, Jan 19, 2016.

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  1. Blue Ghost, Jan 19, 2016
    #1
    Blue Ghost

    Blue Ghost Member

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    I always see props side by side, but I've never seen a setup of two props on the same shaft, much less spinning in opposite directions for more thrust.

    Can one of you smart types explain the engineering of why this is so?

    Thanks much for any reply.
  2. Pascal, Jan 19, 2016
    #2
    Pascal

    Pascal Senior Member

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    They ve been around forever, first on torpedoes.

    Volvo was the first to bring the technology to recreational boats with their duo prop stern drive, in the late 80s. Mercruiser followed a few years later with the bravo 3 drive and all its corrosion and reliability issues...

    Then a few years back they used similar technology aimed at larger inboard boats with the IPS pods, although the props were mounted facing forward. Merc followed with the Zeus...

    The smaller counter rotating props are more efficient and allows the use of smaller props. I m sure you can find technical references on line covering the hydrodynamics .

    Years ago when duo prop came out I had a small 25 footer single stern drive. I upgraded the drive lower unit to DP and the difference was huge with much betters acceleration and planning
  3. Blue Ghost, Jan 19, 2016
    #3
    Blue Ghost

    Blue Ghost Member

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    To me this is incredibly interesting because in all the years I've been around boats I've never heard of it being done. I guess it depends on the mags or sites you read.

    I did not know that about torpedoes. Very interesting.

    It just seems like whenever I see a video of a boat under construction and see the underside, it's like there's only one prop per drive shaft. It seemed like a waste of energy, and that a counter rotating shaft would give you more kick.

    Thanks for the rundown. Much appreciated.
  4. Marmot, Jan 20, 2016
    #4
    Marmot

    Marmot Senior Member

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    The added cost and complexity of shafting, seals, and reduction gearing very quickly outpaces the relatively minor benefits that might accrue to a recreational vessel using contra-rotating props.

    There are (or were) exceptions:

    turbinia.jpg
  5. Blue Ghost, Jan 20, 2016
    #5
    Blue Ghost

    Blue Ghost Member

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    Very interesting. Thank you.
  6. Capt J, Jan 20, 2016
    #6
    Capt J

    Capt J Senior Member

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    The PODs all use the coaxial propellors, but I think they're more for maneuverability in docking situations with the joystick and the coaxial propellors on each pod give them a little more control versus a single propellor on each pod.
  7. Blue Ghost, Jan 22, 2016
    #7
    Blue Ghost

    Blue Ghost Member

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    One of the reasons I ask is because some Russian helicopters have coaxial counter-rotating rotary wings, and it appears that it's done for two reasons; to cut down on the need of an extended tail with a vertical propeller (this keeps the aircraft from spinning in the opposite direction of the propeller spin), and supposedly there's some added lift bonus. But it appears that the supposed added lift is countered by the extra mechanism's weight to get the counter-rotating wing working on the same shaft.

    But I thought that maybe it might have proved useful for a water application in some situations. What those would be, I don't know. As alloys get stronger and lighter, I wonder if the concept hasn't been rethought or revisited somewhere down the line.

    Just curious. Thanks for the replies.
    Last edited: Jan 22, 2016
  8. Marmot, Jan 22, 2016
    #8
    Marmot

    Marmot Senior Member

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    Not just Russian. Here is a link to a good explanation of why they are used in helicopters. http://www.gyrodynehelicopters.com/coaxial_benefits.htm

    Except for outboards (pseudo-pods and IOs included) where it allows for smaller diameter props for a given power output, the technique doesn't offer much to counter the added complexity and life cycle costs.
  9. Blue Ghost, Jan 22, 2016
    #9
    Blue Ghost

    Blue Ghost Member

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    What a great site. I'm embarrassed to say that I had no idea about all the other benefits of coaxial designs. Thanks for the link. Highly educational.
  10. ddw1668, Jan 22, 2016
    #10
    ddw1668

    ddw1668 Senior Member

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    I have owned three DuoProp equipped small boats. They have all been outstanding performers for their HP and that includes fuel mileage advantages. The last of the three is a 22' Donzi Classic with a 385 HP fuel injected BBC that will run the owners age on GPS.......(74.8). The boat runs like it is on rails at any speed and so far, the drives have all been bullet proof. I understand that the DPX drive on the Donzi was about a $3000.00 option over the black drive in 1996 which may account for the very few so equipped. I highly recommend the Volvo drive for small boat propulsion.
  11. Kevin, Jan 24, 2016
    #11
    Kevin

    Kevin YF Moderator

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    The Russians do like their counter-rotating props, don't think? The Tu-95 has been the nuke carrier of choice since the 1950s and it's powered by four of the most powerful propeller engines in the world. I can't even begin to imagine the maintenance on those.
  12. Marmot, Jan 24, 2016
    #12
    Marmot

    Marmot Senior Member

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    The TU-114 civil airliner version set the speed record for propeller driven aircraft. It reached Mach .78 (.73 in some articles) or 540mph in level flight.

    The noise level produced by the supersonic propellers induced genetic defects in people and plants under the flight path. (just joking but the sound pressure of that engine/prop combination was reported to be recorded by the subsea surveillance systems used to detect submarines.

    http://www.aerospaceweb.org/aircraft/jetliner/tu114/
  13. Blue Ghost, Jan 24, 2016
    #13
    Blue Ghost

    Blue Ghost Member

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    Every book I read on naval aviation has F-14 and F/A-18 drivers saying how they can actually hear those props when they intercept and escort Russian Tuplovs at altitude. The idea being I guess is that normally you can't hear anything but your own jet fighter, which means those beasts must really be loud.
  14. Capt Ralph, Jan 24, 2016
    #14
    Capt Ralph

    Capt Ralph Senior Member

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    You ever heard of a dual prop pod raising fish?????
  15. Blue Ghost, Jan 25, 2016
    #15
    Blue Ghost

    Blue Ghost Member

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    Raising fish? I don't really understanding what you mean. Do you men racing?
  16. Capt Ralph, Jan 25, 2016
    #16
    Capt Ralph

    Capt Ralph Senior Member

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    Old fishing stories about the sounds of 3 or 4 bladed propellers, Detroit or Cat engines, Bertram or Hatteras.
    Every fisher person says their boat makes the right sounds thru the water to raise and catch fish.
    That is all great and I guess they catch allot of fish.

    Above, comments on how loud an airplanes dual props are.

    Then I realized, I have never heard of anybody with DP or coaxial pod stating they catch allot of fish or any at all..
  17. Capt J, Jan 25, 2016
    #17
    Capt J

    Capt J Senior Member

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    I really do not know the answer as to whether they raise fish or not. Very few Sportfish have pods, the only 2 I can think of were 40' Cabo's and some Jarrett Bays and Tiara's. But have never heard any reports one way or another from owners.
  18. Kevin, Jan 26, 2016
    #18
    Kevin

    Kevin YF Moderator

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    Eight props with four blades each, all breaking the sound barrier together...

    Would that be 32 simultaneous sonic booms? I'm not sure how this would be calculated.
  19. Capt Ralph, Jan 26, 2016
    #19
    Capt Ralph

    Capt Ralph Senior Member

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    Huckins assembled and delivered (early 2015) a custom 45+ foot SF with Zeus pods.
    Fairly fast, no big fish reports yet.
  20. Capt J, Jan 26, 2016
    #20
    Capt J

    Capt J Senior Member

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    Cabo built quite a few 40's with Zues. I've run at least 1/2 a dozen different ones. I guess it also depends as Cabo used transom exhaust with their Zues and not underwater exhaust like others and IPS boats. Tiara has built a lot with IPS. I haven't heard any reports. I know gas stern drives do suck in the fish raising department.
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