Transmission-free surface drive? (jet pump reverse bucket)

Discussion in 'Surface Drives' started by srimes, Oct 19, 2008.

  1. srimes
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    srimes Senior Member

    Please excuse me for posting while intoxicated.

    Would a reversing bucket like that used on jet pumps work on a surface drive? A bucket isn't really feasible on regular inboards cause it needs to be out of the water when going forward, but this wouldn't be a prob on a surface drive. I like to potential simplicity of surface drives and if you could do without a transmission that'd be that much cooler.

    For a fixed shaft w/ rudder the bucket would have to turn much like a kitchen rudder. Heck, a kitchen rudder would probably work fine for low speeds but at higher speeds would cause too much drag.

    What do yall think?
     
  2. CDK
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    CDK retired engineer

    Now that your head has cleared you probably already have doubts about the idea. Even on a jet the reverse bucket is a doubtful construction, but at least there is no rudder in the way there.
     
  3. mydauphin
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    mydauphin Senior Member

    Engineering while intoxicated is against the law isn't it... EUI
    \
    ===/===<
     
  4. PetterM
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    PetterM Senior Member

    There is no reason what this should not work, in fact I think Renato Sony Levi tested a similar arrangment at some point....I think it was quite difficult to control though...
     
  5. srimes
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    srimes Senior Member

    cool, thanks. Jets do well backing up w/ their reverse bucket, right?

    Do you know if Mr. Levi used a sloped transom when trying the reverse bucket? I've read that reverse isn't so hot on surface drives w/ rudders unless the transom is angled.
     
  6. PetterM
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    PetterM Senior Member

    I am not sure. It was described in his book “milestones in my design” I think, will try to check at home tonight.
     
  7. PetterM
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    PetterM Senior Member

    A had a look last night, there was no illustrations but I would guess its similar to the LDU?
    If you are planning to design your own surface drive, then I would recommend that you buy Levi’s book “Milstones of my Designs”.
     
  8. StianM
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    StianM Senior Member

    Won't work
    A jet sucks the water from below the boat so the jet pointed backwards make posetive trust and if you direct it forward it will create negative trust.

    A surface drive sucks the water from the front of the boat so directiing the trust forward will only work against the work done to push the water backwards and you will just not get annywhere.
     
  9. srimes
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    srimes Senior Member

    blowing totally beats sucking for creating thrust. As far as thrust is concerned, sucking sucks. find an explination of "pop pop boats" for a better idea.
     
  10. MattZ
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    MattZ Junior Member

    Actually, if the reverse buckets reduce the flow into a smaller area it will work. There's a device called a Kitchen Rudder, invented by Admiral Jack Kitchen of the British Royal Navy that does exactly that.

    Also, the thrust reversers used by commercial jets draw air in the front yet provide negative thrust by pointing the thrust forward.
     
  11. StianM
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    StianM Senior Member

    At speed the engine is feeded with air and therfor the negative trust only work against the moving force of the airplane. Atleast I gues so since I allways see the aircrafts pushed into the airport by viecles and don't use there own thrust to get out.

    Just thinking out loud (no facts presented in this post)
     
  12. MattZ
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    MattZ Junior Member

    Commercial jets can back up from a standstill with the thrust reversers. Boats with a Kitchen Rudder can back up from a standstill. Here's a link to a pulsejet powered go kart that draws air in from the rear, forces it out the rear, and still develops a forward thrust: http://www.youtube.com/watch?v=5U-grFuXZ9U

    Stricly speaking, it's not the 'suction' or the 'discharge' that drives you forward. It's the transfer of momentum to the fluid, whatever it may be (air, water, rocket fuel.)

    Here's a simple calculation to make my point: (I'm Canadian so I'm doing this in metric.) Lets say you have a jet pump sucking in 100L/s of water. It 'sucks' water in the front and 'pushes' it out the front. So the suction would 'pull' the boat forward, but the discharge would push is back, and we shouldn't go anywhere. But there's a little bit more to it. It depends how fast you're sucking, and how fast you're discharging. If the nozzle converges then the discharge velocity is much higher so there's more momentum in the water being discharged than in the water being drawn in.
     
  13. StianM
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    StianM Senior Member

    There is a god after all;)
     
  14. MattZ
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    MattZ Junior Member

    Sorry, I did a quick calculation, but never posted the numbers. Basically it comes from Reynold's Transport Theorem if you've ever studied fluid mechanics. It's a first order approximation in most cases, but it proves the general existence of the phenomenon.
     

  15. Mark Emaus
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    Mark Emaus Junior Member

    Not true,

    Jets do not use reverse thrust to back up anymore because the engines are to low to the surface and thus pick up FOD creating damage to the blades. Jets like the B727 and MD80 can use reverse thrust to back up and they work very well. I did an expermiment with a long tail mudmotor by building a bucket to provide reverse and it worked. We are now looking at making a similar type bucket used by jet skis for a mud motor reverse. I must note that the reverse is only going to be used for docking and slow speeds. We are concerned about weed build up, but the trade off of expense compared to a transmission is the offset.

    Mark
     
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