Curious about Jet drives.

Discussion in 'Jet Drives' started by thudpucker, Jan 11, 2011.

  1. thudpucker
    Joined: Jul 2007
    Posts: 885
    Likes: 31, Points: 0, Legacy Rep: 453
    Location: Al.

    thudpucker Senior Member

    Is there a Video based explanation of jet Drives?:confused:

    I thought it was just a propeller in a pipe that produced force by forcing a lot of water out of the pipe pushing against the static water, as in Thrust from a prop wash or a fire hose.

    Someone told me the thrust comes from the Propellers inside the jet, moving the water inside the jet.
    In other words the powerful exhaust is not what makes the boat go.

    I'd like to know more, but I want to see it as a Movie. Reading is tough for me these days.;)
     
  2. Frosty

    Frosty Previous Member

    If you stand in a dinghy and throw a bucket of water out the back you will move forward. Well kinda if you discount the effct of moving your arms to do it.

    To every motion there is an equall and opposite.

    A jet is often refered to as a pump, which it is.

    The prop in a pipe is an impellor but is more like an archemedes screw,--a pumping device.

    Check out Wikepedia,--must be something there even erroneous that it may be.
     
  3. thudpucker
    Joined: Jul 2007
    Posts: 885
    Likes: 31, Points: 0, Legacy Rep: 453
    Location: Al.

    thudpucker Senior Member

    Good analogy Frosty. I was sure that was the way.
    I had other engineering questions too.
    The Relation ship between Rotor and Stator is one of my great curiosities.
     
  4. Frosty

    Frosty Previous Member

    A rotor rotates a stator is stationary.

    But why do I think you knew that!
     
  5. thudpucker
    Joined: Jul 2007
    Posts: 885
    Likes: 31, Points: 0, Legacy Rep: 453
    Location: Al.

    thudpucker Senior Member

    LOL, I did know that, and its one of the things that perplexed me all these years since I was exposed to a jet engine.
    That relationship between the Stator and Rotor is certainly complex. The Foil, the timing and the expensive parts it takes to keep that relationship.

    I owned a jet boat in AK. Two stage Hamilton. The Jet worked fine, the little Rake I made to get the small rocks and grass out of the grid was 're-invented' several times. Tools are as necessary as bait.

    It's propulsion was poor I think because it used more gas than a much smaller outboard did. I'm pretty certain of that....dont know much more though.

    So if a guy was to make 'jet' out of a prop inside a piece of PVC, would the prop in the water be doing the propelling of the boat????
    Or would it be the pressure of the Exhaust pushing on the water?

    I actually have caused somebody in the boat to fall down, when I threw a bucket of water out the back. Your analogy is proven correct!

    I've been reading on Wiki, Ask, Ehow etc all evening. The information I want is not easily found.
    I'll do more later this week.
     
  6. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    Thudpucker, the principle of a jet is best visualized with a water container on wheels. There is a hole in the rear near the bottom where the water spurts out.

    The water exerts an equal force to all walls of the container, but the rear wall is missing the surface where the hole is, so the sum of forces is smaller than on the opposite wall and the container moves forward.

    As you already found out, there is more wet surface in a jet, so more drag, causing lower overall efficiency.
     
  7. anthony goodson
    Joined: Mar 2007
    Posts: 439
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Thudpucker rather than research on Wicki ,research here ,there are some excellent articles in the archives. To be efficient a fast jet needs to exhaust above the water not below it "pushing on the water" won't work. As Frosty rightly pointed out it's Newton's third law of motion. There is however a little recognised bonus from a submerged jet ,when it comes to bollard pull they punch above their weight. to the threshhold of cavitation naturally.
     
    Last edited: Jan 11, 2011
  8. thudpucker
    Joined: Jul 2007
    Posts: 885
    Likes: 31, Points: 0, Legacy Rep: 453
    Location: Al.

    thudpucker Senior Member

    I did not fail Physics...I did not fail Physics....I did not... :(

    The Water exiting CDK's wagon load is exiting by gravity. I don't see where the energy from gravity, causing the water to flow out the nozzle, would have any effect on the Wagon. I'm pretty old though. I might be missing some easy pieces of this puzzle.

    Water cannot be compressed.
    To move the wagon, the water must be crashing into something to cause the Gravity to stack the water up, and then the wagon will move as the water tries to escape.

    So if the flow is stopped, the gravity "Making energy" pushing against whatever is stopping that water, will move the Wagon away until the water can escape.

    If I'm right, the Water out the nozzle should be stopped by the static water the boat's in.
    That's the way the Reversing works. I've never seen a jet just blowing water out the back. They all seem to point down at the Static water.
    What am I missing?

    Later today I'll get into the resources on this site.
     
  9. baeckmo
    Joined: Jun 2009
    Posts: 1,271
    Likes: 241, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Well, you make me wonder; what level of physics did you actually pass......?
     
  10. anthony goodson
    Joined: Mar 2007
    Posts: 439
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Pop round to your local fire station whilst they are drilling and ask if you can hold one of their hoses, all will be revealed . Be careful how you ask though !!!!!!
     
  11. thudpucker
    Joined: Jul 2007
    Posts: 885
    Likes: 31, Points: 0, Legacy Rep: 453
    Location: Al.

    thudpucker Senior Member

    Baeckmo Dont be naughty now. You didn't respond to my statement. How do we know that you see something more credible.

    60 years ago I was deep into that thought stuff. I was going to be an Industrial engineer. The guy that sets up assembly lines.

    Anthony I don't agree with your analogy. The Water from that truck is under a lot of pressure. Way more than Gravity.
    (PS: I saw the Korean ARMY use the Water pressure on the Crowd during the insurrection of Sigmon Rhee. It worked good. Till the truck ran out of water and the crowd tipped the truck over!)
    CDK says the force of gravity on the walls of the water carrier, is enough Kinetic energy to move the wagon away from the escaping water.
    Somehow I just cannot under stand that.
     
  12. anthony goodson
    Joined: Mar 2007
    Posts: 439
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Sorry I had bypassed the water carrier thing completely , My point was that the fireman's pump and hose combination is in fact a jet unit ,particularly when they feed it from a river or similar. Somebody near here many years ago made a small fireboat ,it transpired that the jet from the hose was more powerfull than the propulsion unit ,the results were hilarious.
     
  13. baeckmo
    Joined: Jun 2009
    Posts: 1,271
    Likes: 241, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Aah, sorry Thud, no offence; more a question on what kind of explanation would be appropriate. I'll be back on that.
     
  14. thudpucker
    Joined: Jul 2007
    Posts: 885
    Likes: 31, Points: 0, Legacy Rep: 453
    Location: Al.

    thudpucker Senior Member

    OK Anthony I understand and agree with you on that one. Good idea that Fire-boat.

    So your are correct when the water is being pumped. It's the inertia of the Pump moving the water that's making the boat move.
    Cool. I was thinking it was something deeper than that.

    A jet Aircraft puts out a column of hot gasses that wont dissipate quickly. The next gasses are pushing on that column, and the pump in the plane is being driven along by all that Hot air stacking up.

    If I understand the Rotor and Stator, the Rotor, causes a gob of air to channel down toward the Stator.
    The Stator forces this gob of air to slam into the next set of Rotor blades.
    So the channels for all this air moving have to be timed correctly and smooth as a baby's butt.
    I also think I read that you cannot have endless stages of compression. Enough's enough.

    I'm glad you stayed with me tonite. I like the tech stuff.
     

  15. anthony goodson
    Joined: Mar 2007
    Posts: 439
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    The stator takes the swirl out of the column of water, well it does it's best to.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.