Reactive thrust of submerged waterjet

Discussion in 'Jet Drives' started by markmywords, Aug 11, 2011.

  1. markmywords
    Joined: Aug 2011
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: UK

    markmywords New Member

    Hi,

    I'm sure that this question has been posted before in many different guises but ......

    I am looking at a jet propulsionpositioning system for a small floating platform. I have calculated the thrust issued from an array of water jet nozzles.

    I know what forces I want to act on the platform and roughly what drag is present.

    I want to work out the reactive thrust developed by these submerged nozzles? Reviewing rocket equations is fine but they are all firing exhausts into an immovable ground plane and receiving an equal & opposite reaction. My waterjets are firing into a body of water (they are submerged below the platform) and I'd like to get a lead on how to work out the resultant reactive force received by the platform.

    Any ideas?

    Many thanks in advance.
     
  2. anthony goodson
    Joined: Mar 2007
    Posts: 438
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    I'm sure Baeckmo will be along with the magic formula in a mo ,however you have not made it clear if these are actually deeply submerged nozzles ,or commercial waterjets at rest,and therefore in a submerged state.The two will not behave in the same way.
     
  3. markmywords
    Joined: Aug 2011
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: UK

    markmywords New Member

    Reactive thrust

    Anthony,

    thanks for prompt reply. The nozzles are positioned flush with the bottom surface of the platform and angled at 20deg below this horizontal plane. I can't deeply submerge them.

    Many thanks

    Mark
     
  4. anthony goodson
    Joined: Mar 2007
    Posts: 438
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Well after that point I am out of my depth but I'm sure that information will be helpful to the hydrodynamicists when they arrive. Bit like buses really ,you wait all day for one and then they all arrive together.
     
  5. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    I'm probably wrong but I believe Newtons laws of action reaction apply. It doesn't matter what the jets push against, air, water, etc. it's the reaction that make the propulsion.

    Thrust should be the same above or below the waterline.

    Steve
     
  6. Frosty

    Frosty Previous Member

    Feel the thrust from a garden hose with your thumb at the end. Adjust the speed of water exiting and feel the equal and opposite motion, then feel the less thrust when doing the same experiment under water.
     
  7. TeddyDiver
    Joined: Dec 2007
    Posts: 2,578
    Likes: 120, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    It's the counter pressure of the water diminshing some of the jet velocity so smaller nozzles help.. anyway so me thinks.. waiting also to hear some real expertise.
     
  8. Frosty

    Frosty Previous Member

    Of course it is, the water absorbs the velocity therefore no equal velocity.

    To every motion is an equal and opposite motion.
     
  9. anthony goodson
    Joined: Mar 2007
    Posts: 438
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    I have a feeling you're all going to get a good telling off in a minute.
     
  10. Frosty

    Frosty Previous Member

    Ha ha,-- probably.
     
  11. jehardiman
    Joined: Aug 2004
    Posts: 2,737
    Likes: 341, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Yes it will work, and no, it isn't an good way to power a vessel.

    V=body velocity, Vj=jet velocity, rho=mass density, A= jet nozzle area

    mass flow rate= Q = rho*Vj*A
    change in velocity = (Vj-V)
    Thrust = change in momentum = T= Q*(Vj-V)
    Useful work done on the hull= T*V
    Work (kenetic energy) required to accelerate jet internal to the hull= 1/2 m del v^2 = 1/2*Q*(Vj-V)^2
    effeciency = eta = useful work/total work = T*V/(T*V+1/2*Q*(Vj-V)^2)
    reducing
    eta= 2V/(Vj+V)

    So we can see that to have maximum efficency Vj=V which means T=0 or A = infinity. And we haven't even started talking about real fluids and internal losses.

    What it really means is that a similar sized propeller external to the hull is far better than having a hole through the vessel.
     
    1 person likes this.
  12. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    you're probably right anthony, thats why I started with "I'm probably wrong" but....

    I've been down this road before. A looong time ago on an island far away the US Navy posted a fleet of Pegasus Hydrofoil Gunboats....

    http://wn.com/Pegasus-class_PHM_hydrofoil

    I was a vendor to the Naval detachment on that island and I had opportunity to tour the first vessel to arrive just hours after she docked.

    I asked the Captain if the power would be more efficient if it "pushed" against water instead of air because the waterjet nozzles are on the transom and not submerged. This launched a discussion involving the previously long deseased Mr Newton and of action reaction with examples of why spaceship's work in space with rocket nozzles "pushing" against a vacuum.

    I was told reaction was same / same and it was the reaction that caused the motion and what it "pushed against" was irrevelant.

    But...like I said, "I'm probably wrong" :eek:

    Steve


     
  13. jehardiman
    Joined: Aug 2004
    Posts: 2,737
    Likes: 341, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Realisticly you aren't pushing against a vacuum, you are pushing against the mass you are expelling. Think of a projectile gun recoil...it is caused by the throwing (i.e. changing the momentum) of a small mass really fast.

    For the really geeky, see if you can come up with what happens if you change from an axial jet (as describe above) to a right angle one (like a Tees White Gill thruster or Hero's engine).
     
  14. yipster
    Joined: Oct 2002
    Posts: 3,486
    Likes: 95, Points: 58, Legacy Rep: 1148
    Location: netherlands

    yipster designer

    Will check these thrusters tomorow. reading this on my mobil. Best explenation I once read was on a jet site that mentioned the waterjet only took flight when someone took it out of the water and pointed the nozzle on the water surface hitting it. Action reaction? forget now why but that was easy to understand for me, will read the form tomorrow. JH, have a thought on a pulsed (hard to get a pulse but thrust may ne up?) waterjet?
     

  15. anthony goodson
    Joined: Mar 2007
    Posts: 438
    Likes: 17, Points: 18, Legacy Rep: 256
    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    JH is right in that a large slow moving prop is good for bollard pull .However don't disregard the jet, in this respect ,they punch well above their weight in static thrust, The reason for this is that a good waterjet ,when stationary ,can absorb in excess of 90% of engine revs and power,{to the limits of cavitation},whereas a standard propeller needs to advance in order to do this.
     
    1 person likes this.
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.