just a question?

Discussion in 'Education' started by xiphion, Nov 11, 2004.

  1. xiphion
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    xiphion New Member

    i have followed kschungs thread and am unable to understand why the kort nozzle should be unable to perform well at higher kt? also in my understanding nozzles are not used on large vessels because the improved thrust does not override the loss in GT. can an aluminium kort solve this problem?
     
  2. CDBarry
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    CDBarry Senior Member

    Read Josip Gruzling's recent SNAME paper (given at the 2004 annual meeting).
     
  3. Dutch Peter
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    Dutch Peter Senior Member

    Xiphion,

    No, the material the kort nozzle is build from will not be of advantage. It's the shape of the nozzle that will improve thrust and efficiency.
     
  4. xiphion
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    xiphion New Member

    then why don't we use an accelerating nozzle on single screw vessels? i understand this might seem like repeated question to that of kschung, but what i wish to know is what is the overriding factor that prevents use of a nozzle on a vessel going to high seas?
     
  5. Dutch Peter
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    Dutch Peter Senior Member

    Nozzles ARE used on single screw vessels. Even on pretty large vessels (>100 m) or prop sizes (>2 m). Does this answer your question?
     
  6. jehardiman
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    jehardiman Senior Member

    xiphion; You are having the same problem as kschung in that you equate high Kt to high thrust and high speed, rather than equating high Kt to high propeller loading at a lower than optimum speed of advance and efficency.

    See the attached figure.

    If we look at true J (i.e. Va/nD) for two similiar props with one in a nozzle, we see that the Torque Hp (Kq) is the same for both props when fully loaded up in the bollard condition, however what the nozzle does is to change the apparent Va which therefor changes the true J. We can see this in that the Kq curves are the same at J=0 and Nozzle Kq is nothing but a ratio of Open water Kq on J. Kt however is increased at J below 0.5 in this example. This is the increase in momentum of the wake caused by the duct increasing the velocity of the gathered inflow water. The ratio Kt/nozzle kt is called the thrust ratio (tau) and is a function of the ratio of the Va to the apparent velocity of advance of the prop and its rpm. Therefor at high slip conditions the accelerating effect of the nozzle is generates thrust while at low slip conditions the prop is overspeed and no thrust is generated.

    But what is the cost of this additional thrust? If you look at the eta curves, you see that at low J, the efficency of the duct is better, but it never reaches the efficency of the open water prop. Why? because if you design the nozzle from maximizing thrust at low speed, at high speed the nozzle will always cause the propeller to operate off its design point. In the figure you can see that there is a cross over point in efficency. This is where you would change from a nozzle design to an open water design.

    If eta will never be as good as an open water prop, why use a nozzle? Because you might be constrained by such things as prop diameter, shaft rpm, and caviation. On the flip side, flow separation in or on the nozzle can greatly decrease the thrust of the nozzle. Flow separation is commonly caused by ship pitch and wave effects which limits nozzles usefulness in oceangoing ships.
     

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  7. xiphion
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    xiphion New Member

    thanx for all your help. it was really useful!! thanx
     
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