How to find Delivered Power

Discussion in 'Props' started by Mitch1990, May 29, 2020.

  1. Mitch1990
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    Mitch1990 Junior Member

    Hi,

    I am trying to find the delivered power using the attached equation. I have Kq from a prop chart and all the other values. The value I get for power delivered is less than the effective power of the hull. Can this be correct.

    upload_2020-5-29_15-54-58.png
     
  2. gonzo
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    gonzo Senior Member

    Do you mean power of the engine?
     
  3. DCockey
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    DCockey Senior Member

    What is the source of the " effective power of the hull"? How accurate is the number?
     
  4. Mitch1990
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    Mitch1990 Junior Member

    What I am trying to do is choose a prime mover for a propeller. The effective power of the hull is 600kW at 14kn. When I use the above formula to determine the power delivered i get 135kW which is most certainly wrong
     
  5. Mitch1990
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    Mitch1990 Junior Member

    By delivered power I mean the power delivered to the propeller
     
  6. DCockey
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    DCockey Senior Member

    How do you know the effective power of the hull is 600kW at 14kn? Is this an exercise and you were given that information? Did you do detailed calculations? Did you use a formula for estimating effective power of the hull? Is it possible the effective power of the hull is less than 600kW at 14kn?
     
  7. Mitch1990
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    Mitch1990 Junior Member

    It was given as part of an exercise which has many components. I have sized a Wag B prop with a BAR of 0.6 and 4 blades, I have the required coefficients at the highest open water efficiency from the prop chart.
     
  8. Mitch1990
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    Mitch1990 Junior Member

    Is it possible for the effective power to be less than the delivered power? That doesn't sound right to me
     
  9. Mitch1990
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    Mitch1990 Junior Member

    The idea is that I should be able to develop a theoretical propeller curve based on the design point of effective power at 600kw and 14 knots
     
  10. Mitch1990
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    Mitch1990 Junior Member

    Is it correct then for me to use the propulsive efficiency to find PD?
    upload_2020-5-29_17-7-3.png
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    What other values are you referring to?

    Is this an equation given to you.. or... an equation you have assumed is to be used?
    Makes a big difference...

    It would help if you posted the whole question and any variables/assumptions given.

    To calculate the effective power is very easy and straightforward - given known data...or assumptions of the hull.
    But it seems you're only providing just snippets of the question and thus it remains unclear ... providing the whole question, shall shed light on your enquiry.
     
  12. Mitch1990
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    Mitch1990 Junior Member

    The question is:

    Select an engine. A quick survey of similar vessels establishes that the effective hull
    power will be around 600 kW at 14 knots. The vessel is twin screw CPP with a displacement of 700 tonnes and is approx 70m long

    I have the P/D = 1, Jadvance = 0.67, Kt = 0.1936, Kq = 0.0327 and eff= 0.6317 for a wag b prop with z = 4 and BAR = 0.6.

    I have assumed a wake factor of 0.12 and thrust deduction coeff of 0.15. I do not have Cb to find the wake factor, however I was very conservative i feel.

    I used the first equation i posted from my text book which now i look closer refers to it as propeller power. Once I have the propeller power using
    upload_2020-5-29_19-36-51.png
    I will use the below develop a propeller curve to match to an engine with a mechanical loss of 0.95 through the gearbox and prop shaft.
    upload_2020-5-29_19-37-21.png

    That looks like everything I was provided.

    Mitch
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Thanks for your reply Mitch. But Im still not clear... as you state:

    This suggest you have made these assumptions/calculations etc.....correct?

    If so, what is the question and data provided, ONLY... not any of your assumptions or thoughts.

    Many thanks.
     
  14. Mitch1990
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    Mitch1990 Junior Member

    Sorry,

    Anything below the line that says I have assumed is assumptions, above this line is what was provided in the question. It is worth noting that the prop data is from an spreadsheet not a chart
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    ok... so the question is this

    So given this data and ONLY this data, you cannot make leaps like you have. Since everything will be based on what??... just pure assumptions - which is not the MO of the question.

    Thus in order to answer the question, you first need to know the relationship between Effective Power (Pe) and Delivered power (Pd).

    This can be simply brokne down int what is called the PC - Propulsion Coefficient.

    This is simply PC = Pe/Pd
    Which you note above as your Nd.

    The full definition is rather lengthy, but has all these elements:
    appendage coeff x hull efficiency x prop efficiency x RRE x shaft efficiency

    So, given your question... what do you need to calculate the Pd??.... you need the PC.

    Thus... how would you establish the PC... given the little data you have?
     
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