Thrust and Horsepower. Kort Nozzles

Discussion in 'Propulsion' started by Yobarnacle, Dec 29, 2011.

  1. Yobarnacle
    Joined: Nov 2011
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    Location: Mexico, Florida

    Yobarnacle Senior Member holding true course

    I see various posts in this forumn and other sites on web, assigning values of thrust=horsepower. I have been given formulas to calculate Thrust and Horesepower and RPM and Torque.
    Still, I see rules of thumb, like, on average 1 HP = 22 lbs Thrust.
    I understand there are loses in in system due to frictions, heat, inefficiency.

    I presume static auxilary components , like nozzles and prop shrouds, don't add any HP to a systems output, because they don't move and do no work.

    I presume any increase in thrust is from improved efficiency and reduced friction, like negating tip vortexes. All output HP was input into system, and because of physical law of conservation of energy, you can't get more out than you put in.

    My question.
    Are the formulas I was given hard fast laws of physics? Or coventional system estimators?
    Are claims of greatly increased thrust using nozzles or other enhancements bogus? Or are there different formulas used to determine POWER, THRUST, TORQUE, RPM for these improved systems?
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The relation between hp and thrust cannot be captured in a formula because there are too many variables. Only static thrust (zero velocity) can be calculated based on prop diameter, pitch and rpm; even then, assumptions like blade efficiency and friction loss make the outcome deviate considerably from reality.

    As soon as the hull starts moving, the complexity increases with additional factors like flow disturbance from the hull shape and a changed propulsion angle. Because these numbers change continuously with speed, formulas are meaningless.

    A rule of thumb from someone with lots of experience is the best you can get.

    Any object in the water, used to increase efficiency by shaping the flow of water before, around or behind the prop, also brings its own drag. If there is any positive result at all, it only applies to one specific case.
     
    1 person likes this.
  3. Yobarnacle
    Joined: Nov 2011
    Posts: 1,733
    Likes: 122, Points: 63, Legacy Rep: 851
    Location: Mexico, Florida

    Yobarnacle Senior Member holding true course

    Last edited: Dec 29, 2011
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