Translate Newton to kwh

Discussion in 'Hydrodynamics and Aerodynamics' started by Pierre Talbot, Nov 9, 2020.

  1. Pierre Talbot
    Joined: Nov 2020
    Posts: 2
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    Location: montreal

    Pierre Talbot New Member

    I'm new on this forum and the boat design is not my core competency so my question might be basic. I have used the software Yachtlines and entered the values for a yacht that has a waterline of 12.10m, waterline beam of 4.2m, draft of 1.1m, 13500 kg, speed of 6.2 knots and other coefficients. The software returns 1389 Newton of output resistance. Test results of this boat with 2 electrical engines, 2 straight shafts, 2 propellers, gives total of 22.9 kwh at 6.2 knots. I have tried to find a way to correlate/convert the 1389 Newton to the 22.9 kwh but I`m missing something. I don't need precise correlation but a rough order of magnitude.

    Is there someone who can help me?
     
  2. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Welcome to the forum Pierre,

    Power (naked) = resistance x speed. Speed is in m/s
    Thus Power = 1389(N) x (6.2x0.5148 m/s) = 4433W or 4.43kW

    But.. the power required, at the prop, is not the same as Power (naked) or EHP as it is also called.

    The shape of the hull influences the flow of water into the prop...and then there are mechanical losses etc to take into account. Collectively called the PC, Propulsive coefficient.

    However, the numbers you have suggest a PC of just 0.19 (4.43/22.9)...so something does not add up.

    Since the PC is a roughly first stab for a prop boat can be in the order of 40-50%, we can say that the power required, at the prop is the following, if say 40% for the PC..or 0.40:

    Power = Power (EHP)/PC = 4.43kW/0.4 = 11.1kW.

    This is roughly half the amount, so i suspect there is a doubling up of numbers somewhere? ... is the resistance per hull..or total?....if per hull..there is your answer....if total..something is not right?!
     
    hoytedow and bajansailor like this.
  3. Pierre Talbot
    Joined: Nov 2020
    Posts: 2
    Likes: 0, Points: 1
    Location: montreal

    Pierre Talbot New Member

    Thanks for the information; it will be useful in my research.
    The boat is a monohull. The Yachtlines software doesn't take into account the drag of the propellers and rudders but probably that doesn't explain the missing factor of 2. I still need to better understand all the different coefficients that the tool need and use the right values.
     

  4. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    ".....gives total of 22.9 kwh at 6.2 knots. I have tried to find a way to correlate/convert the 1389 Newton to the 22.9 kwh but I`m missing something."

    You are comparing "Apples to Oranges" here, and the term "kwh" is not a power, but an energy with no reference to the time to consume the "22.9 kwh". Power is generally Force X Velocity (Newtons and Knots here), and Power is generally kilowatts or horsepower, NOT kwh (kilowatt-hours).

    One useful fact, applicable to ALL of engineering and science, is that dimensional consistency is ALWAYS required, "Necessary but not sufficient" is the rule here. Dimensions, be they energy or power or force or turning torque or length or whatever, must ALWAYS match on both sides of any equation, with no exceptions. This is called "Dimensional Analysis", and must be satisfied, without exception.
     
    fastsailing and TANSL like this.
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