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    Propulsion power: was [Nav-L] Barrels
    From: George Huxtable
    Date: 2005 Jan 19, 23:00 +0000

    This is a response to a posting by Bill.
    
     Who wrote-
    
    >1.  I do not know if fuel consumption is in direct proportion to rpm.
    
    It can't be as simple as that. For power (and hence fuel consumption) to be
    proportional to rpm, that could only be tha case if the driving torque
    remained constant. But to drive a vessel faster requires more torque on the
    screw as well as more speed. So the power has to increase more than
    proportionally with the speed.
    
    Indeed, the drag on any vessel increases according to the square of the
    speed, roughly speaking, up to some limiting speed, depending on length, at
    which wave-making becomes excessive. (For a submarine, travelling deep,
    that wavemaking limit wouldn't apply).
    
    In which case the power required to overcome that drag, being drag x speed,
    will depend on the cube of the speed
    
    So, if the engine efficiency and screw efficiency remained constant,
    halving the vessel's speed would cut the rpm by 2, the shaft torque by 4,
    and the power required and rate of fuel consumption by 8. And a voyage
    would take twice as long so the engine would be running for double the
    time, so the total fuel used for the voyage would be reduced by 4 compared
    with full speed.
    
    These are abstractions that are only approximately met by real ships, which
    don't always read the theory books. And it's only very roughly true to
    assume that engine and screw efficiencies and slip remain constant over
    such a wide speed range. For example, a diesel running at a certain rpm,
    even under no-load conditions on the shaft, requires to develop a certain
    minimum power just to keep itself turning, because it has to chuff through
    itself a constant volume of air per second, compress it and spit it out. So
    that results in a diesel's efficiency being near maximum at close to its
    designed running speed, and falling at lower speeds and torques.
    
    It would, then, be a very poor approximation to imagine that power would be
    proportional to ship's speed. In answer to the exam question, Doug appears
    to be presuming a cube-law, or something very close to it, rather as was
    guessed-at above.
    
    It was interesting that Doug's question presumed a propellor slip value of
    only 2%. I had always though that ship's propellors slipped much more than
    that, but it seems I was wrong, or very out-of-date. Is 2% slip a realistic
    value for the prop of a modern merchant vessel? Doug probably knows.
    
    George
    
    
    
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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