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    Re: awareness / actually: speed from engine RPM
    From: Herbert Prinz
    Date: 2003 Apr 24, 05:54 +0000

    George Huxtable wrote:
    
    > Here I disagree with Herbert. The two, driving propeller and idling
    > patent-log, are not the same at all.
    
    I did not say they are the same. I said that the patent log is also based on
    the principle of a rotating propeller screw, therefore suffers just the same
    from slip (amongst other problems) and has to be calibrated for varying
    conditions, too.
    
    > Those that read the account from the
    > 1890s of a cable-ship in a storm, discussed recently here, will recall that
    > it took all the power available simply to hold station against the wild
    > weather. Presumably, the shaft-counter will have been clicking away without
    > the vessel making headway at all.
    
    I did not follow that discussion, but I get the idea. I bet that if the master
    of the ship would have found the time to bring out a patent log in said
    conditions, the log would also have been clicking away due to wind induced
    surface current. And so would any other fancy modern instrument regardless of
    what principle it is based on, because they all measure the speed relative to
    water in close vicinity of the hull. But there is a lot of water flowing under
    the keel if you go full power against the wind without making headway!
    
    Clearly, any method will fail in extreme circumstances. This does not mean that
    it is useless on a regular basis. The slip factor is certainly not a constant.
    Think of what happens when you reach hull speed. It depends on wind and sea
    state, but also on speed, draft, trim and foulness of bottom, in short on
    everything that affects the resistance of the hull. It must be determined
    empirically for various conditions; using it certainly requires that the master
    is familiar with his vessel in these conditions.
    
    In actual practice, nobody bothers with formulas for propeller pitch, slip, and
    so forth. Rather, one directly produces curves or tables that show speed versus
    engine rpm. The proper procedure for doing so using the Measured Mile markers
    that I mentioned in my earlier post can be found in nautical handbooks.
    Although these markers are not US Coast Guard maintained aids to navigation,
    they are charted and whoever pays for their upkeep must think that they are
    still useful.
    
    Bowditch, 1909, considers counting prop revolutions a "valuable check on the
    patent log" and a "means of replacing it, if necessary". 75 years later, DMAHC
    Pub. 9 says "This method of determining speed is widely used in the merchant
    marine." (Article 616, p. 155). Calibration techniques are described in
    appendix U.
    
    Herbert Prinz
    
    
    

       
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