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    Re: Logs vs Knotmeters
    From: Alexandre Eremenko
    Date: 2008 Mar 29, 22:39 -0400

    
    Dear Lu,
    
    > I suspect patent logs (sucn
    > as Massey's or Walkers) were used
    > strictly for distance measurement,
    > since distance traveled is needed for DR navigation,
    
    All logs described in Leckey (1919) measure DISTANCE.
    But not "because distance traveled is needed for DR"!)
    Just because they did not know how to measure speed
    directly.
    
    A (patent or taffrail)
    log is used to measure the distance over a short period of
    time.
    Then this measured distance is divided by the time to deduce
    the speed. Then when doing DR the speed in
    converted to the traveled
    distance.
    
    There are many reasons why they could not use a taffrail
    (or earlier) lots continuously. The main reason is
    quick wear. According to Lecky, wear of the gear was a major
    problem, even with short time ise of lots.
    
    Leckey also mentions propeller or paddle revolutions
    on steamships to measure speed and says that this is
    not worse than any patent-log.
    
    > "did people switch back to speed measurements before
    > dropping DR altogether"
    
    Yes. Most of the XX century
    (definitely from 1950-s till the time of GPS) they used
    pressure logs that show SPEED rather than distance.
    
    And they did dead reconning until GPS times.
    I used to have a Russian 1970-s book on dead reconing.
    It was mandatory on Russian ships at that time.
    The logs were pressure logs measuring both speed and the
    distance. Taffrail logs were still used as a backup.
    
    Alex.
    
    
    
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