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    Re: Logs vs Knotmeters
    From: James Whitson
    Date: 2008 Mar 29, 23:26 -0400
    Sirs-Any plot on a chart not based on a fix of position is considered to be a DR plot. The technique and formula for DR is the same for calculating and plotting an ETA. Use it all the time. DR is alive and well, not for "Where am I", but, "When will I be there". Granted, it is still a caculated guess.-Jim 

    > Date: Sat, 29 Mar 2008 22:39:34 -0400
    > From: eremenko---.edu
    > To: NavList@fer3.com
    > Subject: [NavList 4773] Re: Logs vs Knotmeters
    >
    >
    >
    > 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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