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Dear Lu,

> A very long time ago, navigators measured vessel speed
> by counting knots
> pulled through one's fingers from a chip log,

Not so very long time ago:-)
In the time of Columbus even this simple lot was not known.
They estimated the vessel speed by the
"look and sound of the stream behind".
Or by throwing small pieces of wood from the bow
and timing when they pass the stern.
(It is amazing how Columbus did his sometimes excellent
dead reconning with these means).
I don't know when the rope-and-knots log was invented
and when it came to the general use.

> Somewhere along the line, patent logs
> (such as the Walker log) were
> invented and distance was measured
> directly by trailing the log's
> spinner.   When?

The first spinning log was apparently invented by
Edward Massey in 1802, it was in general use by 1836
and used until 1961. It was improved by
Alexander Bain in 1946.
First Walker's log was invented in 1861.
All these early logs had a disadvantage that the dial
was close to the screw, so one had to pull the log out
to see the traveled distance.

Then in 1878 the same Walker invented the "modern" version
with the dial attached to the taffrail.
All these logs indicate the distance traveled,
rather than speed, and one has to use a clock with them.

The next invention ("Forbes log", I don't know when,
but suppose this was already in XX century, or very late in
XIX) is a lot that reacts on the pressure of the incoming
stream, with electrical transmission to the permanently
fixed dial which indicates speed, rather than distance.
(Same principle
that was used in the airplanes).
This was probably superseeded only by the satellite systems.

I don't know whether anyone is doing
dead reconing nowadays.
But I will not be surprised if some people
do it for the same reasons they do Cel nav,
that is either for fun,
or because "it is required":-)

Alex.




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