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The following dialogue occurred between Jared Sherman and Herbert Prinz.

>> And, how are you supplied with slip for a particular prop and hull? Or
>>is that something you have to measure and calculate while under way?
>
>Jared, have you ever wondered what all these Measured Mile Markers are
>for? And engine revolution counters and tachometers? A patent log, by the
>way, is just the inversion of the same principle. It, too, suffers from
>slip and has to be calibrated regularly.
>
>Herbert Prinz

Here I disagree with Herbert. The two, driving propeller and idling
patent-log, are not the same at all. 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. And it seems to me that even in less
extreme conditions, winds will affect the turns counted per mile gained,
but to a more limited extent. Which is why I have asked whether the slip
factor (given in this case as 7%) can be taken as constant under all
conditions. To me, it appears to be a doubtful assumption, though perhaps
appropriate for posing an exam. question.

I hope Doug can explain whether he ever has to make allowance for different
slip, under extreme weather conditions, or whether it's found that in such
circumstances turns-counting gives an incorrect answer for distance gained.

George Huxtable.

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