On Tue, 1 Feb 2005 12:41:26 +0100, "Jürgen Spelter"
wrote:

I got a garmin Sounder transceiver witz a built in paddle wheel speed
sensor. I wrote to garnmin and they told me, that the paddle wheel makes a
specific number of elektrical impulses per mile.

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That is true for any given speed but because of friction in the paddle
wheel bearing, pulses per mile is not constant. The more accurate
units perform an internal lookup function based on speed, and also
have a calibration adjustment since different hull configurations and
mounting locations can also have an effect on paddle speed.