[C320-list] Calibrate Knotmeter

[Steve Magennis]

No, Stan, the knot meter DOES NOT use the spinning wheel as a
generator, and it does not create any voltage that is related to speed.
 The way the paddle wheel works is that every time a magnet in teh
wheel passes the sensor in the holder, it creates a voltage pulse.  The
speedo counts how many pulses are created per unit of time, and
converts that to a speed.  The read-out IS linear with the rotational
speed of the paddle wheel.

However, to relate the paddle wheel rotational speed to boat speed
THROUGH THE WATER (not SOG) requires that the paddle wheel spins
freely, and is located in a place where the water flow isn't disturbed.
 If the paddle wheel doesn't turn freely (fouled or the axel is
corroded), you could easily get a non-linear speed read-out.

To minimize this potential non-linearity, you must have a non-fouled
paddle wheel and a smooth axel.  You can purchase a replacement axel /
paddlewheel from Raynarine, or potentially, you could clean it with
emery paper, and then lubricate it (possibly with Sailcote or Boeshield
T-9)

The attached link to the Raymarine site will get you the manual for the
speedo.

http://www.raymarine.com/raymarine/SubmittedFiles/Handbooks/Instruments/ST60/81037_5www.pdf


Steve
Alchemist #909


--- Stan <ola7442000 at gmail.com> wrote:

> I gave up a long time ago.  It's not a linear adjustment. i.e. On my
> boat
> between approximately 4.5 - 5.5 Kts. it matches the SOG.  Below 4.5
> its
> reads low. Above 5.5 it reads high.
> 
> I'm not really sure how this can be as the the little spinning wheel
> is sort
> of a generator.  The faster it spins, the higher the voltage it will
> put
> out.  The output voltage is scaled to Kts.  It should be a straight
> forward
> calibration, but it is not.  As I said, I gave up.  I just use the
> SOG since
> I'm on a lake.
> 
> Stan
> "Our Little Amusement" #744
> 
> PS  Moving to Las Vegas tomorrow.  The boat will follow in Jan. or
> Feb.
> depending when I get a slip at Lake Mead. (4-5 month waiting list)
> 
> 
> On 9/29/06, Irv Grunes <igrunes at comcast.net> wrote:
> >
> >
> > Guys,
> > Have gone crazy trying to understand the manual on calibration of
> the
> > knotmeter.
> > Have not been able to decipher it.
> >
> > Can anyone describe is some detail the process so that the
> knotmeter will
> > read the same as SOG over still flat water.
> >
> > Ours has never read correctly.
> >
> > Irving Grunes
> > Isle of Wight #851
> >
> >
> >
> >
> >
> 
> 
> -- 
> Stan
> C320 "Our Little Amusement" #744
> __/)__/)__/)__/)__
> 


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