In an airplane, you care about both speed through the air and speed over the
ground.

Speed through the air determines whether your wings will generate enough
lift to keep you from falling, so it's the more immediate concern.  For this
you use a pitot tube, which is simply a little tube pointing forward.  Ram
effect of the air going into the tube raises the pressure inside.  The
airspeed indicator on the instrument panel is a specially-calibrated (and
pretty sensitive) pressure gauge.

Speed over the ground is important for figuring out where you are and how
long it will take to get where you are going.  Classically, it is much more
difficult to figure out, since you have to guess at the wind speed and
direction and then do some vector math.  GPS really simplifies this,
especially since all the vector math is hidden inside and done
automatically.

Power boats also use pitot tubes, but the pressure differential at the
speeds of human- or wind-powered boats is so small that it's not practical,
at least with mechanical meters.

Sailboats often use paddlewheel devices, which project through the hull.
One of these might be practical on a canoe, since they only stick out a
fraction of an inch in some cases.  They are pretty rugged, and might even
survive normal canoe use, at least for a while.  They are pretty expensive,
though.  You could buy a halfway decent canoe for the price of some of them.

> -----Original Message-----
> From: Mark Skeels [mailto:meskeels@EARTHLINK.NET]
> Sent: Wednesday, September 05, 2001 10:31 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [OT]: Canoe Speedometer
>
>
> Hi, Kat,
>
> Not sure how that would affect paddling; I s'pose it'd depend
> on how you
> make the paddle wheel assy. Maybe the best way would be to
> hang it off of
> the back of the canoe. Biggest problem I see is speed would
> be relative to
> the water, and in a river, there's current. So, the speed
> readout would not
> be actual ground speed.
>
> How do they do it in an airplane?
>
> Mark
>
>
>
> ----- Original Message -----
> From: "Kathy Quinlan" <katinka@MAGESTOWER.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Wednesday, September 05, 2001 9:37 AM
> Subject: Re: [OT]: Canoe Speedometer
>
>
> > A paddle wheel with two or four magnets attached, to a
> largish shaft and a
> > hall effect device to pick up the magnetic pulses this will
> convert them
> to
> > electrical pulses to be converted to speed by the pic :o)
> >
> > You need an even number of magnets to keep the paddle wheel evenly
> balanced.
> >
> > I know that any currents will upset the reading, but this is how
> commercial
> > boats do it :o)
> >
> > Regards,
> >
> > Kat.
> >
> >
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> >
> >
> > ----- Original Message -----
> > From: "Mark Skeels" <meskeels@EARTHLINK.NET>
> > To: <PICLIST@MITVMA.MIT.EDU>
> > Sent: Wednesday, September 05, 2001 10:37 PM
> > Subject: [OT]: Canoe Speedometer
> >
> >
> > > Greetings, PICsters.
> > >
> > > How would you go about creating a device to measure and provide a
> > real-time
> > > readout the speed of a canoe on a lake or a river? (using
> a PIC, of
> > course?)
> > > (...or not...)  :-) Only thing I can think of is GPS, but
> perhaps sonar,
> > or
> > > something?
> > >
> > >
> > > Mark Skeels
> > > Engineer
> > > Competition Electronics
> > > meskeels@earthlink.net
> > >
> > > Soli Deo Gloria!
> > >
> > > --
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