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Dixon "Short Wave Sportfishing" wrote in
message ...
On Sun, 29 Aug 2004 20:52:50 -0400, "JGK"
wrote:

Just curious as to the accuracy between the two.
The speedometer in my boat is a standard speedo nothing special, the
GPS is a Hummingbird Matrix 25 which I just got a week or two ago.
After a few outings I noticed that there is at times a considerable
difference
between the two and other times they seem spot on. Can't quite figure
that
one out. Would a head current account for the boats speedometer to
appear
at a higher MPH than the GPS?

The GPS is more accurate - the speedo is relative and usually works on
air - much like a pito tube in fact, only in this case, it's water
compressing air in the line. The only real true speedo, is the paddle
type as that works on wheel RPM and doesn't rely on air compression.

I have wondered if the paddle type is flawed in theory. If you placed the
paddle in uninteruped water, such as say alongside or in front of the boat
it should be accurate except for water current. I've been suspicious about
the water the boat is displacing being accelerated along the bottom of the
hull.

dixon