Join Date: Jul 2000
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Paddle wheel transducer
With due respect to Jeff''s experience and obvious expertise in many areas, I must take exception to his statement that the GPS speed readout is not a useful indicator of boat speed.
GPS reads speed over the bottom when in a general nav mode (i.e. not navigating to a waypoint position). If you are indicating 5 knots, that is your speed from your present lat/long position. If the GPS is set up to navigate to a given waypoint, then the speed indication will only be accurate if you are heading directly to or away from that waypoint. This may lead to some confusion if that point is not understood.
I use my GPS constantly as an idicator of sail trim and it works great! How many paddle wheels will give you an accurate change of .1 knots? Plus GPS doesn''t pickup seaweed or critters to screw it up.
What GPS will not give you is speed through the water, which usually isn''t significant especially for sail trim. However, one must remember, if you have a tide or river flow against you of five knots, the GPS could be reading zero if you are making five knots through the water (the same speed as the flow against you). The result is zero speed over the ground. If you turn around and go with the flow, the GPS will show ten knots!(speed through the water + current flow = 10 knots over the ground) Hey! this boat is FAST!!
Hope this helps clarify the matter