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Author Topic:   Accuracy of Paddle Wheel Speed
lurkynot posted 05-21-2007 09:30 PM ET (US)   Profile for lurkynot   Send Email to lurkynot  
I have an 01'[ I don't think this is intended to mean "one foot"; probably meant to write "'01", that is, a contraction of "2001"--jimh.] with SmartCraft and am always amazed at the port gauge in the paddle wheel mode giving me speed readout. When compared to my Garmin GPS SOG readout they are nearly dead on. That technology never ceases to amaze me. I am contemplating adding the original two of the original analog gauges temperature and tachometer that came with boat from the dealer. I still like the looks of analog readout, although the digital age sure provides some useful data.
jimh posted 05-22-2007 12:14 AM ET (US)     Profile for jimh  Send Email to jimh     
[Separated this article from an older discussion comparing the network topology and complexity of installation between the two CAN-Bus digital instrumentation systems, the open-standard NMEA-2000 and the closed, proprietary standard SmartCraft. The comparison between SmartCraft and non-digital instrumentation systems did not seem to be on-topic for the other discussion.]

In situations where there is no current effect, it should not be too hard to obtain accurate readings of speed over ground from a paddlewheel speed transducer. The biggest influences are the mounting location of the paddlewheel and its calibration setting. I had a LOWRANCE SONAR unit which supported speed measurement with a paddlewheel transducer that was mounted on keel centerline and ran in solid water all the time. After a few tweaks of the calibration I was able to get the paddlewheel readings to agree with a GPS speed reading over a range from idle speed to wide-open-throttle with a maximum variation of about 0.5-MPH.

lurkynot posted 05-22-2007 11:52 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
Jim, I had it on the Detroit Straits this weekend in a current upbound and downboand not in a lake without external forces.
jimh posted 05-22-2007 10:09 PM ET (US)     Profile for jimh  Send Email to jimh     
Are you making a claim that a SmartCraft paddlewheel compensates for current effects and shows speed over bottom?

And are you sure you are really reading the paddlewheel? It would not be the first time someone became confused about what sensor was being displayed.

BarryGreen posted 05-23-2007 08:33 AM ET (US)     Profile for BarryGreen  Send Email to BarryGreen     
On SmartCraft it will say "PADDLE" (or something similar) next to the digital speed display if it is using paddlewheel data.
lurkynot posted 05-26-2007 09:04 AM ET (US)     Profile for lurkynot  Send Email to lurkynot     
I do not think there was any intent of a claim mentioned. I was simply stating a reference of the Smart Craft gauge to the Garmin GPS. Also I am darn sure I was referencing the paddle wheel from the Smart Craft system. Furthermore I am not a hair dresser, and certainly not confused.
jimh posted 05-26-2007 02:37 PM ET (US)     Profile for jimh  Send Email to jimh     
This is an interesting situation. The observation is that a GPS and a paddlewheel sensor used with SmartCraft instrumentation give speed readings which are "nearly dead on" even though the measurements were made "in a current" in both "up bound and down bound" directions.

It seems to me that the paddlewheel must read speed through the water at all times, while the GPS will read speed over ground. Mathematically this means:

If X = speed of boat through the water, and

Y = speed of the current, then

The speed measured by the GPS going upstream would be:

GPS(Upstream) = X - Y

and the measured measured by the GPS going upstream would be

GPS(Downstream) = X +Y

In either case the paddlewheel speed is just

Paddlewheel = X

So the only case where Paddlewheel and GPS can be nearly dead on is that which is when Y (the current) is near zero.

Can you explain the mechanism used in the SmartCraft system which allows its paddlewheel to read the same speed as a GPS even though in current? I would be interested to know how they do that. That is technology that never ceases to amaze me.

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