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Author Topic:   Paddle Wheel Speed Sensors: Why So Few
pglein posted 02-12-2007 02:25 PM ET (US)   Profile for pglein   Send Email to pglein  
When I bought my Garmin SONAR five years ago, I paid about $150 for a small basic black-and-white SONAR that included a transom-mount transducer with an integrated paddlewheel speed sensor. At the time I thought that was a lot of money for such a simple device.

I was looking for a friend today and it seems like the cheapest SONAR I could find--looked at several brands, but not all--that would even accept a paddle wheel speed sensor add-on was $200, and you had to pay an extra $50 for the paddle wheel speed sensor, which mounts separately from the SONAR transducer, doubling the amount of work one has to do to install it. What's more, it says it's only good for freshwater use. Mine can be calibrated for either fresh or salt water quickly and easily. To be fair, the new SONAR is a color unit, but it was the only one that said it was speed sensor capable.

I thought marine electronics were supposed to be getting smaller, cheaper, and more integrated, not the other way around.

What gives?

A2J15Sport posted 02-12-2007 03:54 PM ET (US)     Profile for A2J15Sport  Send Email to A2J15Sport     
Why so few? Answer: GPS.
David Pendleton posted 02-12-2007 04:04 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
My Raymarine transducer had the paddle wheel. It was almost impossible to calibrate accurately, so I took it off.

A GPS tells you speed over ground (SOG), which is different than what a paddle wheel is supposed to tell you--speed through the water--but I don't know that it matters.

Use a GPS.

A2J15Sport posted 02-12-2007 04:16 PM ET (US)     Profile for A2J15Sport  Send Email to A2J15Sport     
Paddle wheels are great for detemining "water speed". That is a great asset for those that fish water speed sensitive fish.

Navigationally, they are not accurate for determing speed or position.

GPS is the answer. Just like "airspeed" and "groundspeed" most frequently conflict. Only a good navigator could keep an aircraft on track. Thus came LORAN. A very useable concept and system, before GPS.

pglein posted 02-13-2007 05:52 PM ET (US)     Profile for pglein  Send Email to pglein     
Having a GPS that gives you SOG in no way eliminates the need to know your speed through the water. If I had to choose one or the other, I would much rather know my speed through the water than over the ground. Granted, both are valuable.

Knowing your speed through the water is necessary for fishing, watersports, docking, obeying no-wake zone restrictions, and calculating fuel burn. Aside from that, it is much cheaper to acquire than a GPS.

Knowing your SOG is useful, but only when used in conjunction with a knotmeter to measure current. Lets say you're on your way from point A to point B and then continuing on to point C. Between point A and point B, the current is in your favor by 2 knots. Between point B and point C, it will be against you by 1 knot. If you are cruising along on the first leg at 10 knots through the water, your SOG will show 12 knots. If you didn't have the knotmeter, you wouldn't know that you were picking up that 12 knots from current and might erroneously assume that you would arrive at your destination on time. Knowing that you're picking up that extra two knots, and that you're a knot of speed once you start that second leg can be very important for determinging if you have enough time and fuel to make the trip.

I guess it comes from growing up on larger boats, but I don't buy the argument that a knotmeter has been made obsolete by GPS. You can still calculate your SOG by using a chart and triangulating your position over time.

2manyboats posted 02-13-2007 10:11 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
This is a joke right?

SOG is what gets you there not the speed through the water.

Calibrating a paddle wheel for salt or fresh water?

Tom W Clark posted 02-13-2007 10:25 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Q: Why so few?


hauptjm posted 02-14-2007 11:55 AM ET (US)     Profile for hauptjm    
I think there may some confushion over what is going to give you an accurate assessment of your your boat speed. GPS will always give you your actual speed, not the knotmeter or spedometer.

Using an axample like you own:

You travel through a pass going east while there is a 3 kt. current traveling west. If your Spedo/knotmeter shows 10 kt/mph, you are only traveling roughly 7 kt/mph. If your local constable requires a max speed of 10, you're still under the limit by 3 kt/mph. Likewise, if you travel back out of the pass, and your spedo shows 10, you're speeding because you are actually traveling 13 kt/mph.

hauptjm posted 02-14-2007 11:57 AM ET (US)     Profile for hauptjm    
Sorry for the typos. I hit enter by accident
BOB KEMMLER JR posted 02-14-2007 12:56 PM ET (US)     Profile for BOB KEMMLER JR    
Paddle wheel speedometers and the pitot tube speedometers usually make thier owners very happy, because they are usually reading 10 mph (at WOT) over what they are actually going. I think a lot of people are using these types of speedometers when giving thier speeds and saying they used a GPS because they assume they are just as accurate.On my 15 my "happy" speedometer is reading 58-63 mph while my GPS is reading a little over 50 mph.
bluewaterpirate posted 02-14-2007 01:20 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
In nautical terms it called set and drift. That's why you use a paper charter to plot a DR. The plotter positons would give you course/speed made good then you would determine a compensating course. That's what auto pilots compute. The Track Crossing Angle (TCA) is the term used in the owners manual.


fishnbanjo posted 02-15-2007 09:08 AM ET (US)     Profile for fishnbanjo  Send Email to fishnbanjo     
Well I currently have a BottomLine Tournament and had a Hummingbird prior and both use paddle wheel speed indicators. I also have a speedometer and tachometer on my 13'6" SS and a GPS so I have more indications of speed then necessary. The speedometer and GPS generally indicate the same speed with the GPS more definitive due to it being digital and the BottomLine speed I use more for trolling speed indication since neither the GPS or the speedometer are accurate enough under 5 mph to be useful.
JMARTIN posted 02-15-2007 12:16 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
I have a paddle wheel, but have not dialed it in yet to the GPS. The only reason I would use it would be for determining trolling speed. Are paddle wheels very accurate at slow speeds? John
simonmeridew posted 02-18-2007 01:24 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
I have found that there is a calibration factor in the fishfinder/paddlewheel done electronically via the software in the fishfinder. However in comparing with the GPS speed, the calibration is not linear, so that you should pick the speed range that you want accuracy with the paddlewheel. For example the paddlewheel can be made quite agreeable with the GPS at 5 MPH but will be off by a percentage at say 20 MPH, and more so at 30 MPH. But the unit can be calibrated to be accurate at 20 MPH but would not be reliable at 7 MPH, by the GPS. Of course river flow or tides will make you make a bidirectional test. Nothing new there.
jimh posted 02-18-2007 01:29 PM ET (US)     Profile for jimh  Send Email to jimh     
This discussion of small boat electronics has moved to the SMALL BOAT ELECTRICAL discussion. Please use the SMALL BOAT ELECTRICAL discussion for topics involving electrical or electronic devices used on small boats.
jimh posted 02-18-2007 02:27 PM ET (US)     Profile for jimh  Send Email to jimh     
My last Lowrance SONAR had an option for a paddle wheel speed sensor. I installed one. The SONAR had electronic calibration adjustments to help improve the accuracy of the paddle wheel reading. After some experimentation I was able to get the paddle wheel and the GPS to agree on the boat speed (in water with little or no current) to within 0.5-MPH from dead slow to wide open throttle. I sold that gear with the boat, and don't have a paddle wheel on the new boat.

The cost of a GPS receiver which will provide simple speed measurements has fallen to about $50. This very low price had made use of a GPS on small boats very common, and this, in turn, has probably reduced the interest in boat speed measurements using a paddle wheel sensor.

David Pendleton posted 02-18-2007 06:11 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
My paddlewheel speed sensor was also adjustable via the Fishfinder controls, but I could only get it to read accurately under certain circumstances, e.g. very slow or very fast speeds. Never from idle to plane.
aja posted 02-20-2007 12:58 PM ET (US)     Profile for aja  Send Email to aja     
From my experience, paddlewheels work well at low speeds but tend to lose accuracy as the boat speeds up and planes. This is especially true depending on the placement of the transducer around chines, water flow off the transom etc. Although the absolute reading may not be entirely accurate, it is consistant enough to give you a relative idea of what you are doing through the water, taking into account effects by wind, current waves etc which is essential for trolling.
We have both a GPS and paddlewheel on our sailboat and find the GPS speed reading is nowhere near as sensitive as the paddlewheel to changes in steering, sail adjustments, sea state etc. When motoring along in calm conditions with no current they read the same, but when variables are tossed in the paddlewheel speedo is the only one to look at. Of course the values of the GPS come up as a chart plotter and to display VMG and other tactical info, but that is another topic...

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