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Author Topic:   Transducers--SONAR and PaddleWheel
jimh posted 10-23-2001 11:31 PM ET (US)   Profile for jimh   Send Email to jimh  
Transudcers--SONAR and PaddleWheel:
--Some observations--

Over the course of the boating season I installed two new transducers on the transom of my Whaler 20-Revenge. One was for SONAR and the other a speed paddlewheel. I noticed that the performance of these devices was subject to some variation, which I explain as follows:

The SONAR Transducer
The SONAR transducer was mounted at the keel centerline of the boat. This is generally not the location used on single engine boats, because of the presence of the lower unit of the outboard in the same area, but my boat has twin engines, so the center section of the transom is available for mounting the SONAR transducer.

I mounted a LOWRANCE "skimmer" transducer precisely at the keel center line, and set the height of the bottom of the transducer to be just at the depth of the keel. In this position the results were not terrific.

At slow speeds the SONAR worked well, but at planing speeds the transducer did not hold the bottom well and often lost echoes. Fortunately, when drilling the mounting holes I left room for some downward adjustment.

Moving the SONAR transducer down about a 1/4-inch produced a marked improvement in the ability of the transducer to remain effective at higher speeds. The SONAR now works as well at 30 MPH as it does at slow trolling speeds. That slight additional immersion really made a huge difference.

The Speed Paddlewheel
I mounted the speed paddlewheel a few inches off of the keel centerline to the starboard side of the transom, setting the transducer to mount as flush with the bottom of the boat as possible, but due to the radius of the curve of the transom-bottom joint, the transducer frame does not extend below the plane of the bottom, although the paddlewheel does. This puts the axis of the wheel slightly above the plane of the bottom of the boat.

The results from this location were not perfect initially, either. After installing a GPS (with WAAS augmentation) and comparing the speeds indicated by the two devices, the paddlewheel speed was always slow.

Fortunately the LOWRANCE X-65 Sonar permits speed calibration adjustments. To get better agreement, the readings from the paddlewheel had to be increased by 13%. At this setting there was good agreement between the GPS and the paddlewheel at most speeds.

The only speed where there was a discrepancy was around 10-14 MPH, a speed where the boat is no longer in displacement mode but is not yet on plane. The attitude of the hull is pitched very bow-up at this speed, and the transom is dug in. At this speed range the paddlewheel reads higher in comparison to its results at other speeds. When the 13% compensation was applied, it produced readings at this speed range which were too high.

I did some thinking about what might be causing this variation and came up with this:

The paddlewheel tends to indicate a lower speed through the water than the actual speed because of its location very close to the hull-water interface layer. As the boat moves through the water, the hull pulls some water along with it, dragging the water forward with the boat. This layer of water is located close to the hull.

The paddlewheel operates in the layer of water that is at the hull-water boundary, and thus the water going past the paddlewheel has been accelerated forward slightly by the action of the hull going forward. This makes the speed reading sensed by the paddlewheel tend to read lower than the actual speed.

This boundary layer effect persists until the boat's attitude climbs to a very bow high orientation, at which time the water passing the paddlewheel at the transom is making a pronounced bend around the hull-transom curve. The boundary layer effect is broken, and the water passing the paddlewheel is moving (relatively) faster than it would be if still being affected by the boundary layer effect.

Once the boat gets back on plane, the boundary layer effect resumes, and the water passing the paddlewheel retards in speed.

This analysis would explain the variations seen in the paddlewheel calibration and the need to add 13% to the readings to get accurate results.

Other factors that influence the degree of boundary layer effect may be:

--the presence of bottom paint or other roughness on the bottom. A rougher bottom that occurs with bottom paint may drag more boundary layer water along with the boat. This explains the need to calibrate upward on my boat (which has bottom paint) and also explains why a boat with bottom paint runs slower (because it is dragging more water along with it.)

--the longer the hull length, the more the boundary layer sets up, so a boat with a 20 foot hull may experience more boundary layer effect than a boat with a 10 foot hull.

One speed range where the paddlewheel always reads much lower than the GPS is at extremely slow speeds. This may be due to the fact that at these speeds the entire paddlewheel is immersed in water, while at higher speeds only the bottom half of the wheel is in the water. The greater friction that occurs when the entire paddlewheel is immersed accounts for the low readings at the lower speeds.


llewellyn posted 10-24-2001 09:23 AM ET (US)     Profile for llewellyn  Send Email to llewellyn     
I'd like to adjust my Lowrance X85 paddlewheel so that it indicates more closely to the GPS speed(and distance travelled?) readings. How do you do that, please? Did you notice a difference in the distance travelled(trip log) too? Thanks, Lew
bigz posted 10-24-2001 01:12 PM ET (US)     Profile for bigz    
Jim, as you have stated the paddle wheel digital sensor works in the real environment which is composed of wind driven water, currents and tides. Have you factored or rightly stated have factors to adjust for these elements for your calculations ?

Have you also noticed that when sitting idle say at the dock or on the trailer your GPS shows a SOG figure? If so have you calculated this deviation into this comparison?

I have found if the transducer is properly positioned that speed readings at all ranges are on the average no more than of 1 to 2 mph off the GPS with activated DGPS readings whether high or low. The units we have been using are dual freq.. range 50/200, surface water temp and the digital paddle wheel sensor all in one type.

Frankly believe either gives you a pretty good estimate of your speed. Though the GPS seems over all the most reliable.

jimh posted 10-24-2001 01:42 PM ET (US)     Profile for jimh  Send Email to jimh     
For llewellyn: the LOWRANCE software is wonderfully designed and provides easy access to the adjustments for speed compensation. See your owner's manual if you need explicit instructions on how to adjust the particular model of LOWRANCE you have.

For bigz: sounds like you are not buying into my hull-water boundary layer idea. As for effects of wind or current, we did make runs into and against the wind in the process of tweaking the speed compensation of the paddlewheel to match the GPS.

The GPS is a WAAS unit, which claims fairly small dimensions of error in position, which in turn should imply small errors in deduced speed and course.


bigz posted 10-24-2001 02:16 PM ET (US)     Profile for bigz    
Now Jim, you know old bigz almost always buys into your ideas --- key word is almost!
Did take a 10 day canoe trip back in '68 into the "Boundary Waters" I must add -- beautiful area -- ;)

Current WAAS status
I'll stick with our DGPS for now.

One factor also to consider Jim is the shape and construction of the transducer. Both of ours a Raytheon triducer and the other a InterPhase triducer are made by Airmar. These have as I said performed exceptional well -- They are not the same as Lowrance units -- we do use the GM 2000 CP coupled with their GPS and DGPS receivers, decided on a separate sonar unit I didn't care at the time for the Lowrance add on setup components.

jameso posted 10-24-2001 04:08 PM ET (US)     Profile for jameso  Send Email to jameso     
I am running a Zercom Clearwater Pro with temp and speed. After a bit of tweeking (transducer placement) the unit reads within 1 to 2 mph of my gps. It is seemingly accurate at all speeds, I like it because I can read the speed of the tide while anchored.Yes I did several runs and compared winds ect, but with a calm day, inland lake, no tide or currents, over 10 miles each way the unit was very accurate. The transduce ris about 3" off center to starboard.
simonmeridew posted 10-24-2001 09:05 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
on the X-85 unit, press the menu button, go to system setup menu, highlight the calibrate speed label, press the right arrow key. and then:
if the digital speed display is slower than your actual speed, add a positive percentage to make it read faster If the digital speed display is faster(as mine was)than the GPS shows, use a negative percentage to make it read slower. ( I think I needed about -.87 or so)Use the arrow keys to change the speed, then press the ENT key, and Press theEXIT key to erase the menu.
Jim Bennett posted 10-24-2001 09:36 PM ET (US)     Profile for Jim Bennett  Send Email to Jim Bennett     
OK guys, where does the plain old pitot tube type speedometer figure into these comparisons, calibrations, etc. On my 68 Sakonnet I have a Teleflex pitot tube typa and an Eagle Accura 240 Fishfinder with a speed-temp transducer and skimmer(same as Lowrance); this unit also allows for calibration but haven't done it yet but have noticed 2-3 mph difference on the two speedometers. Would like to know if should calibrate sonar speed to pitot speed. Plan to install same transducers and sonar(until I upgrade to GPS) on my OR 21 and forego use of Pitot type.
jimh posted 10-24-2001 11:12 PM ET (US)     Profile for jimh  Send Email to jimh     
TO jim bennett: you must let us know how the pitot tube's accuracy compares to a good GPS/DGPS reading. It would be interesting to have you collect some data. As Mark Twain said, "A man who has two watches never knows what time it is."

Now back to the "boundary water" theory...

In "Oceanography and Seamanship", author William Van Dorn writes regarding resistance and propulsion:

"The arrows [in an illustration] indicate the direction and magnitude of fluid velocities important to propeller action, as measured relative to the vessel's hull. At some distance from the hull, the relative velocity is directed aft and is equal to the ship speed....Close to the hull there is a smaller relative forward flow...called the wake stream, arising from water frictionally entrained by the hull and augmented by the orbital motion of the stern waves."

In an abstract of U.S. Patent 4,086,867 (which can be read on-line at, the inventor states:

"The relative velocity of the water in the boundary layer very close to the hull of the marine vehicle, will always be much less than the velocity of of the vehicle itself."

So I don't think I am completely off the mark with my analysis above regarding why the paddlewheel calibration was lower than actual speeds.

Another point that should be made: in my set up the paddlewheel is very close to keel center line, just inches off center. If any water was affected by the boundary flow effect it would be this water. The farther you move outboard from center line the less of the hull is in the water and probably the less the boundary effect obtains.


jimh posted 10-25-2001 09:21 PM ET (US)     Profile for jimh  Send Email to jimh     
I was reading further in the patent info (cited above) and found it includes an equation which approximates the thickness of the boundary layer of water attached to a hull.

(Too much math in the equation to reproduce here; see the article.)

The boundary layer thickness varies in direct proportion to the wetted hull length upstream of the location.

As vessel speed increases the boundary layer thickness decreases. Also, as vessel speed increases the wetted hull length may also decrease, in turn causing another decrease in the boundary layer.

And the boundary layer decreases as well when the bow of the boat tilts up instead of being level.

All of these relationships work well with my analysis of the paddlewheel calibration results I saw in real-world application.


Dr T posted 10-26-2001 11:30 AM ET (US)     Profile for Dr T  Send Email to Dr T     
When I mounted my paddlewheel to the transom of the 13, I placed it several inches off center. The transom of the 13 has a slight curve, and I would guess that the centerline of the paddlewheel is a couple of degrees off the centerline of the keel. This would introduce a systemic error that would cause it to read low in this application. So maybe I am getting a bit better than the 27 mph indicated at WOT (5400 ft altitude, Johnson 35 hp, 10.5 x 11 prop).
jimh posted 07-22-2004 07:56 PM ET (US)     Profile for jimh  Send Email to jimh     
[Fixed broken hyperlink.]
JohnBT posted 07-25-2004 10:39 PM ET (US)     Profile for JohnBT  Send Email to JohnBT     
I have noticed a very interesting thing about transducer performance on my Montauk. I repowered last year and mounted the new engine on a 9" setback bracket from Stainless Marine.

My transom-mounted depth/fishfinder transducer reads correctly at much higher speeds - up to a maximum of 35mph or so depending on conditions - where prior to putting the engine on a setback bracket, the depthfinder rarely worked at much higher than low planing speeds.

The setback bracket has improved my boat's performance tremendously in several areas, but this improvement was unexpected and therefore I thought it noteworthy.

where2 posted 07-27-2004 05:23 PM ET (US)     Profile for where2  Send Email to where2     
Lots of interesting thoughts, including the boundary surface layer concept. Unfortunately, the boundary surface layer concept fails to account for why my Lowrance reads equivalent to my DGPS until I reach 27mph, and why at 42mph my paddlewheel is reading 18% higher than the DGPS.

Sal DiMercurio posted 07-28-2004 02:25 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Keep in mind the only time a GPS will give you the correct speed "OVER THE TOP" of the water is on a lake where the is no current or horizontal movement of water due to tides.
The gps gives you the correct speed over the ground thats under the water, not the speed you are actually moving over the top of the water.
If you could stop the horizonal movement of the water, then you would have the correct speed on GPS.
My transom mounted Furuno Sonar LS6100 reads perfect [ picture & speed ]at 60 mph.
The bottom of the transducer has to be at least 1/4" below the bottom of the boat, or you will loose it as soon as you get on plane.
where2 posted 08-01-2004 11:03 PM ET (US)     Profile for where2  Send Email to where2     
I'm well aware of the meaning of speeds when it comes to DGPS. I am also assured the lake I was testing in had no drastic wind or tidal effects to account for +18% variation over DGPS speed, and the paddlewheel is obviously in engagement with the water surface if it is reading 18% higher than the GPS at the time of measurement.

My Paddlewheel is separate from the FF Transducer, and I took care to mount it at an attitude in which it would maintain contact with the water surface so long as the hull maintains contact with the water. It's mounted roughly 8" off the centerline of the boat.

For a day job I use survey grade GPS receivers that cost $17k each (Trimble 5700's). The GPS I use at work will put me back on top of your favorite can of beer if we bury it in your yard today and want to dig it up next summer. The Standard Horizon CP150 on my boat is just a fun little toy, and can get me back to a spot roughly the radius of the 15' boat it's mounted on. The CP150 lists your boat speed as Speed Over Ground abreviated "SOG=" in the display, unlike my Garmin 45XL which simply said "Speed=".

jimh posted 08-02-2004 08:12 AM ET (US)     Profile for jimh  Send Email to jimh     
Some models of the LOWRANCE fish-finder/SONAR have calibration controls for the paddlewheel input.

I never had one going over 35-MPH. My guess is that at high speeds like 40-MPH or more, the paddlewheel may not be in solid water, and it is able to spin faster because only the tips of its blades make contact with the water, the rest is spinning in air. The change in resistance to spin due to the much lower resistance of the air would account for the over-speed indication.

I think the paddlewheel is probably best at lower speeds, like those used in trolling. The traditional pitot tube speedometer would be completely insensitive at those speeds, so the paddle wheel speed reading would be valuable.

The availability of speed indications deduced from GPS data makes all other means of speed determination somewhat less important than they used to be.

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