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Author Topic:   Speedometer Comparisons Revisited
Jim Bennett posted 07-22-2004 07:00 PM ET (US)   Profile for Jim Bennett   Send Email to Jim Bennett  
A few years ago there was a thread regarding comparison of transducers - sonar vs. paddlewheel - for speed accuracy.

I would be interested in knowing if anyone has made comparisons of sonar vs. paddlewheel vs. pitot tube. I believe, as related by others in previous thread, paddlewheel readings were somewhat less than sonar/GPS. My two Whalers are still configured as before - 68 Sakonnet with pitot tube (Teleflex) and paddle wheel(Eagle Accura 240 with Lowrance Skimmer/S/T) and 73 Outrage 21 only with the paddlewheel setup. I have separate but same transducers on both boats and just transfer the Eagle readout box. Recently, with reconditioned prop and freshly waxed hull, my Sakonnet clocked 39-40mph on paddlewheel and 44-45mph on pitot tube(Merc 115 at 5000RPM WOT). My Outrage (with Merc 150 at 5800RPM WOT) hits 48-49mph by paddlewheel reading only. Am I really over 50mph with Outrage? Hope to have someone with GPS calibrate me at East Tennessee Rendezvous in September. Meanwhile would appreciate comments.

jimh posted 07-22-2004 08:00 PM ET (US)     Profile for jimh  Send Email to jimh     
[Fixed broken hyperlink.]
LHG posted 07-23-2004 03:48 PM ET (US)     Profile for LHG    
Jim - those readings on the Outrage sound about right. I now have one with the 2.0 liter 150, smaller block than yours, and that's about the speeds I'm getting. I do know, however, that paddle wheels tend to "float" around 50, so I would guess the reading off the engine's speedo water intake, ON A MERCURY GAUGE, would be more accurate. Then, of course there is DGPS for the best accuracy, but it takes a few minutes to get an accurate reading.
JohnJ80 posted 07-23-2004 06:23 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I took a shot at calibrating my paddlewheel on my Fishfinder, my pitot tube off my motor (Evinrude), and my GPS.

There really is no calibration that works - both of the other methods are strongly nonlinear with respect to the GPS.

The Pitot tube reads nothing until about 10mph. Then it seems low (using GPS as standard) until about 35mph. From there it goes high until it is 7mph high at about 40mph.

The fishfinder seems very accurate above 2mph and under 10mph but it gets flaky above about 30mph. Eventually being 20-30% off at the top end.

best I can tell, all of the in the water methods are impacted by the hyrdodynamics. If you tilt the motor, depending on where you mount the paddlewheel - all of that matters and some of it is variable.

So, the best way to figure this out is to use a GPS over a long course and then repeat back along the recipricol (sp?) course and average the two readings.


LHG posted 07-23-2004 06:35 PM ET (US)     Profile for LHG    
Many do not know that air pressure activated Speedometers, with the water being picked up by either separate plastic pitot tube or engine water pickup (such as Mercury's have), have to be matched to the pickup style.

That's why I indicated a Mercury Speedo is necessary for an accurate reading off a Mercury lower unit water pickup. It seems that the engine water pickup is much more efficient than the separate pitot pickup, and hence requires an OEM calibrated Speedo to work with it to avoid a higher-than-actual speed reading. Putting an aftermarket Speedo, such as Teleflex, on an installation with the engine water pickup, will give you considerable higher, inaccurate, speed readings above 25 mph.

AQUANUT posted 07-24-2004 07:51 AM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
I have a magellen marine gps, mercury speedo guage[ attached to my 2004 mercury 115hp EFI 4s, and a lowrance speed wheel on my transom of my 2004 montauk 170.[attached inline with the strake.]

I use the magellan as my reference point.
I recently upon clompletion of installing a furano 1850 with dgps, used my magellen on the lake at the same time...readings were identical

My speed wheel on my lowrance wheel is usually off, depending on actually speed I am traveling, sometimes as much as 3mph.

my mercury speedo is usually useless unless I am excedeing 35 mph.
thats the way it is here

Jim Bennett posted 07-24-2004 06:46 PM ET (US)     Profile for Jim Bennett  Send Email to Jim Bennett     
Larry, John, and Aquanut,

Thanks for your responses. I found them not only informative but when you take all the info together, it's also quite humorous and makes one wonder if we ever know what speed we are going LOL!!

For what it's worth, my pitot tube/Teleflex speedometer on Merc 115 is a separate pitot and not the engine's intake as Larry recommends. Actually, Larry, does my old 79 115 have an intake for a speedometer? Also, when you say my Outrage paddlewheeel numbers correspond with what you are getting on your "new" Outrage, do you mean paddlewheel or GPS?

John, your observations with pitot vs paddlewheel are pretty simular to what I see on my Sakonnet with the 115Merc. Thjey both track together pretty well until around 30mph then pitot tends to get higher above that speed. My 115 does not tilt so I am at full down position with my measurements.

Aquanut, is your Lowrance paddlewheel speed 3mph higher or lower than Magellan reading? Also good to know that Magellan tracks with your Furuno 1850 DGPS. I have Furuno 1650 DGPS on my FourWinns 248 in Florida and have been curious how accurate it is.

All in all, I got more than I bargained for with this question. Hope you all found this as informative and humorous as I have. Really do hope to calibrate one or other of my boats with someone's GPS at our rendezvous!

JohnJ80 posted 07-24-2004 10:30 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Knowing what speed you are going on the water IS a problem.

There is someone who now makes a speedo, looks like a teleflex one, but it is really a GPS! Funny.

You really need to know to speeds, your speed over the bottom for good navigation and your speed through the water (depends on direction of current against your speed). For example, you could get a nasty surprise if you boat in areas with a current and didn't know it - like when you use 50% of your gas going downstream and really need another 25% to go back upstream and there are no marinas around. Oops!

If you calibrate with a GPS, you will only be able to calibrate for a small range on whichever mechanical unit you select.

The next fishfinder I get will have a GPS built in - they don't cost that much more - and to heck with the instrumentation (speedo). Tach, trim gauge and voltmeter are useful but the speedo is basically a waste of money.


dfmcintyre posted 07-26-2004 08:43 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
If your seriously thinking about an accurate speedometer reading, look for the water type used on competition water ski boats.

Way before loran and GPS, I mounted the Airguide (I think) units on a Montauk and Outrage. Gave me damn accurate readings.


AQUANUT posted 07-27-2004 12:01 AM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
interesting comments don,

also interesting enough is the fact that most high end competition ski boats such as ski natique, ski centurian,
sanger,moomba, malibu, sky supreme,etc, usually have two speedometers. I.E. [not all of these are considered high end]

is it because 1 is so accurate?

incidently many or them are adjustable from the helm seat.

times change, new technology hopefully improves accuracy

ah yes...but I "remember, in the day"


Jim Bennett posted 07-27-2004 10:59 AM ET (US)     Profile for Jim Bennett  Send Email to Jim Bennett     
Thanks to all for further valuable info.

John, I concur with your opinions on stream current effects but that probably not big issue on my big lake. Also agree that calibrating with GPS will probably only be good for small ranges on mechanical unit. I intentionally didn't install speedo on my Outrage to (1) conserve console space and(2)already planned to use Eagle/Lowrance FF with paddlewheel on both boats. It is the discrepancy I observed with paddlewheel vs. pitot on my Sakonnet that made me wonder what really have with Outrage.

Don, I presume the Airguide speedo you refer to is a newer technology system. Funny, when I stripped and revarnished my Sakonnet console , 6 - 7 years ago, I am pretty sure I replaced a broken Airguide pitot speedometer with the Teleflex one - uses similar pitot tube.

Terry and Don. We seem to have plenty of the waterski/wakeboard type boats in area. In fact, Mastercraft has factory just few miles up lake from me (SeaRay also).
Maybe I'll just motor up to a skiboat someday in my Outrage and see if they'll "clock" me; i.e. if they can keep up with me HA!!

dfmcintyre posted 07-29-2004 03:45 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Terry -

Drawing back into some memories of three decades or so, when I did some serious waterskiing with a guy who went professional... I think that it wasn't or isn't an accuracy problem for needing two, it is something about having one still reading if one drops off line due to picking up debris in the water.

There was also a 3/4" x 8" brass tube mounted on the transom that the water pickup is plumbed to, and off the top is a fitting that connects to the hose running to the speedo. And the speedo is adjustable.


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