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Author Topic:   Accuracy of SPEED Measured with GPS
jimh posted 09-05-2006 08:58 AM ET (US)   Profile for jimh   Send Email to jimh  
[This discussion strays far off-topic. For a more concise look at GPS speed accuracy I recommend another discussion on this same topic.--jimh]

Yesterday I was conducting a little drag race with Steve and BACKLASH. We both ran our boats up to wide-open-throttle in relatively flat water, and found there was only a slight difference in speed. After the test we compared the speed measurements we obtained from our GPS receivers. I observed on my GPS that we were going about 38.5 to 39-MPH. Steve observed on his GPS that we were going about 44-MPH. I was quite surprised at the 5-MPH difference in speed deduced by the GPS receivers. (Curiously, the YAMAHA pitot tube speedometer on BACKLASH was in very good agreement with my GPS reading.)

My GPS is a current model Standard Horizon CP-150. It is a 12-channel receiver with WAAS enhancement. During the test the Horizontal Dilution of Position error (HDOP) was about 1.1-feet. [With the benefit of seven years of addition study, I now realize that the HDOP is a figure of merit, not a dimension in feet--jimh]

Steve's GPS is an older LOWRANCE unit with differential GPS (DGPS) enhancement. I believe it is a 5-channel unit. I am not certain if the DGPS receiver had a correction signal or not.

A speed of 39-MPH is


A speed of 44-MPH is


Thus the differential speed between the two boats would be about 7.3 feet per second. I do not believe that there was that much difference in speed. We were more or less keeping pace with each other, although BACKLASH may have had a slight speed advantage. I would judge the difference to be less than one foot per second. That is a difference in speed of


From this brief test I was made aware of a potential for error in speed measurements by GPS receivers. It was quite surprising to see such a large variation between two devices. This brings to mind Mark Twain's observation:

"A man who owns two watches never knows what time it is."

[This citation may be in error. Many sources now cite a different author for this familiar quotation.--jimh]

For many years I have been observing that my boat could only reach 40 to 41-MPH, and, to tell the truth, feeling a little bad about that. Now I find that perhaps all this time what was holding me back was my GPS!

an86carrera posted 09-05-2006 09:05 AM ET (US)     Profile for an86carrera  Send Email to an86carrera     
I am surprised by this as well. I wonder if it is the sampling rate or the location accuracy that caused this.

This is really going to throw out any speed data we have all taken for gospel.


Sal A posted 09-05-2006 09:33 AM ET (US)     Profile for Sal A  Send Email to Sal A     
I should be spanked for suggesting this, but assuming you were going about the same rate of speed, doesn't 38.5-39.0 knots/hour equal about 44 MPH? Could it be that the setting on your GPS receiver is for knots/hour and not MPH?
Moe posted 09-05-2006 09:38 AM ET (US)     Profile for Moe    
The difference is mighty close to that between nautical miles per hour (38.5) and statute miles per hour (38.5 x 1.15 = 44.5 kph). Could the GPS units have been set to read different units?


kingfish posted 09-05-2006 09:38 AM ET (US)     Profile for kingfish  Send Email to kingfish     
Hmmm...been there, done that-

What do you think, maestro?? ;-)

Moe posted 09-05-2006 09:39 AM ET (US)     Profile for Moe    

38.5 x 1.15 = 44.5 kph should read 38.5 knots x 1.15 = 44.5 mph

jimh posted 09-05-2006 10:54 AM ET (US)     Profile for jimh  Send Email to jimh     
The notion of a confusion between statute and nautical miles per hour came immediately to my mind, too, and I checked the settings on my GPS receiver. It was set to statute miles and miles-per-hour. Perhaps it is a software glitch. I will experiment with changing them and see what happens.
crabby posted 09-05-2006 11:22 AM ET (US)     Profile for crabby  Send Email to crabby     
Man with one GPS always knows how fast he is going, man with two GPS units never really sure...
erik selis posted 09-05-2006 12:20 PM ET (US)     Profile for erik selis  Send Email to erik selis     
Great subject Jimh !

I have often wondered about the accuracy of GPS speed measurement. Not only in the marine environment but also on navigation instruments for automobiles. My buddy has a built in navigation system in his car (Becker)and I have a mobile system in my car. I once took mine along with me when driving in my buddies car and compared the displayed speed on both units. Much to my surprise there was a difference of 8% at 120km/h (about 75MPH). I have no idea who's was correct. The speed on the speedometer was in-between the difference. (4% off of both measurements).


highanddry posted 09-05-2006 12:49 PM ET (US)     Profile for highanddry  Send Email to highanddry     
My Humminbird 987c GPS, Magellon Sport track, Garmin Pilot III (aviation model) and my Delorme GPS running on my lap top all agree within one MPH at any given moment which BTW generally agrees prety close with my car speedo and the SmartCraft display, rarely more than a few MPH difference there.
JBCornwell posted 09-05-2006 12:52 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
One unit displaying knots and the other displaying statute mph would make sense.

It is also possible that the DGPS was intermittent or not working at all. That should have produced erratic readings on Steve's unit.

There is another possibility. GPS measure location in 3 dimensions, thus vertical as well as horizontal distance and speed. Could one of the boats have been on the edge of porpoising, thus moving up and down as well as forward?

If none of those possibilities is the case, I am baffled, too.

Red sky at night. . .

JOHN W MAYO posted 09-05-2006 01:13 PM ET (US)     Profile for JOHN W MAYO    
I have compared 2 Garmin GPS many times in side by side operation. Almost always they showed almost identical. The update rate is once a second, so I could see how they show a slight split between the two, but not a major differance.

If there was an antenna mounting interferce some with one of the units, this might account for a less than accurate reading maybe.

Backlash posted 09-05-2006 01:14 PM ET (US)     Profile for Backlash  Send Email to Backlash     
I too was surprised by the GPS speed differences. My GPS is an older Lowrance LMS 350A, 5 channel unit without DGPS enhancement.

I have previously checked my WOT speed with my Garmin handheld, 12-channel, with WAAS enhancement and observed 42 MPH at 5700 RPM with a light load. Yesterday with a medium heavy load I was only able to reach 5500 RPM. I did not play with the engine trim at all. At a steady 3500 RPM, my speedometer was showing 25 MPH, while the GPS showed 26.5 to 26.7 MPH.

I can only conclude from this test that Jim's 12-channel GPS is probably much more accurate than my old 5-channel unit.


Backlash posted 09-05-2006 01:27 PM ET (US)     Profile for Backlash  Send Email to Backlash     
I think I have solved the mystery of the GPS speed differences.

The manual for my LMS-350A shows under CALIBRATE SPEED:

The speed display on the sonar screens comes from the optional speed sensor - not the GPS receiver.

So the speed is actually coming from the paddle wheel!


davej14 posted 09-05-2006 02:37 PM ET (US)     Profile for davej14  Send Email to davej14 with two or more GPS' still knows what speed he is going.
kingfish posted 09-05-2006 02:42 PM ET (US)     Profile for kingfish  Send Email to kingfish     
My GPS (Garmin 235 Chart Plotter) reads STW (speed through water) and SOG (speed over ground). Speed through water is sensed from the paddle wheel and speed over ground is sensed from the GPS (and in my case, DGPS). There is an adjustment procedure to bring the STW to a higher level of accuracy, using readings from SOG at different speed ranges.


swist posted 09-05-2006 05:48 PM ET (US)     Profile for swist  Send Email to swist     
Slightly off topic, but I boat in an area of considerable currents, tides, and winds (Maine coast). The difference between speed over ground and speed over water is pretty useful information. But the only way I know of gettting speed over water is with a low-tech pitot or paddlewheel pickup. Would be nice if there was a more accurate way - boat speedometers are famous for telling lies.

On the other hand I might observe that airplanes have the same problem in an even more unforgiving environment. Their airspeed indicators are pitots, and clearly are better equipment than your avergae boat.

bsmotril posted 09-05-2006 05:53 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Some GPS have a user configurable parameter for "Averaging" which is used for speed calculations. It sets the number of samples the GPS takes to calculate a speed. You'll get best results with is set to FAST or HIGH. Many older models have this feature because they were CPU constrained and if you set the averaging to slow or low, you could then get faster updates to the course or crosstrack indicators. BillS
23walkaround posted 09-05-2006 10:18 PM ET (US)     Profile for 23walkaround  Send Email to 23walkaround     
[CHANGED TOPIC TO COMPLETELY UNRELATED DISCUSSION] The speed of the whaler hull designs has impressed me, I have a '92 23' Walkaround with hard top, canvas, galley and head. A few weeks ago while in the San Juans with my wife we were going to our moorage at Lopez Isl. from Friday Harbor after having dinner with friends. Sea conditions were mirror flat so I opened it up all the way trimmed it just shy of porpoising and my Raytheon Pathfinder GPS plotter/radar held steady at 57 knots. this was with 130 to 150 gallons still in the tank all of our fishing and cruising gear, six cases of Heineken and a fat golden retriever. When I got home I took it out on Lk Washington on flat water and about 40 gallons of gas left, nothing in the boat but myself and it went 63 knots. It is a blast to blow the guys in their competition ski boats out of the water in a "big ol fishing boat".
kingfish posted 09-05-2006 10:29 PM ET (US)     Profile for kingfish  Send Email to kingfish     
57 knots and 63 knots in a 23 WA? Ain't buyin' it, buddy - that's 65 and 72 MPH respectively, and that just isn't going to happen.


Tom W Clark posted 09-06-2006 10:58 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
63 knots? Hahahahahahahahahahahahahahahahahahahahahahahahahahahaha!


If you're not measuring speed with a GPS, don't bother. Those paddle wheel speed sensors are a joke (unless you are trying to measure trolling speed). Like the old fashioned pitot tube speedometers of years gone by, they are just about worthless for accuracy.

Sal A posted 09-06-2006 01:01 PM ET (US)     Profile for Sal A  Send Email to Sal A     
[GETS BACK ON TOPIC OF SPEED MEASUREMENT ACCURACY AND TECHNIQUES] I got into boating only fairly recently, so please forgive the following dumb question. What is a pitot and how does it work to determine your speed? What is a "paddlewheel" and how does it work to determine your speed? All I have ever known is GPS measured speed.
Binkie posted 09-06-2006 01:27 PM ET (US)     Profile for Binkie    
Competition ski boats, (inboards) are not propped to go very fast, maybe 45 mph, they are propped to pull a few skiers at 35-40 mph. How many cases of those Heinekens did you drink?
kingfish posted 09-06-2006 01:46 PM ET (US)     Profile for kingfish  Send Email to kingfish     

A pitot tube is a small rubber or plastic tube, maybe 1/8" inside diameter that runs from the backside of a hole that faces forward, either on the front of your motor or on the front of a bracket that extends below your transom, to what is effectively a pressure guage that is calibrated as a speedometer on your dashboard. More speed, more pressure, higher MPH readings on the speedo.

A paddle wheel is exactly that, a paddle wheel that is mounted on the back of the transom, low enough so the water running under the hull turns it, and the MPH readings on the speedometer are based on the number of revolutions of the paddlewheel.


Sal A posted 09-06-2006 01:51 PM ET (US)     Profile for Sal A  Send Email to Sal A     
Thank you Kinkgfish.
crabby posted 09-06-2006 08:46 PM ET (US)     Profile for crabby  Send Email to crabby     
My apologies for my earlier non-informative post...

I have had the opportunity to use many different handheld GPS units since the early 1990's and I have taken all of them out on the road to check them out for speed accuracy relative to my vehicle's speedometer. My original Magellan5000 was not very good (too long ago to remember the details, but it was truly a POS and it cost me over $1000US back in 1994).

More recently my old trusty Garmin48 (circa late 90's) has always tracked very close to what my speedo said. Ditto for a GarminMap76, a Garmin etrexVista, and a Garmin72 (all circa 2003). There is (as someone mentioned previously) an update lag with the G48 (it is quite outdated compared to the new models) but once you hold a steady speed for about 5 seconds the GPS computer catches up. The lag is not quite as noticeable on the newer units.


jimh posted 09-06-2006 08:47 PM ET (US)     Profile for jimh  Send Email to jimh     
Pitot tube speedometers are used on aircraft, too. They deduce speed from measurement of a pressure differential.

Paddlewheel speedometers usually are electrical. The paddlewheel contains magnets which induce a pulse in an attached coil. The faster the paddlewheel spins the higher the pulse frequency. Various electrical techniques are used to derive a speed reading from the pulse frequency.

Both of these techniques can be quite accurate if carefully implemented, but the devices used in recreational boats are not generally produced and calibrated with enough precision to be as accurate as a run-of-the-mill GPS.

where2 posted 09-06-2006 09:08 PM ET (US)     Profile for where2  Send Email to where2     
Anyone care to decipher the clock algorithym used to compute positions from 4 or more moving satellites in real time? (I've actually had a class in college which discussed this particular topic) That is exactly what is going on inside a GPS receiver... It knows precisely how fast it is going. Whether or not you trust it's output is another story.

I'm willing to trust my GPS receiver's computations far more than any other source of speed on my boat. Then again, I work with GPS receivers that cost $30k and am quite familar with the pitfalls and shortcomings of GPS.

RJG posted 09-06-2006 09:32 PM ET (US)     Profile for RJG  Send Email to RJG     
My 1988 22 Revenge WT/WD with a 1988 225 Yamaha Excel tops out at 40-41 MPH measured with my GPS. Yamaha SWS 15 1/4 X 15 prop at 5800 RPM. Sorry Jim, but your GPS seems to be right on. I,too, thought I would get a little more top end with a 225.
swist posted 09-07-2006 08:09 AM ET (US)     Profile for swist  Send Email to swist     
Jimh - I was going to make the same point. 747s use pitot airspeed indicators. The measurements can be very accurate if the equipment is good, which it has to be on a plane where airspeed is as important, if not more so, than groundspeed.

But with the advent of GPS, and groundspeed being good enough for most recreational boating purposes, it is not likely anyone would bother making a high-accuracy marine unit that measures speed over water.

jimh posted 09-07-2006 01:42 PM ET (US)     Profile for jimh  Send Email to jimh     
My understanding is that boats used in water skiing competition have rather accurate pitot tube speedometers. The competitors all have to be towed at the same speeds through the water.

Also, somewhere I recall reading an accident report on a airplane near disaster in which the culprit was found to be a blockage in a pitot tube sensor. It was throwing off the plane's computer controls.

As for the GPS, I am actually somewhat pleased to learn that there was some non-GPS problem on BACKLASH contributing to the variance in the speed readings. I do feel sorry for Steve--losing 5-MPH off your top speed is never a good outcome! :-)

We'll have to repeat the test and see how the two GPS-derived speeds compare in the future.

jimh posted 09-07-2006 09:15 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is some information on a pitot tube speedometer problem which nearly caused a crash of a Peruvian airliner:

It appears that pitot tube speed measurement devices are routinely used in modern aircraft.

swist posted 09-08-2006 07:45 AM ET (US)     Profile for swist  Send Email to swist     
Yes they are. I know nothing of the technical issues, but it does seem like a crude device for measuring an important parameter in a commercial airliner. I do believe that in some modern aircraft, there are multiple airspeed pitots, and an alarm goes off if they disagree, which gets around clogging or certain other failures of just one pickup.
Peter posted 09-08-2006 12:09 PM ET (US)     Profile for Peter  Send Email to Peter     
Garmin's specifications state 0.05 meters/second (0.11 MPH) steady state is the accuracy one should expect for the speed readout based on GPS position determination under their assumptions on typical signal quality. What that means is that two GPS units side by side on the same boat should display the same speed to within 0.1 MPH.
kingfish posted 09-08-2006 12:28 PM ET (US)     Profile for kingfish  Send Email to kingfish     
If each GPS were accurate to 0.11 MPH, couldn't then one be reading 0.11 MPH high and the other reading 0.11 low, so the difference between them could be as much as 0.22 MPH?

Not to be picky- ;-)


Peter posted 09-08-2006 01:05 PM ET (US)     Profile for Peter  Send Email to Peter     
I suppose that is true, it is +/- 0.11 MPH. You'll probably never be able to truly see the difference in accuracy unless both are updating in-sync.
highanddry posted 09-08-2006 01:42 PM ET (US)     Profile for highanddry  Send Email to highanddry     
Since I happen to be, among other things, an A&P and sometimes work on aircraft including the various bizjets---a typical Cessna Citation X, the worlds fastest bizjet, uses three pitots. One is for the pilot side display, the second is for the copilot display and the third is for the standby display. The pitots provide their data to an ADC(air data computer) which calculates speed and other things and displays it as a tape on the three MFDs (multi-funtion displays). All aircraft that I know of use a pitot/static system as primary reference to their speeds through the air and altitude. While not absolutely accurate--think about it--the pitot sees the same air conditions as the wing and it is the wing that is important--not absolute accuracy. Therefore all flight manuver related speeds are from the pitot systems. Other speeds are provided by DME (distance measuring equipment) which is gound based VOR and now of course GPS. There are another system called AHRS which is attitude, heading and rate and it is used to provide aircraft position such as pitch, roll, heading, ROC etc. or older aircraft may have various gyro instruments for this function.

The pitots are heated and yes on rare occasion one can be plugged thus the redundancy. Techinically, airplanes don't care about ground speed. The wing needs airspeed and that is the data provided by the pitot and is super critical to the pilots. Imagine landing with a 50 kt tailwind and the stall speed is 110 kts and approach is usually 1.3 times that and trying to use a GPS to provide AIRSPEED--no way----the pilot would see a approx 150 MPH ground speed but the wing would see only 100 knts and --can you say STALL? BUT--the good old pitot does not care about any that--it only sees the speed of the aircraft via pressure in relation to the air through which it moves and therefore --by it's nature and operation and simple physics---only provides AIRSPEED, the speed of the aircraft through the air. The DME, GPS and on some aircraft Inertial Guidence are used to calculate ground speed--for navigation purposes but for flight purposes--airspeed by pitot/static system.

Oh, I am a pilot with 3500 hours, PP, Inst, Comercial. I work on these everyday.

chopbuster posted 09-09-2006 12:02 PM ET (US)     Profile for chopbuster  Send Email to chopbuster     

"They deduce speed from measurement of a pressure differential."

Inanimate objects do not deduce, "to deduce" is a function of intellect and reasoning.

Pitot tubes do however "measure".

runpasthefence posted 09-09-2006 04:17 PM ET (US)     Profile for runpasthefence  Send Email to runpasthefence     
This was posted on another [internet] forum site back in January. Think it's pretty relevant.

[Removed the cut-and-paste which was made without any proper citation of its source. The information it contained, a mention of the intentional dither of positional information from the GPS system in order to spoil its accuracry ("selective availability") was not believed to be current. The information provided was fourth-hand at best: a guy quoting a guy who was quoting a manufacturer's representative quoting the government.--jimh]

Tom W Clark posted 09-09-2006 05:42 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Oh brother. This is why it is so silly to quote other forums or other second hand [actually fourth-hand] information. It is often bad.

Selective Avalability (S/A) has been turned off for years!

Royboy posted 09-09-2006 10:15 PM ET (US)     Profile for Royboy  Send Email to Royboy     
Let me see if I have this straight: two boats are travelling at different speeds measured on different speed measuring devices and someone is at a loss to explain why the readings are different? Let me be the first to propose a possible explanation:

The two boats are travelling at different rates of speed at any given moment.

Now, if the two speed measuring devices were on the same boat, we might have a conversation about the accuracy of the two devices. When they are on different boats there is no possible correlation between their readings, because at any given moment they are indeed travelling at different speeds. That fact that one or more of the participants thinks they are going the same speed is irrelevent.

Did I miss something here?


bluewaterpirate posted 09-09-2006 11:11 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Roy .....

This is one terrific thread!

It began with 2 similar Whalers drag racing with an observed 5 mph difference in speed both who thought they were using GPS's to measure SOG (sorry Jim for using abbreviations). Then the plot thickened with the possibilty the boats might be using different speed measurements (knots -vs- mph). But thanks to the truthfulness of one the drag racers the difference was found to be a paddle wheel sensor that had been used to measure SOG (mystery solved, BUT). The story got even better, a Whaler that could do 63 knots (followed by a BS call) which was soon followed by a plane crash and finally a military issue brought about by some poor sucker who posted some inaccurate info from another board. Thank God someone pointed that out.

Can't wait to see where this thread goes next. Maybe a classic Whaler armed with cruise missles & a GPS thats positional data is only good to plus or minus 11 feet.

WoW (semi-shout) ..... one last idea. Maybe someone can find a way to incorporate the ongoing Boozin & Cruisin Foodfight Thread into the story!

Cheers .......

jimh posted 09-09-2006 11:15 PM ET (US)     Profile for jimh  Send Email to jimh     
When inanimate objects are formed into a measuring device, the designer of the device imparts his intellect into the device. That is how the device is able to deduce the measured quantity from the physical action that occurs. This is especially true when the device contains a processor which executes stored code instructions. The intellect needed to make the measurement is stored in the code.

You can say that a anemometer measures the wind speed by rotating in proportion to the speed of the wind. When that rotation is connected to a gauge and a pointer that show the wind speed on a scale, it is the intellect of the person who designed the instrument that measures the speed.

That is how the speed is deduced from the measurement of pressure differential. It is not a direct measurement of speed at all. The speed has to be calculated or deduced from the pressure differential.

swist posted 09-10-2006 07:26 AM ET (US)     Profile for swist  Send Email to swist     
Wow, what an elitist view of measurement techniques :-)

Also do not forget the Heisenberg uncertainty principle, which has an important corrolary that it is impossible to measure something without perturbing what you are measuring. In this case some of the forward energy of the boat is being diverted to apply pressure into the pitot tube. So if you have identical boats and identical engines, but one has a speed-over-water measurement device and the other does not, then the former will go slower.

jimh posted 09-10-2006 09:06 AM ET (US)     Profile for jimh  Send Email to jimh     
I am surprised at the trend the discussion has taken. The topic now seems to be more on the course of the discussion itself--a sort of meta-discussion--than on the original discussion, the accuracy of GPS speed measurements. But I won't put a stopper on it, it is sort of fun. Let's see how the critics are doing:

"Let me see if I have this straight: two boats are travelling at different speeds measured on different speed measuring devices and someone is at a loss to explain why the readings are different?"

This analysis was going well right up to the point where the critic wrote

"...and someone is at a loss to explain why the readings are different?"

No, there is no lack of understanding why the measurements are different. This is the entire focus of the inquiry. Let me explain:

The underlying assumption has been that measurements of speed by GPS techniques were very accurate, so much so that it was folly to question them. An example was provided in which there was a gross error in measurement of speed via GPS techniques. Further investigation allowed for the discovery that the error was not due to variation in GPS accuracy at all, but was caused by another situation. It remains for further testing to discover if two GPS units will measure the same speed for the boats involved in the test.

Next we have a summation of the thread which speculates on where it will go next. This critic correctly summarizes that the discussion of the thread has been changing, however, in terms of his prediction on where the next topic of the thread would turn, this critic is in error. He fails to anticipate that the next topic of the thread would become a discussion of its critics.

Thus we see that both these critics of the discussion have failed to contribute accurate information to the discussion.

chopbuster posted 09-11-2006 12:51 AM ET (US)     Profile for chopbuster  Send Email to chopbuster     
Reminds me of the current inane IBM boob tube commercial that depicts a some what anal retentive and white collar type lamenting that the "servers wish to serve us" imparting a science fiction like organic animation to its function.

Buckda posted 09-11-2006 01:06 PM ET (US)     Profile for Buckda  Send Email to Buckda     
JimH -

We should have tested this with the four boats we had on Lake Erie on Saturday. I'm sure the results of that test would have certainly introduced a new dimension to this discussion!

jimh posted 09-11-2006 01:42 PM ET (US)     Profile for jimh  Send Email to jimh     
I guess the full moon brought out all of the critics. I am not sure why this discussion seems to have earned all this attention.

Dave--It was too rough Saturday for testing. We want to pare this accuracy down to the 0.1-MPH range. There was so much vertical motion on the boats that it would have affected the speed computations. With the wave height on Lake Erie (2-4 footers) we'd have needed a SPORT 150 to take on those monsters and run at 25-MPH and stay on an even keel.

I hope those perch were tasty.

cwolf posted 09-11-2006 01:46 PM ET (US)     Profile for cwolf  Send Email to cwolf     
I've been following this thread with some interest regarding accuracy of engine speedometer vs. gps readings. I always assumed the engine supplied reading was not as accurate as the gps. This weekend I made it a point to compare the speedometer reading of an 06' Yamaha F150 equipped with Command Link gauges (not yet connected to gps) and my two year old Garmin GPS. Both showed exactly the same speed I would say 90% of the time. The rest of the time they differed by only 1 mph. It looks like the engine speedometers have improved greatly over the years.
swist posted 09-11-2006 03:30 PM ET (US)     Profile for swist  Send Email to swist     
You might be missing the whole point of this discussion. The fact that your engine gauge reads close to your GPS doesn't necessarily mean anything because they are measuring two different things unless there is zero current or wind effect.

I will agree with you that I found the Yamaha built-in pickup to be pretty accurate UNTIL the first time the tiny pitot opening clogs with something and you have to ream it out with a drill or whatever.

PeteB88 posted 09-12-2006 04:12 PM ET (US)     Profile for PeteB88  Send Email to PeteB88     
Who cares? My 13 is so golllldanngggg fast I can't revert my eyes from straight ahead to look at the speed. Ya!

What is most important here and not discussed as I scan this thread is absolute accuracy of TROLLING SPEED!!! I swear my buddy is messed up because he trusts his Lowrance GPS for trolling speed read-out and the last two times we been skunked while rods were a -poppin all around us.

What about trolling speed? THAT is much more critical than accuracy at WOT.

Ya gotta win that race - it's who crosses the line first, Jack!

But trolling speed accuracy, now that's an issue.

erik selis posted 09-13-2006 02:20 AM ET (US)     Profile for erik selis  Send Email to erik selis     

You need to be careful using a GPS speed-readout (directly) for trolling as you are measuring the speed over ground. You need to know the speed through water to find the correct trolling speed. You can compensate for this by adding or subtracting the drifting speed to the GPS readout and also check you paddlewheel speed to confirm.
I.E. if you are drifting along with the water flow at a speed of 2 knots your GPS will show a speed of 2 knots. The paddle wheel should show you a speed of 0 knots. Throwing out a lure in this case would have no effect and most likely not catch a fish. If you want to troll against the current (which is always better than with the current)at a waterspeed of 3 knots your GPS should be showing a speed of 1 knot and the paddlewheel speed should be 3 knots.

The accuracy of the paddlewheel measuring device will most likely be less accurate at slower speeds, often making it difficult to find the sweet speed. Checking out the lure action beside the boat is the best way to find the speed you need for a certain type of fishing lure.


erik selis posted 09-14-2006 07:33 AM ET (US)     Profile for erik selis  Send Email to erik selis
Chuck Tribolet posted 09-14-2006 09:17 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Erik, ionosphereics are a minor error. They are most of
reason that current nonWAAS, nonDGPS, GPS accuracy is 45'
and not 15'. WAAS and DGPS correct for ionsphereics, and
some other things.

Even if jimh and Steve didn't have WAAS or DGPS on,
ionosphereics wouldn't account for the difference they saw.


DillonBW posted 09-14-2006 09:50 AM ET (US)     Profile for DillonBW  Send Email to DillonBW     
This is a great subject and I find I must throw into the mix the possibility of water density while useing a pitot tube.
Does salt water have a different viscosity / density than fresh water? Would this not effect the sensitivity of the readings? I also wonder about air pressure, for airplanes, highanddry may know. Thinner air at higher altitudes....less air going into the tube etc?
I just know my Whaler goes fast enough.
Happy Whalin'
Royboy posted 09-14-2006 10:25 PM ET (US)     Profile for Royboy  Send Email to Royboy     
Jimh, I was trying to point out the bad science in your test. The one and only way to compare the readings of the various speed measuring devices is to have them on a single boat. There is no way on God's green Earth that you are going to get two boats to travel at the same speed at the same time. At least, not outdoors where we like to do our boating.

What can I deduce from your experiment? That Steve's boat is faster than your boat.

"I would judge the difference to be less than one foot per second"

Be for real. Exactly how did you manage to gage that while going at WOT? Could it have been half a boat length per second? It doesn't take long for half a boat to go by when your blasting along on the water at 44 mph. If it took "ONE-ONE-THOUSAND" for ten feet if Steve's boat to go by, you have the following result:


Wha-La, mystery explained!

Steve spanked your monkey and you're trying to muddy the waters with all of this mumbo jumbo, all the while avoiding the real pickle: that Continuouswave needs a repower, or rather, more power to keep up with Backlash.

The truth ain't pretty


jimh posted 09-15-2006 08:35 PM ET (US)     Profile for jimh  Send Email to jimh     
Roy--You have it backwards. At the beginning of the test there stood one premise: BACKLASH was faster. After the test, this was in doubt. Oh, maybe one mile-per-hour faster, but certainly not five to seven miles per hour faster.

The point of the article was really not at all about the boats, but about the measurement techniques. As it turns out, GPS has been absolved of any fault in the measurement error. BACKLASH's speed was being measured by another device, not a GPS.

AZdave posted 09-17-2006 11:57 PM ET (US)     Profile for AZdave  Send Email to AZdave     
I have found speed data can be suspect when units are first turned on. An older Lowrance unit had us going 95mph when we were really doing about 65. Admittedly this was on a 6% downgrade which is really unlikely in a boat!
cooper1958nc posted 07-22-2007 12:14 PM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     

I saw this interesting thread and thought I would comment on speed measuring devices.

Water skiers have traditionally used pitot speedometers. As pointed out these have a small bracket with a hole facing the direction of movement. They measure the dynamic pressure of water, which is 1/2 Ro v square, with ro being the water density. A good rule of thumb is that the dynamic pressure of water is about 700 pounds per square foot at 20 mph. The size of the hole in the pitot is immaterial, as is the depth of submersion. Because dynamic pressure increases as the square of speed, these devices can be quite accurate at high (>20) speed, but bad for lower speeds. If you look at a cheap pitot speedometer, the distances between displayed speeds gets larger as speed increases, making it less prone to error at higher speeds.

The pitot unit of choice for water skiers was the Airguide Jeweled Contralog speedometer. It had a "movement" within the head that rectified the exponential spacing of of cheaper units brought on by the physics of squaring the speed. They had in addition an "averaging tube" which was an air tank that kept the movements of the dials dampened and smooth. The contralog units were very easy to read in the critical 15-36 mph range where all skiing occurs. They were in additional adjustable and were in fact always adjusted so that the trap times through a measured distance (the slalom course usually) agreed with the display. Adjustments were necessary for water temperature and air pressure. Obviously, current would affect trap times over the bottom, but the discussion of how to compensate for current when skiing a slalom course is beyond this discussion. Two units were used in most boats. In practice, even after adjustment, it is not uncommon to see a 1 mph difference between these units.

Pitot speedometers are especially vulnerable to debris fouling, which may affect response time and reading. Pitots almost never read high, except when they are plugged and the boat slows down and they keep their old reading.

Lately, high end skiers have been using paddlewheel speedos. They have the advantage in not being exponential in read out and being more resistant to fouling. They are also electronic, and their output can thus be used in speed control servo devices. I have never seen a paddlewheel speedo read high but I suppose anything is possible with electronics.

jimh posted 07-22-2007 10:43 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the interesting information on the refinements used in pitot tube speedometer measurements
pglein posted 07-23-2007 04:13 PM ET (US)     Profile for pglein  Send Email to pglein     
I have a Garmin 12 channel WAAS GPS in my car that displays my speed over the road in 1/2 second intervals. It is always within 2mph of my car's speedometer...less at lower speeds. The difference seems to be due to the fact that my tires are larger than what came with the vehicle originally and thus my speedometer is not calibrated properly. From this, I have determined that the speed displayed by the GPS is extremely accurate, especially at higher speeds.

On our Albin, I've noticed quite a bit of variation between the SOG (taken from the GPS) and the Speed (taken from the paddlewheel). Being in Puget Sound, it's pretty much a given that anywhere you go, there will be some amount of current. It's often hard to determine exactly how much of the difference is due to current and how much is due to the paddlewheel's inaccuracy. Since current here rarely flows in a straight line, you don't know when you're really going with or against the current, as opposed to going in some tangent direction to the current.

What I have done is take the boat out in open, relatively steady water and run speed trials in large circles. I've come to the conclusion only that the Speed reading is consistently lower than that of the GPS. By how much is hard to predict, since the variation seems to itself vary by how much growth there in on the bottom of the boat and the paddlewheel itself. Further complicating matters is the fact that the boat is cleaned bi-monthly by a diver, and it's often difficult to tell if the bottom has been cleaned since your last use. You also have no way of telling whether the diver bothered to clean the paddlewheel properly on that particular visit.

It's kind of always a guessing game, but I always know that the SOG is telling me how fast I'm going towards my destination which, ultimately, is what matters.

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