Accuracy of water pressure Speedometers - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: The Whaler GAM or General Area
  Accuracy of water pressure Speedometers

search | FAQ | profile | register | author help

Author Topic:   Accuracy of water pressure Speedometers

TransAm posted 06-21-2009 08:21 PM ET (US)
I have often wondered just exactly how accurate the water pressure speedometer that feeds my Yamaha digital multi-function speedometer. Since this is the speedometer that is tied to my fuel management gauge, I thought it would be important to know its accuracy. I have compared the readings of the Yamaha speedometer to 2 separate GPS units and the water pressure speedo seems remarkably accurate. Its readings were generally within 1/4 - 1/2 MPH of the GPS readings all through the RPM range. I was surprised that a comparatively simple, low tech device would be this accurate. Obviously, it is good to know that the fuel measurements are being computed from a solid speed recording. Unfortunately, for engine speeds below 1,000 RPM, the pressure speedometer does not function very well. It would probably make sense to tie the fuel management gauge to the GPS for this reason.
LuckyLady posted 06-21-2009 09:29 PM ET (US)
My speedo is 2kts slower compared to the GPS.
Peter posted 06-21-2009 09:33 PM ET (US)
I found the speedometer on my Yamaha multi-function gauge connected to the Yamaha 225 Ox66 to be pretty accurate relative to the GPS. I could usually tell by the positive or negative difference between the reading on the multi-function gauge and the GPS whether I was running with or against the tide.
SC Joe posted 06-21-2009 10:29 PM ET (US)
The water pressure speedometer on my Mercury 90 FourStroke is off anywhere from 4-6 mph from the GPS reading. I can calibrate it on it's display on my GPS, but it doesn't stay correct. The paddlewheel speedo on the transducer is more acurate, but it is still incorrect. It's readout can be calibrated as well, but often it seems the calibration changes.
jimh posted 06-21-2009 10:47 PM ET (US)
The GPS and the pitot tube speedometer are not measuring the same speed.
Most GPS receivers show a speed over ground as a calculation using a time and distance formula solution between position fixes. The speed is only as accurate as the position fix. At slower speeds where the distance between two positions is relatively small, the error in the fix can be large in terms of a percentage of that distance. Many people think that a GPS receiver can give perfectly accurate speed, that is, with almost no error. An error of 0.5-MPH is about the best that can be obtained with the typical unit, and that is only with WAAS precision fix enhancement. See:
http://continuouswave.com/ubb/Forum6/HTML/002179.html
for a good discussion of GPS receiver speed accuracy.
A pitot tube speedometer shows speed through the water. This can be substantially different from speed over ground due to current effects. Several factors can influence the pitot tube speedometer accuracy. Placing the pick-up in aerated water can affect the accuracy. Accuracy can also vary as a function of the quality of the unit, how well it was made, how carefully it was calibrated.
Jerry Townsend posted 06-21-2009 11:09 PM ET (US)
Your water pressure speedometer will give accurate indication of speed - assuming the probe is clean and not fouled and is not subjected to turbulent flows.
A true pitot tube provides for the measurement for the total and the static pressure. The dynamic pressure is the difference or Total pressure minus the static pressure. The velocity of the fluid is related to the square root of the dynamic pressure.
Basically, the accuracy of the pitot tube velocity measurement is then that of the square root of the pressure guage which will be between 1% and 10% - so we can expect the velocity measurement to be between 1% and 3+%. --- Jerry/Idaho
Jerry Townsend posted 06-21-2009 11:19 PM ET (US)
I neglected to mention the point that Jim brings up - the pitot tube speedometer gives the speed of the boat relative to the water - while the GPS gives the speed of the boat relative to land. They can be quite different. Thanks Jim ---- Jerry/Idaho
R T M posted 06-21-2009 11:58 PM ET (US)
Water pressure speedos can be all over the place, The one on my 13 footer is right on the mark with the GPS, at 20mph and top speed 29mph. The speedo on my race boat is 5 mph faster than the GPS, but it makes me feel good. If your going so slow that you can break your concentration long enough to look at the speedo, your not going fast enough. Use a GPS that locks in top speed, and you can check it later.
rich/Binkie
TransAm posted 06-22-2009 07:05 AM ET (US)
Whatever the methodology I was just happy to see some consistency between 3 different instruments. Prior to removing my paddle-wheel transducer, that measurement too was +/- 1/2 mph of the GPS and tube speedo readings. The largest variation in readings comes with comparing my Garmin auto GPS computed speed and the speedometer in my truck. That variation was between 1 & 1 1/2 mph.
pglein posted 06-22-2009 09:29 AM ET (US)
I find a paddle wheel sensor to provide the most accurate measurement of boat speed. GPS provides only SOG, which, of course is quite useful. But, a proper instrument suite will provide both.
Tohsgib posted 06-22-2009 11:38 AM ET (US)
I only had 2 speedos in all my boats(that worked) and both were within 1mph of GPS usually.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Accuracy of water pressure Speedometers
TransAm	posted 06-21-2009 08:21 PM ET (US) I have often wondered just exactly how accurate the water pressure speedometer that feeds my Yamaha digital multi-function speedometer. Since this is the speedometer that is tied to my fuel management gauge, I thought it would be important to know its accuracy. I have compared the readings of the Yamaha speedometer to 2 separate GPS units and the water pressure speedo seems remarkably accurate. Its readings were generally within 1/4 - 1/2 MPH of the GPS readings all through the RPM range. I was surprised that a comparatively simple, low tech device would be this accurate. Obviously, it is good to know that the fuel measurements are being computed from a solid speed recording. Unfortunately, for engine speeds below 1,000 RPM, the pressure speedometer does not function very well. It would probably make sense to tie the fuel management gauge to the GPS for this reason.
LuckyLady	posted 06-21-2009 09:29 PM ET (US) My speedo is 2kts slower compared to the GPS.
Peter	posted 06-21-2009 09:33 PM ET (US) I found the speedometer on my Yamaha multi-function gauge connected to the Yamaha 225 Ox66 to be pretty accurate relative to the GPS. I could usually tell by the positive or negative difference between the reading on the multi-function gauge and the GPS whether I was running with or against the tide.
SC Joe	posted 06-21-2009 10:29 PM ET (US) The water pressure speedometer on my Mercury 90 FourStroke is off anywhere from 4-6 mph from the GPS reading. I can calibrate it on it's display on my GPS, but it doesn't stay correct. The paddlewheel speedo on the transducer is more acurate, but it is still incorrect. It's readout can be calibrated as well, but often it seems the calibration changes.
jimh	posted 06-21-2009 10:47 PM ET (US) The GPS and the pitot tube speedometer are not measuring the same speed. Most GPS receivers show a speed over ground as a calculation using a time and distance formula solution between position fixes. The speed is only as accurate as the position fix. At slower speeds where the distance between two positions is relatively small, the error in the fix can be large in terms of a percentage of that distance. Many people think that a GPS receiver can give perfectly accurate speed, that is, with almost no error. An error of 0.5-MPH is about the best that can be obtained with the typical unit, and that is only with WAAS precision fix enhancement. See: http://continuouswave.com/ubb/Forum6/HTML/002179.html for a good discussion of GPS receiver speed accuracy. A pitot tube speedometer shows speed through the water. This can be substantially different from speed over ground due to current effects. Several factors can influence the pitot tube speedometer accuracy. Placing the pick-up in aerated water can affect the accuracy. Accuracy can also vary as a function of the quality of the unit, how well it was made, how carefully it was calibrated.
Jerry Townsend	posted 06-21-2009 11:09 PM ET (US) Your water pressure speedometer will give accurate indication of speed - assuming the probe is clean and not fouled and is not subjected to turbulent flows. A true pitot tube provides for the measurement for the total and the static pressure. The dynamic pressure is the difference or Total pressure minus the static pressure. The velocity of the fluid is related to the square root of the dynamic pressure. Basically, the accuracy of the pitot tube velocity measurement is then that of the square root of the pressure guage which will be between 1% and 10% - so we can expect the velocity measurement to be between 1% and 3+%. --- Jerry/Idaho
Jerry Townsend	posted 06-21-2009 11:19 PM ET (US) I neglected to mention the point that Jim brings up - the pitot tube speedometer gives the speed of the boat relative to the water - while the GPS gives the speed of the boat relative to land. They can be quite different. Thanks Jim ---- Jerry/Idaho
R T M	posted 06-21-2009 11:58 PM ET (US) Water pressure speedos can be all over the place, The one on my 13 footer is right on the mark with the GPS, at 20mph and top speed 29mph. The speedo on my race boat is 5 mph faster than the GPS, but it makes me feel good. If your going so slow that you can break your concentration long enough to look at the speedo, your not going fast enough. Use a GPS that locks in top speed, and you can check it later. rich/Binkie
TransAm	posted 06-22-2009 07:05 AM ET (US) Whatever the methodology I was just happy to see some consistency between 3 different instruments. Prior to removing my paddle-wheel transducer, that measurement too was +/- 1/2 mph of the GPS and tube speedo readings. The largest variation in readings comes with comparing my Garmin auto GPS computed speed and the speedometer in my truck. That variation was between 1 & 1 1/2 mph.
pglein	posted 06-22-2009 09:29 AM ET (US) I find a paddle wheel sensor to provide the most accurate measurement of boat speed. GPS provides only SOG, which, of course is quite useful. But, a proper instrument suite will provide both.
Tohsgib	posted 06-22-2009 11:38 AM ET (US) I only had 2 speedos in all my boats(that worked) and both were within 1mph of GPS usually.