Speedometer test

acdc96

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Apr 23, 2013
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412
How could I test my speedometer on land?
My speedometer uses a pick up tube on the transom. It's not hooked up to the motor I believe.

I know it uses air pressure to move the dial. Could I use my compressor with a air blower set at a low psi (like 5-10psi) to blow air into the pickup?
 

ronward

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Jun 24, 2013
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Re: Speedometer test

Yes that'll work but it only tells you if the tube is blocked or not. I did the same thing with mine and I think it took slightly more psi than 5-10 but not sure. Start low and increase as needed. The only real test is under actual operating conditions. Remember though, just because it works does not mean it is accurate. GPS is the way to go if you want accuracy
 

Brian 26

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Aug 14, 2013
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Re: Speedometer test

Yes you should see the speedo move up. If it's been disconnected or looks like there might be crud in the tube I blow from the speedo side out first to clear the tube of contaminants.
 

ronward

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Re: Speedometer test

Yes you should see the speedo move up. If it's been disconnected or looks like there might be crud in the tube I blow from the speedo side out first to clear the tube of contaminants.

Good point. Worst case, buy new tubing at auto parts house
 

smokeonthewater

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Re: Speedometer test

Actually certain pressures relate to certain speeds.... If you spend a little time with google you can probably find a chart.

EDIT: see below
 
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QC

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Re: Speedometer test

You can probably get it to go with just your mouth if you can test with some short tubing right at the head unit. Then blow backward through the hose and make sure it is clear. I have no idea what the max pressure is on those things. I am pretty sure you are safe at five. But real easy to test with it out of the boat even.

Oh, and don't forget that GPS is not accurate for boat speed in current ;) Pitot style are IF the thing is accurate to begin with. Despite all of the comments here, I have found standard pitot speedos to be within 5% of GPS without current.
 
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gm280

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Jun 26, 2011
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Re: Speedometer test

Yes those in dash boat speedometers are nothing more then pressure gauges supposedly calibrated for the correct speed. But everybody knows they are not very accurate because of so many variables, but merely an indication... I often wondered what speeds were represented by different PSI pressures myself. Be interesting to see a graph of such... Hint, hint... :cool:
 

JimS123

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Re: Speedometer test

Not many people know how accurate a pitot tube speedometer really is. Many people think they are inaccurate because they weren't installed or maintained properly. The only disadvantage is the limitation of low speed readings.

A GPS is a fancy gadget to find fishing holes or see your position or rate of travel across the earth. But if you want to know how fast your boat is going you need a pitot or paddlewheel speedo.
 

UncleWillie

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Oct 18, 2011
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Re: Speedometer test

I have posted it here on Iboats multiple times.

For your Reference...
1.5 psi = 10 mph
2 psi = 12 mph <<< Typical pressure produced by a set of young health lungs.
6 psi = 20 mph
13 psi = 30 mph
23 psi = 40 mph
36 psi = 50 mph
52 psi = 60 mph

Notice that the scale is exponential; twice the speed gives 4 times the pressure.
 
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gm280

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Re: Speedometer test

Ha Smoke and UncleWillie, thanks for the info. I wondered what PSIs would read what speeds. And that has to be the same for all boats too... Thanks again both of you. :thumb: :thumb:
 

QC

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Re: Speedometer test

A GPS is a fancy gadget to find fishing holes or see your position or rate of travel across the earth. But if you want to know how fast your boat is going you need a pitot or paddlewheel speedo.
It could be argued that you need both. GPS is speed over the bottom, and if time is important, so is that. Pitot or paddlewheel is speed over the water, and is important for calc'ng props, and trolling speeds etc. They should be in sync on a calm lake.
 

UncleWillie

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Re: Speedometer test

... I wondered what PSIs would read what speeds. And that has to be the same for all boats too... ...

It will be the same as long as they are floating in water. ;)

Salt water will read 3.5% faster due to its greater density.
 

smokeonthewater

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Re: Speedometer test

According to the link I posted the mercs w pickup in the lower unit will read 10% higher
 

UncleWillie

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Re: Speedometer test

According to the link I posted the mercs w pickup in the lower unit will read 10% higher

Yes! There are Speedometers that are made to match the Merc water pickup points.
To be accurate, a pitot tube should not disturb the flow of the water as it approaches the end of the tube.
A classic pitot is a very thin walled tube projecting a significant distance in front of its mounting hardware.
The Merc drive pickup points are openings on the blunt face of the lower drive units.
They generate a minor, but significant bow wave in front of them that raises the pressure.

Other that bragging rights and OCD, as long as you are going the same speed as you were last month;
Does it ever really matter if you are only going 40 when the gauge reads 44? :joyous:
 

gm280

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Re: Speedometer test

Yes! There are Speedometers that are made to match the Merc water pickup points.
To be accurate, a pitot tube should not disturb the flow of the water as it approaches the end of the tube.
A classic pitot is a very thin walled tube projecting a significant distance in front of its mounting hardware.
The Merc drive pickup points are openings on the blunt face of the lower drive units.
They generate a minor, but significant bow wave in front of them that raises the pressure.

Other that bragging rights and OCD, as long as you are going the same speed as you were last month;
Does it ever really matter if you are only going 40 when the gauge reads 44? :joyous:

I do remember the days back when I had my Mercury 115 inline six that had the speedometer pick up tube in the lower engine section. There were lots of time I would run over floating logs at night coming in to the dock and it would plug the pickup tube. And usually I would notice that I was going 40+MPH at that house the next day. So I would have to get out my tinny little drill bit and twist it in to clear the logged material out of the pickup tube and let the speedometer go back to zero again. Those small floating logs/branches or whatever would be so hard to see until plump, you run over them... Happened too many time to me...
 

JimS123

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Re: Speedometer test

I do remember the days back when I had my Mercury 115 inline six that had the speedometer pick up tube in the lower engine section. There were lots of time I would run over floating logs at night coming in to the dock and it would plug the pickup tube. And usually I would notice that I was going 40+MPH at that house the next day. So I would have to get out my tinny little drill bit and twist it in to clear the logged material out of the pickup tube and let the speedometer go back to zero again. Those small floating logs/branches or whatever would be so hard to see until plump, you run over them... Happened too many time to me...

I've never had a Merc, but I can appreciate the problems one would encounter with the orifice on the motor. After running conventional pitot tubes for years I have never had one that needed to be cleared out. Sure, an occasional weed will cover the tip, but as soon as you stop it falls off. Once I had to replace the tubing, but that was really a manufacturer's defect since it got pinched when the boat was built.

My classic '52 has an antique Airguide (yup, that's a 62 year old speedometer!!) and the pitot is aluminum. In calm winds and no current it's spot on with a GPS.

My depthfinder has a paddlwheel sensor that covers the range not covered by the pitot. Setting trolling speed to 0.1 mph accuracy is a piece of cake. Funny thing about that is that in 25 years it has never even been discombobolated by a weed!
 
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