Speedometer GUAGE having issues

[Pusher]

I know the theory behind the ram pressure providing the means for the gauge to "read" water speed. What I don't understand is how the boat gauges themselves work. Mine will read up to 30MPH at approximately 12 psi on the air compressor, but blowing with my mouth (lips sealed) will not give me anything. I know everyone will say throw it, but I'd rather fix the part that's already made it 40 years.

I can't find anywhere on the workings behind it. From what I can tell, there is a copper spring that expands with air pressure, and contracts as pressure decreases. I have NO idea how the air pressure is conveyed to the spring though.

 

[dwco5051]

The pressure of the water into the tube that leads to the speedo compresses the air in the tubing. Once it reaches the point where the water pressure compresses the air enough that equilibrium as been reached that air pressure at the gauge is equal to the water pressure at the sending unit.

 

[Pusher]

Do you know how that equilibrium is transferred into the spring in the gauge?

 

[dwco5051]

The "spring" is really a hollow metal tube with a flat profile and is what is know as a bourdon tube. As the pressure inside the tube increases the tube wants to straighten out. The end of tube is attached to a linkage that operates a gear that turns the speedometer needle. I got this from a teleflex manual "

1. If adjustable air pressure is available, standard calibration speedometers should react to the following pressures: 5 PSI = 20 MPH; 11 PSI = 30 MPH; 19 PSI = 40 MPH; and 30 PSI = 50 MPH."

It seems by this that your speedometer is working correctly and the trouble is most likely caused by a blocked pitot or a kink or obstruction in the tubing from the pitot to the speedometer.

 

[gm280]

The "spring" is really a hollow metal tube with a flat profile and is what is know as a bourdon tube. As the pressure inside the tube increases the tube wants to straighten out. The end of tube is attached to a linkage that operates a gear that turns the speedometer needle. I got this from a teleflex manual "

1. If adjustable air pressure is available, standard calibration speedometers should react to the following pressures: 5 PSI = 20 MPH; 11 PSI = 30 MPH; 19 PSI = 40 MPH; and 30 PSI = 50 MPH."

It seems by this that your speedometer is working correctly and the trouble is most likely caused by a blocked pitot or a kink or obstruction in the tubing from the pitot to the speedometer.

This is one of the best info posts I've seen on here in a long time. You are absolutely correct and thanks for the conversion from PSI to MPH. I knew there had to be a conversion and even knowing how the gauge actually did work, the conversion was good to read. Knowing this one can easily see how such gauges are mere generality speeds and not hard and true readings. GPS is a better truer option IF you really need to know your true speed. But really who does to the tenth of a MPH?

 

[drrpm]

You won't see a reading when you blow into the tube with your mouth because you can't generate enough pressure. Respiratory pressures are usually measured in centimeters of water pressure. One atmosphere of pressure is 10 meters of water and about 15 PSI. A human can generate about 100 centimeters of water pressure which is about 0.15 PSI. That's also the reason people can't breathe through a long snorkel.

 

[Pusher]

The "spring" is really a hollow metal tube with a flat profile and is what is know as a bourdon tube. As the pressure inside the tube increases the tube wants to straighten out. The end of tube is attached to a linkage that operates a gear that turns the speedometer needle. I got this from a teleflex manual "

1. If adjustable air pressure is available, standard calibration speedometers should react to the following pressures: 5 PSI = 20 MPH; 11 PSI = 30 MPH; 19 PSI = 40 MPH; and 30 PSI = 50 MPH."

It seems by this that your speedometer is working correctly and the trouble is most likely caused by a blocked pitot or a kink or obstruction in the tubing from the pitot to the speedometer.

You DA Man!
I can't believe they can solder a blader that delicate. I would never have guessed that, much less that 40pmh of water pressure could exert that much force down the tube.

(I haven't taken the boat for a maiden voyage yet. I assumed the instrument was shot when I tried blowing on it. All of the aircraft pitot instruments I am familiar with are MUCH more delicate.)

I searched forums for a while and couldn't find anything this detailed. Thanks again!

 

[gm280]

Pusher,I can say with absolute accuracy that my speed gauge in my bass boat would get plugged so often that it was nothing unusual to have to pick out the plugged debris so many times. And it was pretty normal to see me going 45 MPH or more while it was sitting on the trailer at the house. So be careful when reading such speeds. They are merely suggestions and not really true speeds. At least in my opinion. My pivot pickup with in the Mercury foot dead centered...

 

[dwco5051]

Now you got me thinking about airspeed gauges. I too used to blow into my pitot tube and suck on my stall warning horn during preflight and never gave much thought to how much pressure it took. A quick lookup on google and I found that it only takes about .042 psi for the airspeed gauge to indicate 50 mph. 90 mph is 5 inches of water column or .139 psi. As a side note when I first got my bass boat I actually blew the hose off the connector on the back of my gauge and soaked my legs during a high speed run. It all relates to the difference in density between water and air.

 

[UncleWillie]

You won't see a reading when you blow into the tube with your mouth because you can't generate enough pressure. Respiratory pressures are usually measured in centimeters of water pressure. One atmosphere of pressure is 10 meters of water and about 15 PSI. A human can generate about 100 centimeters of water pressure which is about 0.15 PSI. That's also the reason people can't breathe through a long snorkel.

100cm of water column is ~1.5psi.

The heart of the gauge is the Bourdon Tube. It is not as fragile as you might imagine.

My 63 year old lungs can muster just over 52"/132cm of water column pressure. (1.9psi)
Mouth blowing into the gage should result in at least a 10mph reading if you do not have COPD.

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
52 psi = 60 mph

Other values can be calculated HERE.

 

[Pusher]

I know DWCO. It's funny that something as routine as a walk-around can drum up so many good memories. Too bad it's so expensive.

Thanks for the extra info Willie. My gauge might be off but I'll give it a go and compare it to my speedometer app for the maiden voyage.

Thanks everyone!

For internet searchs: how does a why doesn't my boat speedometer gauge instrument panel dash dashboard work fix repair test pressure