TOPIC: Anybody have info on testing Speedo?

Well, I've confirmed a water-logged floor in my sea king and decided to reflect on my project over a few cold beers. During this time of reflection and generally organizing my workspace, I found the speedometer. I blew some air into it and it moved, but is there a chart on how much air pressure I need in my air compressor and what the speedo should read?

Best way to test a speedo nowadays Erik is to get a GPS unit. You can get them under 100 bucks, I know I did a year or two ago. Mount the speedo in the dash if you want authenticity, but rely on GPS for actual speeds.

Frank

I thought the airguide speedometers work under a vacuum not pressure. Is that right?

Greg Anderson
Wisconsin

I just don't want to mount the old speedo if it doesn't work. I know Mercruiser can have you test it by putting air on the built in pitot on I/O drives. I didn't know if somebody had a method to do the same on smaller speedo's. No big deal I guess. I could just mount my GPS where the speedo went. The older speedometers are cooler looking than my Garmin. I assumed that they worked on a vaccuum until I read the manual...

Usually if they move with air put to them they will work. May not be that accurate, but then again sometimes they are. If your's is a standard size round unit put it in and run it... Check it with your GPS and if it's off by too much just pick up another one on Ebay and try again. They can usually be had for cheap.

The thing I've encountered on a couple of boats is one minute it will be running right, then you look down and it will be running 10-15 mpg slow. Next time I look down it's right again. Not sure what the deal with that is except maybe water turbulance at the pickup tube is affecting the results?

I once knew the # inches of water that equal one knot and had the graph. ... If need be I may be able to find it, but it was in the time before computers and may take as long to find it.. got out of collage, married, moved 4 times, and divorced since the last time I had to use it, and that was in collage

To test you would need a Manometers. Even if you have it set right at home or shop, you can be as much as 3 Knots off on the lake or even the next day because of diff Atmospheric Pressures


for a true test a GPS is the way to go...Meters (working)are still fun to have on a vintage boat

TallTexan
I would think the graph you mentioned would be a cool addition to the FG Archives if you ever happen across it.
regards
lar

boat_geek wrote:

TallTexan
I would think the graph you mentioned would be a cool addition to the FG Archives if you ever happen across it.
regards
lar

I am going to look on line as to see if I can fined one. I as of right now cannot even think of where any of my old engineering books are.....Got use to using a comp. Many of the things that I had to remember, as how to figure a load, are now in the Comp...what are we coming to....what will happen if all comps crashed today

I think that I'm just going to try to clean this one up enough to where it looks decent. I wish I had a better understanding to take one apart and make repairs. I really don't care how fast I'm going, otherwise I would carry the GPS... I'm looking for a "coolness factor" of a working 50 year old gauge. I also thought of removing the chrome ring and trying to find a new gauge to fit inside, but most are too small in diameter and need a standard hole drilled. Somebody could make a killing in reproducing these older gauges.

Be carefull with trying to use an air compressor to check it. You can turn a working one into a non-working unit. I can get to about 10-15 mph on mine with lung power. Never checked what PSI that is but it isn't much.

Mercruiser has around 20 psi as the max on their outdrives. I can get mine to move blowing into it. I can't remember how far, but it wasn't much and I'm not a smoker.

Guess I need to quit smoking! (Duh) I can only blow about 10mph on the airguide speedo for my Cutter. (Haven't had a chance to check the one in the Starflite yet, I'm still banned from going out into my garage.

Hey, I just thought of something. If your bezel is pitted, go to a truck stop that has a chrome shop. You can buy chrome bezels for the taillights, etc that actually have an "eyebrow" over the top like the fender on a '55 chevy. They should have them to fit over your speedo, and they really look cool. I found this site that has them in different sizes. Not sure what diameter your speedo is...

secure.countywebsite.com/victorylane/index.php?cPath=61_103&osCsid=2e68e426422e69f470570d12bc815462

Holy Smokes that's cheap!!!

Doug, that chrome Bezel is kool........

Take extreme caution when puting air to a pressure speedo. The best bet is to run the regulator all the way down to 0, then put the air to the meter & very slowly crank down the regulator to about 10 to 15 lbs. Seems like the Sea Speed gauges ran about 1 Lb. air pressure per mile an hour. I would not advise taking a speedo apart - as they are very fragile inside. Most of the old speedos are surprisingly accurate up to about 40 mph. Airguide Contralog movements were the most accurate. Never let water get into the workings of the meter - it will ruin it. Drip irrigation line makes great pitot hose, and it's cheap! Just be sure to run the hose at a continual and gradual incline from pitot to the gauge. No "dips" or low spots in the house routing, as these can trap water. Try to find a tiny hose clamp to use at the gauge end if the original is gone. The hose can actually blow off the gauge at high speeds, and water will travel up the hose & make a messy surprise for you.

This maybe of intrest to a few of you. This is a link to original Airguide installation instructions and applacation guide for speedometers, tachometers, clocks and barometers.

www.classicboatwork.com/classicparts/gauges/airguide/airguideinstructions/Airguide_Instruct.html

Greg Anderson
Wisconsin

Great info Guys.

Did a little testing on mine last year, and I found that the speedo didn't read at all until about 10 MPH. Was told this was adjustable, but the speedo was dead on at top speed for me at the time which was 43 MPH. It works for me that way and looks good, so I'm happy with it.

Mine is an AirGuide.

I use a GPS for prop testing.

The speedo uses what is known as water column and not PSI.

Blow into a air gauge and you may move it but not much. Blow into the speedo (as many have) and you can move it to near 50 MPH

a Manometers is need to test and calibrate one. a single leg Manometers are not hard to make out of pvc pipe.

You cap one end; drill tap and insert the tubing for the speedo. you then fill with water to X #s of inches above the fitting. If someone has a working speedo and the time they could set it up and give the #s of inches for every 5 MPH

being a single leg unit you will need a stepladder

Maybe I can clear some things up here...a marine speedo is nothing more than a pressure gauge. just like you check your tires with, just like you check your compression with.
You can check it with compressed air if you like. it does not operate off vacuum.
TallTexan, inches of water colum is just another unit of pressure,(ie 1psi=27.7inH20) you can measure it in bar or Pascals if you like.
Virtually all pitot tubes are the same in design, so they all measure the same pressure at a certain speed between all the brands.
Here is a tech document from Teleflex, it applies to all pitot speedometers.
Your old Airguides can be cleaned, repaired and calibrated if you are careful.
I checked mine on the bench and with a GPS and it is +/-1 MPH throughout the range.

Ladies and gentlemen, we have a BINGO!

Since I know better than to wear a Speedo, I have no idea what I would test it for.