How does Speedometrer Work?

[newbassboat]

I hope this is not a stupid question but how exactly does the speedometer work on a boat? I have a 1990 Alumacraft 17' MV Maverick with a 90 hp Johnson V4. What makes the speedometer work?

 

[Bondo]

Re: How does Speedometrer Work?

Air Pressure coming thru the Pitot Tube.......

 

[QC]

Re: How does Speedometrer Work?

Just to elaborate a little more on Bondo's note. Air pressure caused by water jamming into the pitot tube pushing on air deeper in the tube while underway. Higher speed causes water to push harder on the air, pressure rises, registers as increased speed.<br /><br />I am the only person I know who claims his is accurate. I swear, my boat's is within 2% of GPS. Standard Mercruiser setup . . .

 

[studlymandingo]

Re: How does Speedometrer Work?

I must say that is impressive QC. I used to have one of the "cheapie" speedos that I installed. If I was running a certain way with the current, it would not register at all. Now I just use GPS.

 

[JB]

Re: How does Speedometrer Work?

That's possible, QC. With pitot speedos all over the map it makes sense that at least one would accidentally fall into the accuracy spot.

 

[newbassboat]

Re: How does Speedometrer Work?

Ok another stupid question where is the "pilot tube" located and what does it look like? Is attached to the boat or the motor? Thanks

 

[Bondo]

Re: How does Speedometrer Work?

I am the only person I know who claims his is accurate. I swear, my boat's is within 2% of GPS. Standard Mercruiser setup . .

Ayuh,....... Even Mercruiser makes Mistakes sometimes.......... <br />

Ok another stupid question where is the "pilot tube" located and what does it look like? Is attached to the boat or the motor? Thanks

Pitot,.... Not Pilot.....<br /><br />It's Usually a small plastic arm,.. about 4" long hanging off the Transom so it Drags the the water going by.........

 

[craze1cars]

Re: How does Speedometrer Work?

Here's a picture of what Bondo is describing:<br /> http://www.overtons.com/modperl/overtons/detail/pdetail2.cgi?r=detail_view&item_num=28246 <br /><br />Or...if you don't have one of these hanging down on the transom of your boat, your motor may have a built-in pitot pickup. Mostof the larger, newer outboards (past 20 years?) have this setup instead. Look for a tiny little hole in the very forward part of the lower unit. I would suspect your 90 HP Johnson has this type of setup instead of a remotely mounted arm, but I could be wrong.<br /><br />You wanna learn the history of the Pitot tube and how it works? Click here! http://en.wikipedia.org/wiki/Pitot_tube

 

[newbassboat]

Re: How does Speedometrer Work?

Opps "Pitot" - You know I just posted a question about a tiny little hole in the very front of my lower unit just above what I call the Nose of the lower unit and another about halfway up the side on the side of the lower unit. could this be what this is because I have no tubes hanging down off of the transom. Also it appears that the hole in the front is plugged up is there an easy way to clean this?

 

[Pogo123]

Re: How does Speedometrer Work?

That small hole may be the "Static" side of your pitot system .. it's what gives the pressure side a reference to start from.<br /><br />The only way I know to clean one is with compressed air from the inside. Blowing into the static hole from the outside won't do anything to help cleaning it out.<br /><br />You can get the accunulated dirt and bugs out with a fine wire, but only to a depth of 1/2 to 1". After that, the line needs disconnecting at the source and blown out ... gently.

 

[QC]

Re: How does Speedometrer Work?

Yup, even a broken clock is right twice a day . . . Funny thing is, and I didn't want to say, but this is actually the second one for me. I had an Alpha that read that close and this is a Bravo. Both with the built in pitot. It's actually better than 2%, but like I said I didn't want to push it. I am a flippin' guage-aholic and I always use both: GPS and the speedo. These two have been close to dead on . . . And they both disagree with GPS in river current. Faster upriver, slower downriver . . . I think this is about as close as I'm gonna get to hitting the lottery

 

[craze1cars]

Re: How does Speedometrer Work?

Yes, the little plugged hole is your problem. The hole you're referring to is the air inlet. The air goes out the other side into you speedometer tube. So find the speedometer tube and where it connects to your motor (usually somewhere above the cavitation plate), and disconnect it. Air should flow freely between this hole and the small clogged hole. You can alternate blowing compressed air and stick little wires into both ends of the air passage until all the gack comes out and air flows freely. Reattach the speedo tube, and I bet everything will work.<br /><br />You can sort of test you speedo while you have the tube disconnected...blow some compressed air at the end of the disconnected tube and your speedo needle should dance. If it doesn't you may have a leak in your line somewhere else in the boat. A speedometer is nothing more than an air pressure guage.

 

[Pogo123]

Re: How does Speedometrer Work?

Quality speedometers are a version of the "Pitot Static" system. They measure direct water pressure (hold your hand in the water as you increase speed) which is a function of velocity. That's the "Pitot" side.<br /><br />By itself, this measurement is meaningless without a reference, so the "Static" side is a small port open to non pressurized water or ambient presssure. This pressure is the same no matter what your speed and provides the reference from which your dial is calibrated.<br /><br />Your speedometer is actually indicating the difference between the Pitot and the Static presssures.<br /><br />It's not just a dumb guage ... it's actually pretty smart, but both ports need to be clear for it to function properly.<br /><br />There are cheaper models that only measure velocity induced pressure (Pitot). These must be calibrated against a known good speedo, but (in most cases) will be innacurate at the ends of the scale. If you set it to 20 Kts against a known reference, it will be progressively in error the further from 20 kts you get .. either slower or faster... But a couple knots either way won't be important to the average user.