speedo tube clear, what mph can I expect by blowing into it?

[bahamasboy]

Hello all,<br />My speedo isn't working. I disconnected from the back of the actual speedometer, connected a new hose. When blowing as hard as possible, I can only get about 1 mph to register. How much movement should I expect to get by merely blowing?

 

[bh357]

Re: speedo tube clear, what mph can I expect by blowing into it?

Depends on how fast you're moving when you blow <br /><br />Sorry, couldn't resist. I blew into mine but could only get a small amount of deflection (couple of mphs). When I was on the water, the time I looked at the speedo, I believe it was registering 35~40mph. Is the tube and pitot clear?

 

[swist]

Re: speedo tube clear, what mph can I expect by blowing into it?

You don't blow, you suck!<br /><br />Actually to be more precise, you blow across the pitot opening, not into it. That lowers the pressure in the pitot (same principle as airplane wings).<br /><br />You can get the same effect by sucking on it - but gently or you make break your speedo. The amount of vacuum created by water blowing over the end of the tube is fairly small and the gauge is calibrated for that.

 

[BoatBuoy]

Re: speedo tube clear, what mph can I expect by blowing into it?

swist,<br />My pitot (or pickup) tube is an "L" shaped tube with the horizontal part of the "L" positioned to pickup the onrush of water. Evidently mine is different than the one you reference. Are there different types?

 

[swist]

Re: speedo tube clear, what mph can I expect by blowing into it?

You have a differential pitot - it's slightly more sophisticated and more accurate. Vacuum pitots "pcik up" at right angles to the fluid flow. Also I notice from the equations that there is a difference beween incompressible (water) and compressible (air) - so using air instead of water will never give you the right results. <br /><br />I've seen boatyards use compressed air to check it out though.<br /><br />Take a look at:<br /> http://www.efunda.com/designstandards/sensors/pitot_tubes/pitot_tubes_theory.cfm

 

[wvit100]

Re: speedo tube clear, what mph can I expect by blowing into it?

trying to keep these tubes clear is a loosing battle. mine has been plugged up for two years now and i don't plan on fixing it anytime soon. what do you need with a speedo anyway, as long as the tach is working that's all you need to know.

 

[sledhead041]

Re: speedo tube clear, what mph can I expect by blowing into it?

If your line and gauge are working right the pressure needed is as follows: 2mph=0.048psi 8 mph=0.768psi 20mph=4.9psi I hope that helps.

 

[bahamasboy]

Re: speedo tube clear, what mph can I expect by blowing into it?

Thanks all. I've replaced both the speedo and the line to it. I took the boat out yesterday and didnt get a reading until about 10 mph. Although it seems that we were going much faster than that. I didn't replace the pitot but did make sure it was cleared out using stiff wire. I'll report back if I find anything to be unusual in the future, or if I find out that everything is ok and I just dont know how fast I'm going, or not going, on the water.<br /><br />Thanks

 

[TwoBallScrewBall]

Re: speedo tube clear, what mph can I expect by blowing into it?

Mine seems to work on pressure not vacuum. I applied about 10psi and the guage went up to around 20-30mph. Not sure exactly what MPH because I had the face off the guage, but it definitely went on pressure not vacuum.

 

[TwoBallScrewBall]

Re: speedo tube clear, what mph can I expect by blowing into it?

Gee, 553 posts and I still manage to double-post.

 

[Mark42]

Re: speedo tube clear, what mph can I expect by blowing into it?

I have a Teleflex low range speedo, 5 to 35 mph. It does seem to be more accurate at low speeds than some of the 10 - 60 mph speedos. So depending on the range, it may not be too sensitive or accurate at low speeds.<br /><br />Mark

 

[BillP]

Re: speedo tube clear, what mph can I expect by blowing into it?

Angle of attack is critical on most pitot tubes. A very slight angle change can make big speed changes. That's one reason people get varying readings. Sounds like yours is plugged or the instrument is broken.

 

[PatPatterson]

Re: speedo tube clear, what mph can I expect by blowing into it?

Originally posted by wvit100:<br /> trying to keep these tubes clear is a loosing battle. mine has been plugged up for two years now and i don't plan on fixing it anytime soon. what do you need with a speedo anyway, as long as the tach is working that's all you need to know.

I agree. I have unplugged mine several times, and it works for a trip or two, then quits. I don't bother anymore. I just go by the tach.

 

[achris]

Re: speedo tube clear, what mph can I expect by blowing into it?

Bin the pitot, get a GPS.......<br /><br />Chris..............

 

[Capn Mike]

Re: speedo tube clear, what mph can I expect by blowing into it?

3 speedometers on my boat: pitot, which was original equipment, and is hooked up to the large speedometer on the dash. Works from 12 to 40 (top speed) mph, obviously shows speed through water and is surprisingly accurate, according to #2 GPS, which shows speed over land. #3 is a paddle wheel trolling (0-10 mph), with the gauge on the transom, so we can tell our true speed through the current.<br /><br />Which one is accurate? Well, none of them. I boat/fish on the Columbia River, which has current speeds of 1 to 18 mph. Depending on whether I'm going up or downstream, the speed of the current needs to be factored into the pitot and paddlewheel speedometers. The GPS gives an accurate speed over land and that's useful for many things, such as computing travel time, mpg, etc., but does not take into account the speed of the river current. So when trolling or anchored in a hog line, I need to know the speed of the current, thus the paddlewheel speedometer. My GPS says I'm going 0....but the paddlewheel may say 8 mph!

 

[bahamasboy]

Re: speedo tube clear, what mph can I expect by blowing into it?

Hello Again. I'm the origional poster in this long strand.....<br /><br />I bought a GPS yesterday and have yet to use it ont the water, but will be very curious to see how it matches up with the speedo. The speedo on the boat is a 10 to 60 mph. I'll let you know how accurate/inaccurate the new speedo and tube is after my next outing. I just find it interesting that all these companies make speedo equipment and they haven't been able to master the accuracy issue. GPS is the way to go if you really want to know.

 

[BillP]

Re: speedo tube clear, what mph can I expect by blowing into it?

My handheld gps gives avg speed toward destination, expected time of arrival and minutes in route to destination. Setting the refresh rates low makes a difference when checking speeds and course changes. I think most gps units have this. Mine is set at 5 secs for low speed (boating) and 30 secs for high speed (flying).