Speedometer calibration

[PDS]

After first doing a search and not finding quite what I was looking for(under speedometer,in general) I have a couple of ? about speedometers.1 How far does the water go up the tube, at like 40mph? I am guessing it doesn't go all the way to guage, but air compresses some kind of a spring? 2 Is there any way calibrate a speedometer. Oviously the best solution is for me to get a GPS, but not in budget just now. Besides, like most of you, I would like to know that something I own is working properly. It maybe old, but it's still mine. Thanks for any and all replys PDS

 

[Pursuit2150]

Re: Speedometer calibration

The only way to get "TRUE" speed is to have someone with a speed radar gun check it.<br />My GPS will show a difference of 4 to 6 MPH vs. the wheel on my depth sounder.<br />You can also ckeck it out on an accurate measured mile post. <br />They are located on some bay areas, check your charts.<br />They are really ment for checking compass bearings.

 

[JB]

Re: Speedometer calibration

Howdy, PDS.<br /><br />While a RADAR set can give you a pretty accurate reading, so can a borrowed hand held GPS, and the GPS is a lot easier to use as well as more accurate.<br /><br />By tinkering with the mounting of your pitot tube you may be able to get a reasonably accurate reading out of your "speedometer" in still water. It will still be inaccurate in moving water unless you will settle for water speed vs. ground (true) speed.<br /><br />Because the pressure type meter is nonlinear it is unlikely to be accurate at all speeds, but the wheel type has its shortcomings, too.<br /><br />Only the GPS gives true, exact over the globe speed. <br /><br />Good luck.

 

[allanwh]

Re: Speedometer calibration

Yep JB has summed it up well. The Pitot tube type speedo is inherently inaccurate especially at low speeds and any GPS will give you very accurate speed readings...

 

[PDS]

Re: Speedometer calibration

Thanks, guys for your replys and I am still wondering how speedometers really work. Is it air pressure from the water being pushed up the tube? On my search I saw some people saying stuff about water comming out of their dash(because the hose came off of the back of thier speedometer)never happened to me yet but this is an old boat(1976) and I hope it won't but old age has a habit of breaking stuff. So calibration is out of the question? PDS.

 

[mparks]

Re: Speedometer calibration

Do you see any type of means for adjustment? At the meter? or at the tube?<br /><br />If not, then that would be a fairly good indication it's a "fixed" or "closed" technology at work.<br /><br />I, too, would like to know how accurate my old tube is at measuring actual speed across the water. GPS sounds like the best way... but a bit pricey for me.

 

[SCO]

Re: Speedometer calibration

Another way...you could calibrate it yourself by marking on the dial when you are going a known speed. You could determine your speed by timing yourself at constant speed past two bouys with known gps positions. Better accuracy if they are a mile apart or so on a calm morning. Time it and mark the dial position on the same run. It is air pressure at the meter that you are measuring, but water is what is picked up by the pitot tube which runs up the tube and compresses the air. You may have a diapgram in the meter that inturn moves the speedometer needle. A quick calc shows that at 30 mph, if you stuck a tube into the water off the back of the transom with the end pointed in the direction of the boat travel, and pointed the other end straight up, the geyser would go up some 30feet. this is what a pitot gauge measures, and if there is a leak in the tube at the gauge, then it is more than enough pressure to flood out at the dash. With no leak though, the air in the tube is compressed. I should say pressurized. my calc says 13psi at 30 mph, so I guess it does compress significantly if this calc is correct(boyles or charles law??) but forget about that.The only thing the compression of air in the tube does is let more water come up the tube. Since the 30mph column height is some 30 feet, an extra foot or so of column height due to compression wont affect the reading much.

 

[KeltonKrew]

Re: Speedometer calibration

Another question for speedo,<br /><br />instead of the water pickup tube, my Lower Unit has the tube built in and a hose coming out of the LU to go to the speedo.<br /><br />Is this any more/less accurate than the speedo tubes mounted to the transom?<br /><br />BTW, how does that work. I don't think anyone answered the question. <br /><br />Does water go all the way to the speedo or is it pressure?

 

[93evinrude]

Re: Speedometer calibration

Speedometers vary in accuracy from manufacturer to manufacturer and if you want a very accurate one like a Gaffrig(+-1/2%),it will cost more.Many out there are lucky to be within 2-5 mph of actual.Even with a very accurate unit,it is only as good as the signal it receives and various things can effect the flow at the pick-up tube like:<br />-air in the water flow possibly caused by positioning or a step style hull<br />-water current<br />-partially plugged tube <br />-debris in the water etc.<br />Your accurate reading will definitely come from a gps.<br />Paddlewheel style units should be better than pitot type at low speeds and I am not sure but I believe some are adjustable,probably using the display panel buttons.<br />Re lower unit pick-ups:I have heard they could be good but they have supposedly been somewhat troublesome as debris has a habit of getting in especially if you bump a chunk of wood or something similar.Regular pick-ups are less likely to plug.

 

[SCO]

Re: Speedometer calibration

This was redundant.

 

[TxRiverRat]

Re: Speedometer calibration

He didn't mention what kind of boat he owns, but if it happened to be an inboard, he could ruin it through the slalom course. At 36 MPH it should take you 16.03 seconds. <br /><br />A PerfectPass slalom timer with magnet system automatically calibrates both speedos every time you go throught he course - problem is it's exopensive - 1500 bux installed.<br /><br />I agree about the GPS...