speedometer question

sparticus

Petty Officer 3rd Class
Joined
May 31, 2007
Messages
92
As a recent first time boat owner I have many questions. I have a speedometer on my dash (its the only gage there). I noticed on its maiden voyage on sunday that the gage didnt work. No big surprize its a 1965. I followed the black tubing all the way to the back of the boat and it ends in the water. The gage says "airguide". So my question is....... How the heck does it work? Perhaps I can make it perform its function if I understand the principle behind it. I am guessing water pressure but I have been surprized before. I understand that they arent accurate at all and most probably ignore them but its against my nature to see something broke and not at least take a half hearted attempt to fix something. Thanks for the inputs!
 

Gary H NC

Fleet Admiral
Joined
Dec 1, 2005
Messages
8,972
Re: speedometer question

They do work on pressure.Be sure the tube has no kinks or wire ties or anything restricting it.The pitot on the back of the boat has a very small hole in the front edge of it.It may be clogged or the gauge is bad.
Put about 5 psi of air in the tube and see if the gauge moves..
 

sparticus

Petty Officer 3rd Class
Joined
May 31, 2007
Messages
92
Re: speedometer question

Very cool Gary! Thanks. I was tempted to apply some pressure but without an idea of how much I was hesitant. As a technical instructor for aircraft I much prefer technical manuals when doing maintenace but I will wing it (no pun intended) if I have to.;)
 

diaric

Chief Petty Officer
Joined
Jul 8, 2006
Messages
532
Re: speedometer question

these are usually accurate to about +or- 50 %, but they are cool when they work
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: speedometer question

FWIW, some of the Airguides are actually pretty decent. I belive they make/made a lot of the Ski boat units that are especially accurate.

Also, I know no one will believe this, but I have had an Alpha and a Bravo, last two boats and they are within .5 - 1 MPH of my GPS . . . day in day out, I am not as lucky in Vegas if you're wondering :eek:
 

sparticus

Petty Officer 3rd Class
Joined
May 31, 2007
Messages
92
Re: speedometer question

Gary. You mentioned a small hole at the front of the tube. Does this mean that the end of the tube is plugged? Mine is open at the end.
 

Coors

Captain
Joined
Dec 8, 2006
Messages
3,367
Re: speedometer question

I think you are missing the pitot, that has 3 screws, and is fastened to the transom?
 

sparticus

Petty Officer 3rd Class
Joined
May 31, 2007
Messages
92
Re: speedometer question

So it would seem I am missing the pitot. Anyone have a clue as to where to look to find one? Or......what a boat pitot would look like? A pic perhaps?
 

bigfoot2184

Cadet
Joined
May 30, 2007
Messages
6
Re: speedometer question

I did a quick search on ebay and this came up.... looks right to be, but I'm a newbie to this. I'm sorry if posting links is not allowed here. I'm NOT the seller of this item. Just what came up on Google.

5815_1.JPG


http://cgi.ebay.com/ebaymotors/Boat-Marine-Speedometer-Pickup-Pitot-Assembly_W0QQitemZ260128264393QQihZ016QQcategoryZ26451QQcmdZViewItem
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,219
Re: speedometer question

It doesn't matter which brand of pitot you use. The one in the picture is slanted back to shed weeds (yeah, right!!). Nor does the size of the hole matter. I'll now probably be the rest of the day trying to explain why not, but it does not matter.
 
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