Pitot-tube Speedometer Diagnostic Methods

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Bigbear
Posts: 21
Joined: Sun Sep 17, 2017 8:55 am

Pitot-tube Speedometer Diagnostic Methods

Postby Bigbear » Mon Aug 13, 2018 6:36 pm

[I am diagnosing] the malfunction of speedometer system. If I hook a pitot tube to the speedometer and draw a vacuum by sucking the tube, should I be able to move the needle on the speedometer? Thanks

Yellowjacket
Posts: 24
Joined: Mon Jul 31, 2017 9:33 am

Re: Speedometer not working

Postby Yellowjacket » Mon Aug 13, 2018 9:00 pm

No, you need pressure to check the gage, and the amount isn't that small. That is, you probably can't blow on the tube enough to make it register. Here's a typical calibration for Teleflex speedometer:

    5 PSI = 20 MPH 
    11 PSI = 30 MPH
    19 PSI = 40 MPH
    30 PSI = 50 MPH

If you have a pressure gage or hook it up to a pressure source you can check your calibration.

Boat speedometers are just good old fashioned Bourdon-tube pressure gages, calibrated to read in speed rather than PSI.