Speedometers?

[onebohemian]

Can somebody give me a quick crash course in how boat speedometers work? I assume there are exceptions but just in general, how do they usually operate? Are they connected to motor or somehow just based on water rushing by boat?<br /><br />I've got an '87 Evinrude 90 V4 on an '87 Crestliner 17ft. Speedometer is in the dash but hasn't worked since I bought the boat used. Sometimes the needle will climb a little, but for the most part it is just dead. Use the fishfinder for watching speed so haven't cared much about dash unit, but now would like to fix dash unit if possible. Inspected back of the guage by looking under dash and poking around a little but realized that I have no clue how it is supposed to work in the first place. Don't have boat owner's manual either, although I'm not sure that would help.<br /><br />Thanks, <br /><br />Mark

 

[MGuckin]

Re: Speedometers?

Most speedos work off water movement. Should be a pick up tube somewhere on the hull normally mounted at water level on the stern. Water turns a little wheel and sends the water thru a hose to the speedo. Could be as simple as the hose disconected or plugged up. Very basic but mostly used as a reference, not very accurate.

 

[wilde1j]

Re: Speedometers?

Boat speedometers work on the pitot tube impact pressure principal. The dash instrument is just a pressure gauge with a square root scale dial which indicates the measured pressure and is scaled into MPH by the square root dial. The tubing from the stern to the panel may be plugged ... remove it at the speedometer and shoot some compressed air thru it to clear it. If you rig has a nylon pickup at the stern, these often get kicked back from hitting debris. If so, just reposition so the pickup hole is horizontal.