Speedometers

[bugmud777]

Have a newer Wellcraft inbd/outbd. The speedometer is inoperative. It's the one with the plastic pitot tube (I think). How do they work and what should I look for?

 

[Oldsaltydog]

Re: Speedometers

My outboard has a small hole inlet at the front of the prop housing unit for picking up moving water for the speedometer. The hole sometimes becomes clogged. If you have the same setup, you could probe the hole with a thin wire and see if it helps. Or the plastic tube could be plugged up. Some people use a GPS for determining boat speed.

 

[bugmud777]

Re: Speedometers

I have checked the tube all the way to the speedometer. Air clears through just fine. The plastic piece which drags through the water apears to have a metal plug in the bottom. Resembles a nail head. Should the tube be hollow all the way through?

 

[swist]

Re: Speedometers

There are two kinds of speedos on most small boats. One if the paddlewheel on the transom, usually part of the depth sounder transducer. It's function is obvious - look for anything keeping the thing from spinning freely (or falling off altogether).<br /><br />The other kind is a pitot tube pickup - based on the fact that a fluid rushing past a small orifice which has atmospheric pressure air in it will reduce the pressure in the orifice in proportion to the speed of the fluid (Bernoulli principle?). The speedo dial is actually measuring increased vacuum as speed goes up. (The "fluid" can also be air itself - that's how airplane airspeed indicators work). These openings are very small and and can clog easily. On Yamaha engines they are located just above the bulbous area on the lower unit front - a very bad place for picking up crud. I'm sure there are various other places they are located with other installations.<br /><br />With the advent of GPS, these kludgy mechanical device have become less useful, although GPS gives you speed over ground instead of speed over water. Up here in Maine with 10 kt tidal flows in some places, that can be a BIG difference.

 

[Mark42]

Re: Speedometers

Hi Bugmud777,<br /><br />Sounds like that is a Teleflex model pilot tube. The metal plug should be there, the hole in front and above it. Mine sometimes gets seaweed blocking it, but a quick drop in speed then back up again usually dislodges it. Also make sure it is 1.5" below the bottom if the boat when in the full down position.<br /><br />If the speedo is also a teleflex unit, be careful not to blow compressed air into the guage, because it will damage it (thats what the instructions say). <br /><br />Also check that the hose will hold air pressure or vacuum, not just that it is not clogged. Plug one end and apply pressure/vacuum and listen/check for leaks. <br /><br />You might just want to replace the entire length of tube. It is inexpensive stuff.<br /><br />Good luck,<br />Mark.