This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
- Bass Rogue
Typical boat speedometers are water/air pressure driven devices. They
usually starts indicating speed when the boat reach the 20-25 mph range.
The way these speedometers work is a pick-up tube (pitot tube) is placed
in the water with an opening facing forward. This tube is connected to
the speedometer by a small rubber or plastic hose. As you drive along,
water is forced into the tube opening which squeezes or compresses the air
in the hose. This, in turn, moves a piston to rotate the speedometer
With that simple description, you can probably come up with a couple of
ways for things to fail. First, the pick-up tube opening can get plugged.
This opening can be on the leading edge of the big motor lower unit, or it
can be a hard plastic device, usually white nylon, mounted to the transom.
Either one of these openings can be cleaned with a small piece of wire.
Be careful not to change the size or shape of the opening when cleaning it
or you'll change the accuracy of the speedometer, which probably isn't
very accurate to begin with. If while cleaning, you manage to lose some
of the stuff down inside the opening, disconnect the hose at the
speedometer and blow into it.
If the pick-up opening appears to be okay, maybe the hose has a break in
it. To check that, disconnect the hose as close to the pickup tube as
possible and blow into it. You should be blowing into a dead-end. If the
air just keeps on going and going, find where the hose is broken or
disconnected. I guess another way to put this blowing stuff would be to
say, under normal conditions, blowing from the speedometer end should be
easy, while blowing from the pick-up end should be difficult.