Boat speedometer often have a tube (called a pitot tube) that runs to the back of the transom, or through the lower unit of the outboard motor. When the boat is in motion, water flows into this tube, compressing the air in the tube. As the air in the tube is compressed, it moves the needle on the speedometer, much like the needle on a blood pressure cuff. The most common cause of speedometer failure is a plugged pitot tube.
Look under your boat's dashboard and find the tube running from the center of the back of the speedometer. The tube probably will be black plastic and thicker than most boat wiring. Trace the speedometer tube to the back of the boat, where it will connect either to a pitot tube under the transom or to the lower unit of the outboard motor. Inspect the length of the tube for holes, cuts, kinks or wire ties that visibly pinch the tube. If the tube is cut or has a hole, replace the tube. If a wire tie is pinching the tube, remove and replace the wire tie, making sure not to pinch the tube.
Disconnect the speedometer tube from the pitot tube or lower unit, either by releasing the quick-connect joint to the motor or by using the screwdriver to unscrew the connector at the pitot tube. Inspect the pitot tube for breaks or holes; if any are present, replace the pitot tube.
If your speedometer tube connects to the lower unit of your outboard motor, open the case of your motor's lower unit and visually inspect the tube for cuts, breaks, holes or kinks. If any are present, replace the speedometer tube.
Tell your assistant to watch the speedometer and let you know if the needle moves, then blow into the tube for 10 seconds. It takes very little breath to make the needle move as high as 5 mph.
If the needle does not move when you blow through the tube, disconnect the tube from the back of the speedometer (it should simply pull off) and use a compressor--at a very low pressure, under 10 psi--to blow any debris out of the tube. When the compressed air blows freely through the tube, reconnect the tube to the back of the speedometer and blow through the tube again while your assistant watches the speedometer. If the speedometer still refuses to move, replace the speedometer unit on the dash. This unit is not repairable by the consumer.