Re: Another speedometer question (?)
Speedo is nothing more than an air pressure gauge. Azfyrfyter63 has it pretty accurate, except the water does not travel all the way up this tube to the guage. I'd be surprised if more than a few inches of the tube get water at high speed...but at speed, the water in the end of the tube is getting forced in, pressurizing the air in the tube, and causing the needle to move at the guage.<br /><br />If your boat doesn't have the little thing hanging on the transom he mentions, a lot of motors have the hole built right into the leading edge of the lower unit...I don't know about Force.<br /><br /># 1 cause of speedo failure is gunk, lime buildup, or debris plugging the hole where the air pressure tube attaches at the back of the boat. Make sure the line and water intake hole allow air and water to travel freely, and make sure the tube is not kinked, rotted, or leaking anywhere. Usually you just need to disconnect the tube and clear it with compressed air and a stiff wire. The gauge on the dash is probably fine, but the tube is fragile and the opening in the motor or at the transom is prone to blockage.
