How Does a Boat Speedometer Work?
Saturday, October 10th, 2009
The Pitometer
- On a boat, the speedometer is known as a pitometer, sometimes called a pit log. These devices are used to measure a boat’s speed as compared to the water. This differs from a traditional speedometer, which is used only in land vehicles. Both submarines and surface boating craft make use of the pitometer. The device makes use of a tube which goes through the hull of the ship down into the water. This tube has two openings: one measures the speed in which water is moving through the tube, andone
which monitors the pressure of the water. When combined, an exact speed of the boating craft can be determined.
The Impeller Log
- The impeller log is a small propeller that is mounted in the water. The rotation speed of the propeller is proportional to the speed of the water moving past the boat. In this way, the impeller log is similar to an anemometer, which measures wind speed through the use of propeller-based technology.
GPS
- In today’s boats, many sailors use GPS technology not only to determine the location of their craft, but to find out how fast they are traveling. By using land coordinate information provided by satellites, GPS devices can measure how quickly a boat is traveling based on how much distance it is putting between itself and two or more points. If the information in the GPS system is sound, it can provide the most accurate speed information of the three boat speedometers discussed here.
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