Quote Originally Posted by sherman51 View Post
gps speed is very accurate but may vary from a speedometer that uses a paddle wheel. I use to have a depthfinder with the paddle wheel that showed me going 3 mph but my gps showed me going 2 mph. so now I just use my gps. and in the summer months I like to run about 1.8 to 2.0 mph. but there are a lot of guys that troll even slower by there gps. this is only my opinion and others may disagree.
GPS is digital reception of signals from satellites , usually from at least 12 different sources that basically triangulates your position and how fast that position is changing which translates into speed, the faster your unit processes those signals the more accurate it is. The paddle wheel works like the line counter on your trolling reel and calculates the speed of your boat by the number of revolutions over a given time span which is programed into the processor of the sonar. Current speed and direction, debris(moss, zebra mussels) in or on the wheel mechanism and even the location of the wheel on the boat in relation to the prop can all affect the accuracy of the speed, especially at lower speeds that are used in trolling.