I think this would do the trick
sorry I don't have much time for a full explanation
well you can make a half circle out of a piece of plywood
put marks every 1/8 inch
put a line with a weight on it and mount in on the side of your boat
the line will move back to the same spot each time you go the same speed, so if you get someone in your boat with a gps and mark the speed and where the line is you will have a good trolling speedometer
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