Back in the days before the paddle wheel on most fish finders and then the GPS systems, there was a way to judge your boat speed for trolling. Something similar to what the ancient sailors used. The gadget that I used to estimate my trolling speed was made by Eagle Claw. It had a MPH scale (0-6+) and another unit of measurements that you could use for your best bite speed (broken down in smaller units), whether you were using the trolling motor of your kicked motor, it worked. Attached to this scale was a piece of metal which pointed to which ever unit of speed you wanted to watch. Attached to the metal pointer was a lite weigh metal cable, about 8 feet long, which was attached to a weigh. How accurate was it, who knows, but it would put you on the desired trolling speed to catch striper's when you were pulling hard baits.