Has anyone used the Luhr-Jensen Trolling Speed Indicator? It just bolts onto the boat and uses a weighted ball on a pendulum to record speed. Does the thing actually work? Is it accurate? I have a decent fish finder and a handheld GPS, but neither have speed. I would rather not drop $300-$400 just to get my trolling speed if I can spend $50. I'll ask the inland lakes guys too, they might have more experience with this. Any help is appreciated.