Some HB's units have a difficult time keeping the gps location and speed when moving slowly, like when trolling, a red circle (donut) pops up. This usually occurs from .3 up to .8mph. Just something to consider. Been told its a difference in the algorithm between Low and HB. I have a HB and get the donut at slow speeds, its annoying, while my Ipilot and Lowrance keep a accurate speed showing 100% of the time, no matter the speed. The indicated speed on the HB bounces up/down as well due to this "difference". Just FYI.