John - I had the same feeling until we raced Gov Cup two years ago. The impeller hadn't been in for 2 years. When I installed it for the race, I saw that it wasn't working, so we used a GPS for the race. GPS SOG lags actual boat speed it really isn't very good at telling what the boat is doing now. On top of that, we used a handheld and at night had to keep turning on the backlight to see what was displayed. This year, we used the knotmeter in conjunction with the GPS and it really helped us to know whether we were he aided or hindered by the current. At times, the current was ~1kt.
I highly recommend slathering Vaseline on the impeller. I read about it here on Sailnet and decided to give it a try this year. We're based on the Yeocomico off the lower Potomac, and marine growth is insane during the hot months. I only had a small bit of slime on the impeller in August after 2 months in the water. After that, there was nothing on it until we hauled in November.
Sabre 38 "Victoria"