Another Option
Have you considered buying a handheld GPS instead? My 1979 Oday 25 came with a nonfunctioning knotmeter. Last summer I travelled 130 miles down the intercoastal waterway with a friend who brought his handheld GPS. I fell in love with the speed feature that gave us velocity made good over the ground. In tidal waters this was much more useful than our speed through the water since we were often fighting 2-3 knot currents. I liked it so much that I am planning on removing the knot meter rather than replacing it.Another advantage to using the GPS is that you don't have to put a hole in your hull for a thru hull fitting.
