The speed reading on my Raymarine Bidata wasn't working. With the sensor removed, I got a reading when I flicked the paddlewheel, so I assumed the electronics and wiring were ok.

I then removed the paddlewheel by gently driving the axle pin out, to find a tiny bit of scaly build-up on it and also on the side faces of the paddlewheel hub. A gentle scrub with oxalic acid using a blue scotchbrite removed all that and then, happy days, it worked perfectly again. The paddlewheel appeared to be quite friction-free when I flicked it, but even so that little bit of gunk was enough to prevent it from working.

I hope this might help others to avoid wasting money by replacing a basically sound instrument.

Cheers, Graeme

My speed through the water is constantly half to three quarters of a knot slower than my GPS SOG, considering any current of course and freshly cleaned. Thanks, I'll give your trick a try, it may be the solution.

Keeping the hull clean forward of the sensor makes a big difference to the accuracy on mine. If I do that I can usually rely on the reading to indicate a water speed that is close to but not exactly the same as the GPS in still water - typically around 0.1- 0.2 knot. It's close enough to reliably indicate whether I have a significant current ahead or behind me. It does vary a bit on either tack.