Owners Manual?
Nick - The first thing I'd check is the Owners Manual and see what it has to say. Normally there is a trouble-shooting guide. Raymarine web site sorted for "ST60": http://www.raymarine.com/raymarine/SearchResults.aspNoticed the sort didn't list "speed", just wind, depth, and tridata.If you find the Handbook that belongs to your instrument I recommend saving it to a file and adding it to an electronic "Owners Manual" for your boat.Without having read the Owners Manual, the first thing I'd look for is corrosion at an electrical connection between the transducer ant the readout.