check your wiring
st60 will have a 5 way connection as the following..
red : speed +ve
green : speed signal
Brown : thermistor
white : thermistor
A bad thermistor, or lack thereof will not be the problem, it is not required to be present for speed to work.. if your speed log is a combined speed/temp then there should be ~10kohm between brown and white.
check there is voltage between red and screen
, this is the supply the goes to the paddle... cannot recall
the exact voltage you can find it online..i *think* its about 6V or so
If you can pull the transducer, the voltage between screen and green will go from 0V to the voltage previously measured for every 90 degree movement of the paddle wheel
it IS possible you have a bad transducer.. mine died two years ago...but certainly check the wiring
first.. and, if necessary, you can cut the cable near the transducer and install a terminal block.
The other outside possiblility is that if you dont have SOG on the seatalk
then its a possibility that your ST60 has defaulted to reading SOG vs Speed... this is a dealer setup config.. and the natural default for the instrument.