(DG) ST 60 speed

I have a ST 670 speed transducer that has just started to register speeds of approx 60% of true GPS speed (in a zero tide environment).
I tested it with a multimeter and it shows that there are two magnets working.
I assumed that the internal electronics weren't good so purchased a replacement
The replacement does the same thing, registering 3 knots at 5 knots true.

Any suggestions?

Thanks Conrad