[sorry, the earlier message 'got away' from me before I could properly complete.]
When our Sonic Speed failed, we regained Boat Speed by replacing the preexisting depth-only sensor in the underfloor compartment outside the forward head with a paddle-wheel DST800. The DST800 uses a 51mm (2") form factor, and was a drop-in replacement.
The UDST800 is also a 51mm (2") fitting. Noting however that the UDST800 processor requires a separate power supply, and the specs sheet says that this is 12VDC.
Spanish Water, Curaçao