ahab,
Is the Signet user manual silent on this topic? Ramarine has a semi-automated method of calibrating the transducer to bias current. I will attach a URL to the .pdf. The relevant section is on page 31, "Speed Calibration".
http://www.raymarine.com/SubmittedFi...81037_5www.pdf
The process could be simulated with your GPS. Power into the current at a set RPM and note the SOG: turn and return on the same leg and note SOG. Calibrated speed through the water = the resulting average of the two SOG readings and 1/2 the difference is the current offset.
Is the boat
Pequod?
Wayne