I have an old Raymarine ST60 Tridata instrument, with a non-retractable Airmar 2″ through-hull depth transducer (assumedly M78713) and a retractable 2″ speed transducer (assumedly M78712), probably also from Airmar. Now, of course just after lowering, the Tridata surprised me by displaying a depth of —- m, which is a sounder, wiring, or instrument error.…