Yellow triangular fairing block insert
Do I really need to insert the yellow triangular plug into my fairing block?
Yes. The goal is to create a smooth surface so the water flows cleanly over the fairing and transducer. Boat drag will be minimized and, more importantly, water will be free of bubbles and turbulence.
If the transducer is recessed more than 0.5 mm (1/64th inch)inside the fairing, shim the transducer or carefully file/sand the fairing until they are flush.