Dont be shy about it - I had to drill a 2.5" hole for my perfect pass paddle wheel speedo sensor. With some generous silicone, it hasnt leaked a drop
It was worth it for perfect pass alone - its saved my marriage. Additionally, I get a speedo that actually registers a speed below 20mph and water temp.
That said, I think my OEM depth transducer is epoxied to the flat spot on the bottom of the hull. Directly below the engine. Ill try to sneak a peek later in the week.
