Het Rowdy I used those holes to mount my transducer. This was on a bug boat, but same principle. I raise motor just a bit when cruising around scanning with the SI unit. SI doesnt work well on plane anyway, at least for me because my transducer is out of the water on plane. I could never tell that raising the motor changed my image any, so I would have to say I didnt have interference from the motor. I always had some type of lines in my water column, on the left side, not sure where from, but it didnt interfere with my ability to use the unit. I had to send the unit back twice, so I wasnt exactly in the mood to send it back for this when no one knew exactly what it was. No sir, never been on Grand Lake, and my ignorance says I dont even know where Grand Lake is. Let me know if you have any more questions. I can send you a pic of the way I mounted my transducer on my S boat.Britt
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