First make sure your transducer is not inline with anything protruding from the bottom of your boat, such as being inline with a row of rivets etc. Use a straight edge coming out from the bottom of your hull and make sure transducer is approx. a 1/4-3/8" below and tilted slightly forward, in other words so the water coming off your bottom can smack the transducer on its face. Having the transducer tilted even slightly toward the rear will cause air bubbles. You might need to play with the depth and angle of the transducer a bit, it won't take much change to make a real difference in your reading. I have a riveted aluminum hull and it took a bit of adjusting to get it right but it now reads well at 35mph. Good luck.