Your Fish Finder and High Speeds

If your electronics will not read out when your boat is on plane, there can be a couple of reasons. Improper transducer installation is the most common. In fiberglass boats, a puck style transducer is usually epoxied to the inside of the hull in the bilge area of the boat. This gives the best results even at speeds up to 70 mph.
With aluminum boats, the transducer should be mounted on the stern outside the hull, approximately half way between the motor and the outside of the boat on the starboard side.
The bottom of the transducer must be lower than the bottom of the boat (including rivets) -- one-eighth inch will do. The back end must be slightly lower than the front so that air bubbles will not pass under it causing interference. Using your unit in the auto mode may also give you headaches because the computer cannot keep up with fast depth and bottom changes.