Sonar not reading at speeds and speedometer too.

His is probably an age old question and has probably been answered a 100 times but, I'm new to boating so I need some help please. When I'm running at speed in my 19ft Redfin the Hummingbird Sonar 500 doesn't pickup the bottom sufficiently, I get readings all over the place whic is worrying considering the sudden change in depth in and around the bayous I fish, or try and fish.



It was mounted on the boat which I purchased a few weeks ago. I checked the transducer mounted on the transom and it seems solid.



Speedometer also doesn't work, where can I start to troubleshoot these units?



Thanks in advance.