On my 998, I was able to read 2D bottom on my SI transducer to 21MPH. ( My top speed). Your 2D transducer should read much higher speeds. My first suggestion is to lower the back end of the 2D one click. If that doesn't work, try lowering a second click. Still no luck, now comes the dreaded recommendation of lowering the entire transducer to get under the air bubbles caused
by the rivets. My boat has a bazillion rivets, so I've been down this road. The goal is to get a smooth flow of water with no bubbles flowing over the face of the transducer.
Mike