Stymie is right. The stock HDSi transducer that gets shipped with the 898 units is capable of high-speed depth readings but only if your boat hull will allow this. With the Si transducers you not only have to locate it in a position on the transom that has a constant flow of air bubble free water (which not all boats have); you also have to mount it in a location that will allow for unblocked left and right Si sonar. This is not always possible on all boats. Many install the Si transducers up higher on the transom or even on the jackplate to protect it from damage (the HDSi transducer is $250 to replace) and install a second transducer for high-speed water depth readings.