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There is no way to tell for sure that you would be able to get high-speed water depth readings for any location as all boats are different, even two that are set up identically.
If you mount the HDSi transducer under the step you risk damaging the transducer and its expensive to replace, but that may end up being your best option.
Mounting on one side of the motor or the other would cause the motor to block some of the Si sonar to the opposite side (mount on the right side of the motor and the motor blocks the left Si sonar), unless you can trim up the motor when you use the Si sonar that is. That would still leave you with no water depth readings at high boats speeds though.
There is a multi-HDSi transducer installation that we could explore but that means installing a second HDSi transducer and an AS-Si-LR-Y cable but there are no high-speed water depth readings guarantees.