There is no single answer to this question as the proper depth for a trolling motor will vary depending on a number of factors, including the type and size of boat, the water conditions, and the type of fishing you are doing. However, there are some general guidelines that can help you determine the ideal depth for your trolling motor.
If you are using a small boat in calm waters, then a shallow depth is generally best. This allows your trolling motor to work more efficiently and prevents it from getting caught on underwater obstacles. In larger boats or in rougher waters, a deeper depth may be necessary to ensure that your trolling motor has enough power to keep the boat moving.
When fishing, it is also important to consider the type of fish you are targeting. For example, if you are fishing for bottom-dwelling fish like catfish or bass, then you will want to keep your trolling motor at a shallower depth so that it doesn't get snagged on submerged logs or rocks. On the other hand, if you are targeting pelagic fish like tuna or marlin that swim near the surface, then a deeper depth will give you better results.
Ultimately, there is no perfect answer when it comes to determining the ideal depth for your trolling motor. It is important to experiment and find what works best for your particular situation.