Electric power is measured in watts.
Watts is determined by multiplying amps times volts.
If your trolling motor is using 50 amps at 12 volts, then it is using 600 watts of power.
One horsepower is approximately 746 watts.
So, assuming 100% efficiency, which never happens, your trolling motor would have eight-tenths of one horsepower output.
It is probably more likely to be about 1/2 horsepower rating, or maybe 3/4 horsepower at the most optimistic rating.
Now, a fully charged lead acid battery actually has 13.2 volts at full charge.
So in reality, you could be getting 13.2 x 50, which would be 660 watts, or .88 hp at max, less whatever inefficiencies are in the system. So, even at best, you're still going to be 3/4 hp on a good day, with a full battery, and a very high efficiency motor.