Pounds of thrust does not equal ft./lbs. #Thrust is a unit of force, while a foot-pound is a unit of torque.
Trolling motors are rated at their maximum thrust.
Power (expressed in horsepower) is another thing altogether. Power, as it pertains to propeller drives, is torque times rpm. The actual pounds of thrust from any given horsepower will vary depending on a number of factors, including prop diameter, number of blades, inherant flex of the blades, and pitch.
For what it's worth, rating electric motors by power would be more meaningful than thrust ratings. If you change the pitch of the prop, for example, you could increase or decrease the thrust of the same trolling motor. So, a 54 lb maximum thrust motor would not always give you a maximum of 54 lbs of thrust.
I have a working understanding of all of this but not well enough to "teach it"..... so here's a link that serves as a good baseline for choosing the proper system for any need.
http://www.trollingmotors.net/trolling-motor-resources