Hi Daniel and welcome to the forum.
Generally speaking a motor will smoke if it’s badly overloaded or simply faulty.
I would check that the propshaft is not binding, and that any lock nuts are adjusted and nipped up tight. Also check that the motor coupling, is free.
I assume the correct voltage battery was used, and perhaps the 15 minutes was not spent at full throttle.
Also the motor may have been unsuitable for the prop you are using and is overloading the motor. A worthwhile purchase is a watt meter to check the current being pulled by the motor.
Was the motor brushed, or brushless to start with, and could we have some info on motor,battery, prop, size of tug etc.
Simply fitting a new motor may not cure your issues….
Ashley.