I am getting a trolling motor for my jon boat and was wondering could I use a dual purpose motor to crank my engine and use with the trolling motor.  Any impact would greatly be appreciated.

Depends on how much you run your TM. For a small motor, a true deep cycle will start it just fine and will give you a little better service/run time for the TM.   If you don't have a problem with run time on the TM, the dual purpose will work good.    I run two of those big yellow Walley World batteeries on my TM and but I don't fish all day anymore, few hours in the AM and few hours in the PM plus I run a variable speed motor that's very conservative on a battery at moderate speeds.

Thanks Ben, I guess I need to clarify what I am running, my main motor is a 25hp Nissan and the trolling motor is a 40lb Minn Kota Riptide.  By the way I aslo have on of those big yellow walley world batteries.  Had to replace the main battery yesterday.  I usually do not fish all day either, I usually never fishh more than four hours, unless I am saltwater fishing, then I tend to anchor up more.