I read on here awhile back about a trolling motor the makes the battery last longer. so you get more trolling time out of your battery.Can someone tell me the name of the trolling motors ?

Just about any of them that are variable speed.  That's fully varialbe, not one of those 12/24 things with the five position switch.  

The variable speed motors use mosfet controllers to control speed.  By design the speed is controlled by the on/off cycles of the mosfet.  Since the motor is not constantly drawing current because of the off cycle, it uses much less current at slower speeds.  The closer to max you run, the less benifit you get.  At max speed there's little difference between any of them with the same thrust rating.

Minn Kota

I think you are talking about a maximizer, correct? I've only seen it on Minn Kota's models.

ya I found it its the maximizer thanks to everyone.

Fl fisher.  If you get one of those I sure would appreciate a review.   Thanks.

Minn Kota Maxxum or Motorguide Tour Digital.  Quieter running, more efficient, longer running motor.  Two companies version of a similar concept.