I need to replace my trolling motor but am confused about what brand and size to purchase. I have an 18ft Javelin. I usually fish KY Lake but I don't fish in extreme wind. I do spend 5-8 hrs bass fishing at a time, sometimes more. Do I need a 12 or 24 volt? Is there a big difference in Minn-Kota vs Motor Guide? The one currently on my boat is original to the boat. A Johnson/Evinrude 12/24. Any help out there?

I suggest that you see about getting your original OMC TM reconditioned. i have doen this in the past and it saved me $$$$$$$$$$$$$ here in north Alabama.

With an 18' boat I wouldn't go any less than 24v.  I prefer MinnKota.  The Maxxum motors are nice.  They also sell a 70 something pound, 24V. Edge series motor if you want to save a few hundred.  

im with cart.  ive got the 70 lb thrust maxxum (24v)on my new stratos.  very nice trolling motor.  im not a fan of motor guide.  if im not mistaken they are owned by Merc and im not a fan of theirs either.  make sure you check out the thrust/speed control on them if you get a new one.  mine has a little knob that goes from 20 all the way to 100 in 10 count intervals.  i think some of the new ones have a clicker type knob with only 5 different positions which makes it harder to control your thrust.  good luck.

I have a 70 lb thrust 24 volt minnkota on my 17-9 stratos and it moves it along very well.  It also lasts all day on a single charge.  If you increase motor size you may need larger wiring and should probably replace the male and female plugs if they are old, as the contacts oxidize over time.  If you go from a 12 volt system to a 24 volt system you will also need two batteries instead of one.  The extra battery will need a battery tray and strap.