Best comment of 2015!

I have a 24v on my 18 1/2' boat. If I could find room to add another battery, I would switch to a 36v.

I have a Ranger 521 (almost 22'). Some say its a barge; I say its a well-stocked floating tackle supply store. It takes a certain amount of watts (energy) to move any boat. Watts = voltage x amperage. So, the higher the operating voltage for a motor, the less amperage draw that motor will have to generate the same watts of power. Deep cycle batteries are happier when the amperage draw is lower.

Yes, a 3rd battery does add weight, and you might get to a spot 1.087 seconds later. However, with 36V you have brought along more stored energy, and will will draw that energy at a lower amperage rate than if you had a 24V motor. As stated by others, that means you can continue to fish without running batteries down.

Fishing the same in a given day, you'll drain the 24V system batteries deeper in discharge than a 36V system. The batteries in the 24V system get more abuse this way, and won't last as long in service. Every way I slice it, I see the 36V system being better for any boat that has the carrying capacity to handle it.

What all does it take to covert from a 24v to a 36v, other that an additional 12v battery?

A 36v trolling motor and may have to upgrade the wires, depending on what yours was wired with.

If your existing wiring is done with 6 gauge wire, you already have adequate wiring. (A 36V system actually draws less current through the wire than most 24V systems.) When the third battery is added, it is inserted in series with the first two. For this, you'll need one more short #6 wire to jump between battery 2 and battery 3.

It is highly recommended that all 3 batteries be of the same capacity rating, and preferred that they be of the same vintage so that older batteries don't eventually abuse a new battery added to the system.

If you get a 24 volt get batteries with the highest amp hr rating. Sears Platinum Diehard Grp 31 would be good ones. They have 100 amp hours at the 20 hr rate. Most batteries have less than 80. I know several that have a 24 volt motor on Center Console for Saltwater. 36 would be nice for bird fishing where you need to move at a pretty good speed to keep up.

Which battery you get makes a huge difference. I can run my 24volt 80 Motor Guide Tour all day of normal fishing and can't really tell the batteries are getting drained. On a real windy day usually I have as much trolling motor left at the end of the day as anyone.

Downside is they are about $700 for two. They are also heavy at 75# each.

Not near enough. A Fortrex 112 will also let you chop through most any vegetation/pads that you want to go through without bogging down.