I am looking at buying an old bass boat (87 Ranger).  It currently has a 12v trolling motor.  Is it possible to set it up with a 24v trolling motor?  If so, is this a do it yourself kind of job, or do you need a professional.

What size 12 volt motor does it have and what size 24 volt motor are you looking at.  

Are you considering a 12/24 or a straight 24.  

How many cables go from the battery to the front for the TM and what size are they?

What size circuit breaker is in it now?

This all determines how much converting you're going to need to do.

Changing to 24 volt could be as simple as adding a second battery if the 24 volt motor is going to have a similar amp draw as the 12 volt.

If you only have two wires, a 12/24 needs three so you will need to add another.

If you plan on going to a much larger motor on the front, the existing cables will probably need to be upgraded to a larger size and a larger circuit breaker installed.

All this is more than easy enough for most anyone to do.

very simple. i just changed mine to a 24 system on my ranger.

It currently has a 43 lbs thrust TM, if I get it, I want to go to a 71 or 82.  

Straight 24

Have not bought the boat yet, so I do not know the answers to those questions.  Thanks for the information.

Ranger was one of the first to go to 6 ga cable, so it's possible you may already have 6 ga.  Now, if it was me going to that size motor, I would buy a length of #4 ga cable.  I would run one #4 and connect the two #6's (provided that's what it has) together in parallel, this will make them equivilant to a #4 and save you some money.  If it does not have #6, I would run two lenghts of #4.   You will also need at least a 60 amp breaker for the 71 and probably a little larger for the 82, check and see what the manufactor of the motor you're getting recommends for the size motor.