I got this from Cabela's web site while looking for trolling motor information.

Thrust

The size and weight of your boat translates into your need for power (thrust) and endurance. The basic rule of thumb for determining the amount of thrust you need is five pounds of thrust for every 200 pounds of weight. Don't forget to include the weight of your boat's contents in the calculation. To be sure that you have enough power, take the boat's estimated weight and add in its maximum weight capacity. A boat that weighs 1,500 pounds, and has a maximum weight capacity of 1,200 pounds would need a motor with 67.5 pounds of thrust. (1,500 lbs. + 1,200 lbs. ÷ 200 = 13.5 x 5= 67.5)

Volts

"Generally speaking, if you have a boat that is over 16 feet and you're a daylight to dark fisherman, you're going to want at least a 24-volt system. For the angler that goes out for a few hours on the weekend, a few times a year, a 12-volt system is going to be adequate, but for anglers who hit it hard all day, several times a week, a 24-volt system would be the minimum and a 36-volt system would be optimal. It's all about endurance and performance!"

Extended PowerElectric trolling motors have evolved into multi-battery systems because anglers wanted to be able to use their motors at a higher level of performance over longer periods of time. Having two or three batteries in series just gives you more capacity to draw from. Batteries for marine application have also evolved tremendously. In the beginning, the only option was to use the same type of battery that was designed to start a vehicle, which only requires a brief burst of power. Batteries that are designed for starting a motor perform miserably in marine applications, except for firing up your main engine.

Today, anglers can choose from beefier "deep-cycle" varieties including gel and dry cells that cost as much as $250 each. Top end deep-cycle batteries have thicker cell plates that store more electricity and recharge at a faster rate. Gel cell and dry cell batteries are ideal for marine application because they don't require maintenance, don't spill acid, and are more impervious to vibration, a real battery killer. The term "deep-cycle" means that a battery is designed for repeated deep discharging (cycling the battery to as low as 10.5 volts) without premature failure.

Good luck.