I hate to rain on a parade.

You will have to have a solar array larger than the boat to just keep up
with a electric trolling motor.

Now if you just want to charge the battery while the boat is tied up or at a
mooring, then you could get by with a 50 watt panel that could charge a
group 27 battery in a day or so of good sun light.

Before you commit yourself to a project like this, you should realize the
limitation of solar and the number of watts your motor will use, battery
capacity, etc.

Steve
s/v Good Intentions