Anecdote and data often get confused. My 75 watt solar panel, which is 2 feet x 4 feet in size, yields about 4 amps of power in full sun. That's 18-19 volts regulated down to 13-14 volts, to charge the battery. That's in full sun -- no clouds, no shadows, sun directly overhead. For every thin cloud, passing tree branch, and lower angle of the sun, less amperage.
All you really have to do is look at the motor you intend on using, and find out what amount of amperage it uses every hour. Then look at the amp-hour capacity of your battery, and divide by two (since if you drop the battery below 50% charge you'll kill it). That will tell you more or less how many hours you can run without a solar panel before having to re-charge or switch batteries.
Then look at the rating of the solar panel you think you want to use and add some fraction (say 75% due to various inefficiencies) of that back in for every hour you think you'll get sun on the panel.
I can give another example. I also have a 10 watt panel that I use for charging a small gel cell battery for special applications. It puts out -- at best -- 0.4 amps every hour, and is about 14" x 14" in size. That means that on a really good sunny day with 10 hours of honest charging, you could put about 4 amps back in a battery with a small panel.
I doubt that would run a trolling motor very long, because when I did a quick Google search for trolling motor amp-hour usage, I learned that at fast speeds they draw 30 amps an hour and at slow speeds they still draw 5 amps. Now if you had a true 100 amp-hour battery and could only draw it down 50%, you would only get abot an hour and a half at full speed.
If you like, take a look at this site for a little more info. http://www.minnkotamotors.com/support/faq.asp#hi
On the other hand, if you were going to be back in the world every night, you probably could pull this off with just re-charging the two batteries overnight.