You won't learn much from a single, small panel "under test".
You really need more information -- ultimately, an "energy budget" for the boat. Then, you figure out how much of that budget can be supplied by the biggest solar array you can mount.
I remember that Calder's "Boat Owner's Mechanical and Electrical Manual" goes through the details.
The planning is important. "Upgrading" is more expensive than doing it right the first time.