Observations and results
Did the larger boat hull support a greater amount of weight? Did both hulls have a similar density right before sinking, which was roughly one gram per cubic centimeter—the density of water?
When you first put one of the boat hulls on the water, it should float because its total density is less than that of water. As you add pennies to the hull, its density increases and the hull floats lower. Eventually, when enough pennies are added, the hull's density roughly equals the water’s. This happens right before the penny that sinks the hull is added. The hull sinks because its density has finally become greater than that of water. Consequently, the hull’s density right before sinking should roughly equal the density of water, which is one gram per cubic centimeter. Even though the larger hull supports more weight, it also has a greater volume, and both hulls should roughly have a density of one gram per cubic centimeter right before sinking. (Your densities may not have been exactly this, but may have ranged between 0.7 to 1.3 grams per cubic centimeter. Sources of error that could be eliminated to give you an answer closer to the actual density of water include more accurately calculating the volume of each hull, using something smaller than pennies and including the hull's weight in your calculations.)
Be sure to recycle the aluminum foil when you are done testing your hulls.
More to explore
How does a boat float if it's heavy?, from the University of California, Santa Barbara, ScienceLine
Rainbows on Titan, from NASA Science News
Cartesian Diver, from PBS Kids DragonflyTV
How Much Weight Can Your Boat Float?, from Science Buddies