It's a science project, it would be very expensive to buy a bunch of plans just to study the hydrodynamics of them.
Mea culpa. I read the original post too fast. I thought he was looking for a "5th grade science fair project" explanation for an AP/college prep math problem (I'd like to hear that explanation also). I failed to catch that it was literally for a 5th grade science project.
:-[
Shallow Breathe,
Attempt to the greatest extent possible to help him keep the length, width and weight of each model standard across the models to minimize variability. You'll probably have to add weight to two of the hulls to accomplish this. You want the only variable to be hull form if you are looking at draft with added weight. If your son presents a clear hypothesis and can explain the reason to minimize variability and the difference between accuracy and precision in addition to his actual results by the end of this, he will really impress the judges and have a much better grasp of the scientific process.
Have fun!
Nate