In the context of the experiment described, where forces are plotted against accelerations, the relationship is given by Newton's second law of motion, \( F = ma \).
When you graph force (F) on the y-axis and acceleration (a) on the x-axis, you can rewrite the equation as:
\[ F = m \cdot a \]
This shows that the slope of the graph (force versus acceleration) is equal to the mass (m) of the motorboats.
Since the equation doesn't include a constant term, if the force is zero, the acceleration will also be zero. Therefore, when you graph this relationship, the y-intercept will be at the point where both the force and acceleration are zero.
Thus, the answer is:
zero.