Final answer:
The best factor in determining boat speed is the propulsion force, which must overcome drag, and is influenced by the propulsion mechanism and the boat's velocity relative to the water and any movement of the water itself.
Step-by-step explanation:
When determining boat speed, several factors are at play, but one of the most significant is the propulsion force exerted on the boat. In the given scenario with a moderately large barge, the force of propulsion (Fp) is critical to overcome the drag force (FD), which is relatively small at low speeds and is less than 1/600th of the weight of the ship. We must also consider the boat's velocity relative to the water, and in cases where the boat is in a moving fluid, like a river, the water's velocity relative to the bank must also be included. This is exemplified in Figure 28.13 where a kayak's total velocity is the sum of its velocity through the water plus the water's velocity relative to the riverbank.
Additionally, the propulsion mechanism, like propellers, can affect the boat's performance as well.
Therefore, to maintain desired speeds and avoid accidents such as running into docks, matching the propulsion force to the water conditions and ensuring the boat’s hull is well-designed to minimize drag are key aspects to consider for optimal boat speed.