What characteristic of a round bottom hull helps it to move efficiently even at slow speeds?

The characteristic of a round bottom hull that contributes to its efficiency at slow speeds is the displacement hull shape. Displacement hulls are designed to move through the water by displacing a volume of water that is equal to their weight, allowing them to glide smoothly. This design is particularly effective for achieving good stability and a comfortable ride in calm water, which is beneficial when operating at lower speeds.

Round bottom hulls have a deep, rounded shape, which enables them to cut through the water rather than planing on top as flat-bottomed hulls do. As a result, they can generate less resistance and are often able to maintain forward motion more easily, even when the speed is not high. This makes them ideal for various types of recreational boats, especially those intended for leisurely cruising or fishing.

The other options do not accurately capture the reason for the efficiency of round bottom hulls at slow speeds. For example, a flat design typically involves a planing hull, which is more suited for higher speeds, and multi-hull structures focus on stability and speed rather than slow cruising efficiency. A high-powered engine would not compensate for the inherent design characteristics of the hull shape in terms of efficiency at lower speeds.