What feature is critical for determining the stability of a vessel?

The keel is a crucial component for determining the stability of a vessel, as it is the structural element that runs along the bottom of the hull from the bow to the stern. Its design and placement provide the necessary weight and balance that help to keep the boat upright in the water. A deeper and heavier keel generally enhances the boat's stability by lowering the center of gravity and increasing resistance to tipping or capsizing, especially in rough waters.

While features like the gunwale, chine, and transom contribute to the overall structure and handling of the boat, they do not play as significant a role in stability as the keel does. The gunwale is the upper edge of the hull, which helps to contain water within the boat, while the chine defines the shape of the hull and affects its performance in turning and planing. The transom, on the other hand, is the flat surface across the rear of the boat and mainly contributes to the boat's structural integrity and aesthetic design, not its stability. Thus, the keel stands out as the essential feature involved in maintaining a vessel's stability on the water.