Why is a timber deck cargo vessel allowed to have a smaller GM than other cargo vessels?

The correct answer highlights an important aspect of ship stability related to the specific design and characteristics of timber deck cargo vessels. These vessels typically have a timber deck, which allows for a higher freeboard compared to those vessels carrying heavier materials. Freeboard is the distance from the waterline to the upper deck level, providing an additional margin of safety.

Having an increased freeboard means that the area of the deck edge is submerged at a greater angle of heel. This allows the vessel to maintain stability longer as it heaves or rolls in the sea, which can help prevent premature immersion of the deck edge. Consequently, these vessels can safely operate with a smaller metacentric height (GM) because they can tolerate a greater list before experiencing issues related to stability, such as deck edge immersion or capsizing.

The factors involved with timber as a cargo also play into overall ship design, but it is primarily the increased freeboard that allows for a different stability standard in timber deck vessels. Thus, this unique structural feature is why a smaller GM is permissible on such vessels.