What is the most cost-effective method to limit free surface when building a ship?

Increasing the number of longitudinal bulkheads is recognized as the most cost-effective method to limit free surface effects when constructing a ship. Free surface effect refers to the impact on a ship's stability due to liquid sloshing in partially filled tanks, which can lead to a heel or list that compromises safety.

By adding longitudinal bulkheads, the internal volume of tanks can be segmented into smaller compartments. This segmentation minimizes the free surface area where liquid can move freely, which in turn reduces the potential for destabilizing moment created by shifting liquid. The bulkheads effectively create barriers that confine the liquid to smaller areas, thus maintaining the ship's stability even when some tanks are only partially filled.

In contrast to the other options, which may address stability in different ways but could entail higher costs or have less direct impact on free surface limitation, the addition of bulkheads is a practical and efficient solution that enhances both safety and performance at a relatively lower expense compared to the costs of altering tank sizes, using heavier materials, or modifying the overall design of the vessel.