Which fitting device is used to secure watertight doors on a vessel?

The correct answer is steel fittings, as these are specifically designed to ensure watertight integrity in a vessel's structure. Steel fittings are robust hardware components utilized to secure watertight doors, creating a seal that prevents water from entering the interior of the vessel. They are engineered to withstand the conditions of a marine environment, ensuring durability and reliability in maintaining watertight compartments.

Rim locks, toggle clamps, and pin locks, although used in various applications, do not provide the same level of security and sealing capability required for watertight doors on vessels. Rim locks are typically found in doors for security rather than watertight integrity, toggle clamps are generally used for quick adjustments and securing items in place, and pin locks are more suited for locking mechanisms but may not generate the necessary seal to keep water out effectively. Therefore, steel fittings are the most suitable and effective choice for securing watertight doors on a vessel.