What type of boilers are all propulsion boilers on modern ships?

Modern ships predominantly utilize water-tube boilers for propulsion. This boiler type is characterized by its design, where water circulates through tubes that are heated externally by combustion gases. This construction allows for a number of advantages, particularly in propulsion applications, such as higher steam pressures and efficiencies, reduced water volume, and quicker response times to load changes.

One of the key benefits of water-tube boilers is their ability to withstand high pressures, which is often a requirement in marine propulsion systems. They also typically possess a smaller footprint and lighter weight compared to other boiler types, making them more suitable for the space constraints of modern vessels.

Various other boiler types exist, but fire-tube boilers, while historically significant, are less common in modern ship propulsion due to their limitations with high-pressure steam and slower heat recovery. Steam boilers, as a broader category, may include both fire-tube and water-tube designs, but the specific reference in the context of modern ship propulsion points specifically to water-tube configurations. Composite boilers combine both fire-tube and water-tube elements, which is often more applicable in auxiliary applications than for main propulsion systems.