Three-drum boilers are a class of water-tube boiler used to generate steam, typically to power ships. They are compact and of high evaporative power, factors that encourage this use. Other boiler designs may be more efficient, although bulkier, and so the three-drum pattern was rare as a land-based stationary boiler. Firing can be by either coal or oil. Many coal-fired boilers used multiple firedoors and teams of stokers, often from both ends.