Coal bunkers offered conditional protection

How effective was coal-bunker protection on nominally unarmoured cruisers?

Full coal bunkers could provide useful protection and reduce flooding, but that benefit disappeared as fuel was consumed and empty dusty bunkers could add danger.

Designers placed coal around vital spaces because a filled bunker could act as a barrier and reduce the volume available for floodwater. Coal occupied much of a bunker's volume, so a damaged full bunker admitted less water than an empty compartment. The protection changed as the ship steamed, however, because a distant-station cruiser consumed the very material forming the barrier. An empty bunker could leave air space and hazardous coal dust where protection was expected. Fuel state therefore determined how much protection, flood control, and stability the arrangement could provide.