Sounds like penant B - dangerous cargo my dusty memory thinks.
often seen on petrol, oil and other tankers - do the professional seamen agree?
You often see ' Patrol ' Boats (new ISAF Regs say you can't be rescue or safety but patrol boats now) flying 'R' to distinguish them from coach boats at sailing events.
Or there is always that old favorite that all sailing and power boats carry.......
The barrel of burning tar on the foredeck!!!!!!