Hey guys, my first post on here! I did my Boiler watch keeping certificate on the Rothsay in @87 and as a totally manually controlled boiler system (no auto controls for fuel, water or air), a fast backdown was about as complicated as it got for the boiler room crew. As i remember it, we went full ahead to catch up with the RFA and keep the swell between the 2 ships to a minimum duration and make it easier to get along side and lined up right. Then astern was selected, to rapdily slow the forward speed and quickly match the relative speeds between the 2 ships and finally for'ard was selected again with the right revs to match the RFA's speed.
So for the full ahead part, we would have had most if not all burners in the boiler and feed pumps and forced air blowers matched for that amount of fuel. When we were almost along side, but going a lot faster than the RFA, the throttles would have been flipped over to astern. At this point the main engines were not using any steam, as it takes a short while to spin the throttles over to astern and so as not to trip the boiler safetys, most of the burners were pulled out and you almost idled for a few seconds. Now the astern throttle settings on the main engines used a heck of a lot more steam that the equivalent revs for for'ard settings, so as soon as full astern was rung up, it was a mad rush to get all the burners back in and get the feed pumps and blowers back up to speed for all that fuel. This was only done for a few seconds to slow the forward momentum down and match the speeds. Then when the throttles were spun back over to forward, we pulled everything back off for a few seconds again, for the same reason as above, before finally applying whatever was required for the forward revs requested.
All of that happened in maybe 60 secs or so, from full ahead, to full astern, to some revs ahead. Was a blur of boiler front stokers arms pushing burners in and out, lots of shouts and hand signals from me at the control panel and hopefully no safetys got lifted and no black smoke made. And we practised this stuff, a lot!
So here is where i am going with this, all that black smoke says to me that a telegraph signal got made that no-one was expecting and the boiler room did not have their ducks lined up, to be able to keep up with the throttle and hence steam requirement. Thats probably too much fuel and not enough air and hence the black smoke. Bear in mind the boilers don;t know what throttles have been selected on which engine, they just try to keep up with demand. But that much smoke that quickly, says they changed the throttle settings real quick, one way or the other. Just adding a rew more revs to one turbine and not the other, would be unlikely to have that affect. Whacking them open, the steam pressure dropping, more burners getting shoved in the boiler without the correct amount of air would. If the steering gear failed just as they were coming alongside the RFA and they went from full astern to full ahead on the port turbine, to try and steer away, that might do it, but the boiler room would have been expecting a forward command to come. Seems to me the most likely thing was a wrong telegraph command.
Coming into dock could be almost as tricky, with lot of changes to forward and astern commands. Our MEO always used to come down to the boiler room for a chat before any manouvers like this, just to explain what might happen. I never lifted safetys in my time on the Rothsay and only made black smoke once and only for a few secs til the Chief Stoker give me a brief insight as to my immediate future and health prospects!!!