Modern ships are powered with giant slow-speed diesel engines, ranging in size from that of a small house to that of, well, a larger house.
The era of sail may be over, but there are still sailing ships being built. So it goes with steam- the heyday of steam ships ended in the 1970's, but you will still find plenty of ships from that era working. While far less efficient than steam power in terms of fuel consumption per horsepower (burning 2+ times as much fuel per hour when compared to equivalent modern diesel engines) steam power still holds several advantages- a longer service interval for major maintenance, and instant throttle change variability and availability, as well.
Ships' diesel engines don't have transmissions- they're just too big for that sort of thing, and the transmission to handle that much power would be half the size of the engine itself. So, to go in reverse, the engine needs to be stopped and restarted in the opposite direction. A massive reserve of compressed air must be held in readiness for this purpose, but, even so, there is a limit to how often an engine can be shifted in a short time- I've never done anything but simulator time on a diesel plant, but the simulator I trained on was modeled for training on a top of the line ship, and had only 6-7 shifts available in a 15 minute period. This makes port entry a determined process- once started, it's not so easy to stop! At sea speed, modern diesels must be throttled down gently- in the range of 1rpm at a time, literally, when shifting from sea speed to full ahead (full ahead is actually not quite 100% of the usual throttle range- it's the full speed available when throttling may be necessary. As I said, diesels don't like to change speed. Such a large engine must be cooled and warmed slowly to avoid thermal stress to the engine, so going from sea speed to a speed where the engine cools to a point where it can be throttled more rapidly is critical.
A steam plant requires changing the number of burners active, and opening and closing valves. There is infinite number of throttle changes and shifts available. No worrying about running out of air. To go forward, open one valve. Faster? Open the same valve more. Stop? Close the valve. Stop fast? Close the valve, and open the reverse valve. Reverse? Open the reverse valve. Faster reverse? Open the reverse valve more. That's it. 2 valves, and there are your throttle controls.
One is labeled 'ahead' and the other 'astern.' Pretty simple.
The steam revolution didn't stop at the turn of the last century. The last two generations of steamship were turbine powered- no giant cylinders turning a shaft like you saw in 'Titanic'. JUst two turbines in the engine room. For a single turbine array, there is a small, high-pressure turbine that takes steam direct from the boiler system, and then a larger low-pressure turbine that takes the now partially-spent steam from the high pressure turbine and gets some more oomph from it before returning the steam to the condensers and ultimately back to the boilers.
Silver one is the high-pressure turbine, and the larger one is the low-pressure turbine. This was the powerplant on a 700-foot tanker.
The boiler array is far larger than the turbines, requiring it's own room. The boilers feeding the above turbines were the size of a house- 3 stories tall and 60 feet wide, maybe 30 feet deep, and there were two of them.
I started out in the engine room of a steam ship. I planned on staying there. One long weekend spent hanging halfway out of one of these boilers during a breakdown in the Caribbean was enough to send me out on deck forever. Burns, heat, heatstroke, repeat for 3 days.
Connecting the turbines to the shaft is the bull gear, a transmission that takes power from both of the turbines, which are spinning at differing rates. Beyond the bulkhead in this last photo is the propeller and rudder.
I am Paul B, and I spend most of my life at sea. Ships, Science, the life of a mariner, biology and (mostly) true stories of life among the best and the worst people in the world, the United States Merchant Marines. You'll find it here, maybe. You'll definitely find rants, raves and discussion on life aboard a merchant ship. Come back and see the Brazilian girls, too, who show up fairly regularly.