Well, some words to think about, from a steam engine passionist.
First a link:
Swiss Steam Engine Manufacturer for modern steam engines.
Many people watch the old ship, and find, that the engines were large, less powerfull and need hundret hands for maintainace and operation.
So Titanic as Olypmic engines did realy not need much space, what need the space were the 'auxillaries', as boilers, coal bunkers, feedwater tanks and mainanace space. even the engines were large, because oparting with low presures and thus need more space than today constructions.
But a good planned and constructed steam engine can still today easy compete with the gasoline or diesel engines, biuld in mordern ships, and as shown at the link above: First ships were 'rebuild' steamers...
What made the steam engine to abandon? Well, it was the size, the difficulty to operate, the many people to stoke the boilers and the bad enery biliance...
But things changed: Modern power plants changed only their engines, from the recipoking engines to steam tubrines, and still todeay the steam turbine is commonly found in power plants, if not propelled by wind, water or solar radiation.
Well, and that changed again ships propulsion units: Okay, not much 'not-naval-ships' have a nuclear power plant, but many marine ships, especially large vessels, like carriers or special ships, like submarines operate with nuclear power as standart engine. Here we find again the steam engine: The reactor heats up the cooling liquid in the 'hot' area, which will pass a heat exchange unit: Only a 'small' drum, compared to old boilers, but much more efficient. here water is evaporated and send from there to the main engine, commonly a multiple pressure steam turbine. The exaust steam is, as in Titanic or Olympic, given to the condensator, were it is brought back into water, feed again into the heat exchange unit, and then the circle starts again. Many of there power plants are in operation still today. Those propulsion plants have following advantages: If the pressure in the steam circle is kept high, the water temperature can bee keep above 100°C without evaporation, making the use of heat very efficient. Also the boiler has been changed, because nuclar power plants often use a special cooling liquid instead of water, having a better heat absorbtion, so making the heat form the boiler and to the water more efficient use.
Also: Feedwater reseve is today less, because the closed steam circle has less need for additional feedwater, as the closed circle on past times.
But consider: Which passenger would be lucky to stay on a nuclear power plant? None, I knew.
So if we like to consider steam engines in modern passenger lines, we must first look a the boiler. Today boilers use usually diesel or gasoline to produce the heat, so why isn't it better to operate a diesel or gasline engine, instead of 'boiling water'...
First: Heat is the magic word. Modern passenger ships have a large demand for heat. Heat is used to make the cabins comfortable, heat powers the air condition, heat is used for the refridgeration rooms, heat powers the temperature of the all-deamnded swimming pools.
So we need heat at a passenger liner. And many lines have special heationg units, only producing steam for the heat demand. Fuel is 'wasted' just for this demand. A modern ships boiler can deliver both: Steam for the auxillary as for the engines as well.
Second: Diesel engines and gasoline engines have a 'short-Flame-way' so fuel is never copletely burned, so allways some fuel is 'wasted'. Modern boiler have 'long-flame-ways' so all fuel can be burned and the amount of air need for this procedure can be exaclty adjusted, so not as in gasoline engines were air for burning fuel is often too less.
Thrid: A modern boiler can be finely adjusted to the demanded power, so if the engine hewades to a port, and less power is need, often the burned can be nearly completely shut down, because the steam reseve in the boiler is enough for the remaining manoeuvers, so fuel will be not wasted anymore.
Three point for a steam propulsion, but what about the real steam enigne. Here the gasolien and diesel engines have advantages. If you use a boiler, just for the ships heat demand, I can switch it on and off as required, without any guess for the engine itself. Also, modern steam turbines propelling the propeller shaft having the disadavantage, not relay to be able to act in a areas of rounds per minute very economical. In high speed rounds often the steam pressure must be very high, so the boiler must run full power, without that the turbine will be able to use up all the delivered pressure, which must be than 'crushed' in the condensator. Not very economicaly and very fuel wasting.
Next an low round the turbine tends to instead rounds, which is very destructive for bearings, the shaft as the propeller itself, as the turbine will waste steam because the pressure isn't that much required, so the last turbine wheel will even tend to higher vibrations, which is same: Destructive. So a turbine is only effective, when a ship runs long distance a one speed. Gasoline and diesel engines have a broad range of available rounds with a avarage good power output, so require not such a gear or electrical power transmission like turbine propelled ships.
And this is the main reason: We have good boilers today, but mostly the turbine is to complex to act economical in a passenger or short-distance freighter. Turbines are comonly today used to propel a generator, so giving the engeriy to the ship, but a small electic engine will propel the propellers of the ship, very uneconomical and not need, if a more 'flexible' engine is used, like gasoline or diesel engines. So here we can turn on and off the boiler for heat, and can power with a small auxilary steam turbine the auxially generator, but the gasoline engines will propel the propeller shaft and the main generators for electricity.
So what do we need? Maybe the men a DLM had an idea. The small passanger paddle-wheel ship Montreux was rebuild an steam power. A modern boilder was build, and a modern, mainainace-less engine was constructed. So the diesel engine of the montreux was dismounted and the steam power plant was build in. It was quite larger, of course, but it was less fuel consuming and produced less exaust, but was same flexible in engine power output.
Today it seems posible to construct large, fast running recipoking steam engines, fully capsuled as diesel and gasoline engines, so called steam-motors. These maintainanceless engines have the posibility to propel a ship, as modern diesel or gasoline engines can do. but having the advantages of the old steam recipoking engines, of a flexible steam consumption, quick direktion chance ability and direkt access to the main shaft.
Combinated with a modern force-flow boiler the ship is able to support heat to auxillary stations from the main engine room as well as for the main steam engines, without having the need of extra boilers or extra manoeuver engines. this will make an engine compact, maintainanceless and maybe a posibility for further ships. Also the engine power as the boiler power can be fine adjusted to the demand of power of the ship just in time, so less fuel will be wasted as the fuel will be burned much better, resulting in spare of fuel and less exhaust.
But none will try to do it, because steam engines are considered 'old', 'powerwasting' and 'maintainance-eating'.... The DLM has shown these these facts are not proof, so maybe we must think in a different view, to Titanics and Olipics power plant, which was still at their time modern, but was apruplty stopped in development after diesel and gasoline engines were found. With the know-how of today maybe the steam engines can have a new breakthru, if someone will give them the chance to proof these facts.
In railways in switzerland modern steam engines have expeted the diesel trains, as on a paddle-wheeler too.
So I wrote this, just to give you an different view to steam engines and the steam power, if we compare them with diesel or gasoline engines.
