Quoting to some things Jim Hathaway posted:
In the paper he states "a slow speed turbine expands low pressure steam much furthur and more economically than any reciprocating engine. Under this system, the turbine generally is made to develop about 1/3 of the whole power.
He mentions the first combination vessel, the Otaki with twin screws driven by triple expansion reciprocating engines exhausting into a center low pressure turbine driving a screw.
Parson's paper says Otaki consumed 12% less coal than her sister driven by four reciprocating engines.
He goes of to say that Laurentic, in service realized a 14% savings of coal consumed over the same speed as her sister Megantic with a pure reciprocating engine plant.
Well, this sound not realy funny. But a low pressure turbine won't be able to compete with a large live steam reciprocationg engine. Because turbines do only best, if propelled by high temperature, high pressure live steam, and not with the exhaust trash. Even in your exampe: If I got a ship with a triple expansion engine, and a turbine, and another with two expansion reciprocating engines, well, the turbine will do the job well, but you won't have same power output.
This has to do with condensation efficiency. Turbines can much better expand stream in a continous flow, but it is not using the expansion energy. More a turbine works like a wind mill, so stream is impportant, not expansion. So reciprocationg engines lock-up the steam into the cylinder, and the pressure will push the piston, while the pressure sinks due expansion, but the expanding steam propells a reciprocating engine, not the live pressure.
So many reciprocating engines operate in the lp cylinder in very low pressures, so need a large piston surface, and many times the lp cylinder also operated in negative pressure (vacuum levels), as a result and benefit of condensator usage. But negative pressure in lp makes this cylinder only effectivve if the condensator is operating well, and the whole plant build for a small range of revolutions. Not good for ships engines. So most ships use reciprocating engines whithout such effective condensators, and waste heat energy. Heat ennergy is allways direct correlated to fuel. So as more heat a engine plant wastes, as higher the fuel one need to keeep it running.
So here comes Titanics advantage: Two reciprocation engines deliver about 2/3 or 5/8 of the maximum power output, and the lp cylinder operates not on vacuum, but the turbine, which uses the exhaust steam of the reciprocating engine, now expannds the steam from very low pressure to vacuum levels, and thus making the following condensator highly effective. Thus: Maybe Titanic is not so powerfull than a engine with three reciprocating engines, but need about one thrid less fuel. Thats a advantage and benefit in costs, because fuel is tthe most expensive cost calculation in running an engine. And more power still means more fuel costs, Titanic made here a middle way, in more power to less costs.
And so, before comparing engines, we had to comparre the whole plant.... And than we can assume what's better or less good. Because the turbine is not, it is just different.
BTW, if I remember correctly, Titanic and Olympic's engines could produce 45,000hp while Mauretania and Lusitania's could do up to 70,000hp.
A great difference!
Well, hp output is just a term! How much fuel consumed Mauretania? Because this, one has allways to think about: Not a hp term is what makes an propulsion plant.
So whatt would you think: I have a ferrari and have 500 hp, you have a porsche with 250 hp.... My Ferrari wastes about 30-45 Liters per 100 kilometers, and your porsche only 15-20 Liters.
Which engine is better? The one with 500 hp? Not realy, isn't it? Same in ship propullsion plants.
