Do you think it's mainly bureaucratic inertia that keeps it separate? Or is there something more complicated about controlling engine power than I imagine?
On a modern diesel or gas turbine ship, no there's not. Obviously there are limits to what the engine can do still.
They're not direct driving the screw with steam power from the reactor heat-exchange, surely.
You've probably seen aircraft carrier decks covered in smoke. This is because the Nimitz's catapult system is directly steam powered, service steam is widely used on the class. I can't find a citation for this now (this is from memory) but I saw some documentary on it once where it was mentioned in passing that even the washing machines on the ship were steam powered. There's service steam everywhere on the Nimitz.
Having steam power everywhere introduces a lot of complexities, which is why the Navy's moving away from it with their new carrier class[4]. The Gerald R. Ford-class does away with service steam in favor of more powerful electric generators. There'll be no service steam, just steam to drive the electric turbines.
1. http://www.nimitz.navy.mil/FACTS.html
2. https://en.wikipedia.org/wiki/Nimitz-class_aircraft_carrier#...
3. https://www.marinelink.com/news/dresserrand-propulsion302604
4. https://en.wikipedia.org/wiki/Gerald_R._Ford-class_aircraft_...
https://en.wikipedia.org/wiki/Electromagnetic_Aircraft_Launc...
Basically they spin up flywheels that then dump their kinetic energy back into electric power as the catapult is released.
And it allows them to finely adjust the speed of the catapult depending on the kind of aircraft they are launching.
Yes, they are, for the simple reason that it's more efficient. This is beginning to give way to a series electrical design, but that's for reasons of increased stealth and other concerns, at the cost of overall thermodynamic efficiency.
On all existing nuclear carriers steam is used for the catapults (and this is much more demanding than many people are aware of). The Ford class is developing electromagnetic catapults, which are also proving to be demanding to perfect.
Or does it directly power the engines? This would mean that your entire control loop would include the latency of the steam generation process. Move the lever, wait for the water to get hot, wait for the turbines to spin up, and then the screws would start to turn faster. Like the throttle response in an ancient diesel truck.
Given that the original diesel submarines used giant battery arrays to power the electric motors while the engines were off, and that the former gives much better control response, I'd assume that there are still large batteries in the loop. This paper [2] supports that assertion:
> The nominal cell voltage is 2.0 V. [1] The PDX-57 cell designed for Ohio-class submarines weights 2,100 pounds with a capacity of more than 10,000 Amp-hours and stored energy of 2.6 MWh. [1] The ASB-49 cell designed for the Los Angeles-class submarine weights 1,300 pounds with a capacity of 7,200 Amp-hours and stored energy of 1.8 MWh. [1] The LLL-69 type cell weights 1,500 pounds with a capacity of 8,100 Amp- hours and stored energy of 2.0 MHh. [1]
A couple megawatthours is plenty to run the screws while the steam loop and nuclear reactor respond.
Therefore, toomanybeersies comment:
> On a nuclear powered ship, yes, since you still essentially have a steam engine, the engine telegraph (throttle) isn't actually linked directly to the engine.
is technically correct but actually irrelevant. The nuclear reactor telegraph does control the steam turbine and eventually the battery charge, rather than the engine. But somewhere there's an electric motor controller which responds instantly.
[1] http://ieeexplore.ieee.org/document/1177196/?reload=true&tp=... [2] http://large.stanford.edu/courses/2013/ph241/ditiangkin1/
Both the Ohio and Los Angeles class submarines use mechanical linkage propulsion, not electric, if I understand correctly.
Source: https://www.defensetech.org/2013/09/27/ohio-class-subs-to-sh...
Looks like some of the most modern or in-design classes are using electric drive, but most things out there are using direct mechanical linkage to the turbines to drive the propeller.
Right, this is why when BAe lied to the UK claiming that the QE class carriers could easily be converted to cats'n'traps every engineer in the country said WTF? Because with no nuke where do you get either the steam for the catapults or the surplus electrical power for EMALS when you want to steam directly into the wind for deck operations? But our idiot PPE-educated politicians lapped it up and signed cheques for billions anyway...
Can you not vent some of the steam, so the same volume of steam is being produced and the temperature the same but only some of it goes to the drive?
The comment was about the ability to vary power at all, not doing it efficiently.
Are you going to choose to crash into something because it's wasting fuel to vent part of the steam?
The ship command structure was a form of information hiding, whereby the commander only worried about the speed, whilst the engineer worried about how to efficiently deliver the requested speed.
Under emergency you will vent, but under normal operations where you just want to slow down you care very much about efficiency.
Not just for monetary reasons either (though that's a factor) but because the lumps of coal you're not burning now could save your ass in bad weather and make you reach safety instead of being stranded in the middle of the ocean.
Large marine diesels such as used by container ships do need to follow a procedure to change out of the cruise rpm. I don't know how much that can be skipped/abbreviated in case of an impending collision, but I do know that generally it takes more than just someone on the bridge changing a control setting.