For several years now, emails have been circulating pictures and a description of an enormous marine-diesel engine, intended for propulsion on very large ocean-going ships. The accompanying text, among other things, typically includes this line:
Is this true? Google image searches show the Emma Maersk (a ship that is fitted with that massive engine) does indeed have one single large propeller, but the Wikipedia page for that ship describes it has having the Wartzila main engine along with five smaller Caterpillar engines.
If a single prop/engine design is preferable, why? My guess is efficiency, but seeing as how transcontinental aircraft generally aren’t designed with less than two engines, what are the reliability concerns of an open-ocean ship being fitted with a single prop/engine design? Isn’t propulsion important to keep a ship pointed into the heavy waves when bad weather hits? Or is that not such a big deal on these monster ships?
I’m guessing it’s mainly a cost issue for merchant ships. A high-speed ship requires multiple screws, because they become less efficient the faster they turn (due to cavitation I think?). I guess the ships are generally reliable enough that the single-point failure isn’t a big concern - although there have been cases recently where ships have lost propulsion and been wrecked as a result.
My ship I served on (the Nimitz) has four shafts. These are turned by four complete sets of reduction gears + twin turbines.
These in turn are found in two separate engine rooms, serviced by two reactors.
The goals of a military vessel are different from a commercial vessel: the Nimitz needs to be able to function even after sustaining battle damage to major parts. This is achieved by having multiple systems, running redundant piping along opposite corners of the ship, and other such design features.
On the down side, the hardware is much more complex, everything is multiplied by four (or two), there are many more people needed to run the thing, and maintenance is far higher.
Commercial shipping is all about low cost. If you can have fewer people running the power plant and if maintenance is low then there are significant cost savings.
No need to have redundant systems: if the engine stops, they don’t fall from the sky like a plane; they can simply drift as they calmly fix the problem.
A tangential point: mechanical systems on this scale are much more reliable than systems we encounter in our daily lives (such as car engines). This is partly due to a more rigorous maintenance schedule and better quality control.
As an example of this reliability, when we were in nuclear power training, a friend of mine was stationed at a training facility that had been made by bolting a modern reactor to a steam plant from a WWII cruiser that had been moved to a land site. All of the machinery from the cruiser worked just fine and was maintained on a regular basis.
So a single very reliable engine will cost much less to run and service, and will have little risk of failure at any critical moment.
If you’re on a transcontinental aircraft, and one engine fails, you can still limp home on another engine…unless you don’t have one. Flying an unpowered transcontinental aircraft and making a safe landing is a challenge most people want to avoid. With a boat, most of the time you can just float until you get the thing repaired. Aircraft don’t float unpowered (except balloons, but who takes a transcontinental balloon?)
Of course, you are referring to one specific kind of huge diesel engine (closer to the OP’s topic), while I was referring to generalities of this kind of hardware lasting forever.
In a steam plant, the shaft is connected to reduction gears that are hooked up to turbines that are spinning at a substantial rate—it is not unusual for a smaller ship to turn the shaft at ~200rpm, with a ~20:1 gear ratio meaning that the turbines are spinning at ~4000rpm.
Even so, those last many decades.
I’m not sure about the monster in the Emma Maersk, but most large ship-diesels that are direct coupled to the shaft and prop can be reversed simply by stopping them, and re-starting in the opposite direction.
And why only one monster? It’s a cargo ship. The engine on the Emma Maersk takes up room, but a great deal less then if there were two half sized engines. Engine efficiency also scales up with size (strangely enough). That engine is about the most efficient IC engine in the world, and it can run on raw crude.
ETA - The large Warstilla design engines can also be almost completely maintained while underway, to the point of disabling a cylinder, and replacing everything in it should that be required, although for something that drastic a temporary shutdown is usually required to disconnect the crankshaft.
OK, so single large engine is more efficient and gives adequate reliability.
So what’s up with the Emma Maersk? It’s got one big engine putting out 109K hp, and five Caterpillar engines providing an additional 40K hp. That’s not just a little boost, those Cats are proving a quarter of the total power. and it’s not just a single extra engine, it’s five extra engines.
Apparently, electrical power is needed to drive the two auxiliary motors coupled to the main shaft. Also, the ship has four thrusters which could probably eat up some of those megawatts.
The turbine generators on 688s ran at 3600 rpm. Being on a sub they had to be quiet and had close tolerances and therefore had a lengthy warmup.
The turbine generators at MARF were 10,000 rpm units and being WW2 vintage surface units they required very little warm up. The two turbines were about 40 feet from each other. We used to race to see who can start em up the fastest.
There have been cases of ships losing propulsion and being wrecked for all recorded history, from days of sail onward.
It can carry 1000 forty foot reefers (refrigerated containers). Imagine the refrigeration machinery required to keep a forty foot box at minus 20C in the tropics, then multiply that by 1000 and suddeny 30 MW doesn’t sound so outrageous.
I’m not trying to challenge this by asking, as I don’t have much knowledge in this area, but I’m just curious: Couldn’t they just drop anchor if they know they’re being drifted dangerously close to land?