Why does the largest ship in the world* have it’s engines mechanically connected to it’s propellers?
I thought most modern huge ships routed engine power to a generator which provides electrical power to the huge motors driving the props. It seems common to arrange multiple smaller engine/generators working together to feed a single motor for each propeller shaft. Having to design and build a huge custom engine that has to run at varying loads and speeds sounds rather expensive.
I assume the folks designing these things know a wee bit more about their trade than me, so what am I missing or misunderstanding?
On a side note, why don’t they need to build a new (or remodel an existing) drydock every time somebody designs a new, bigger ship?
according to several (possibly outdated) youtube videos
Could be efficiency. As you change the state of energy you loose more and more. The idea of having multiple engines feeding a prop sounds like a good idea but once you factor in your gross vs net gains it becomes less and less.
Also, that ship is designed to carry freight in the most economical way to have the highest ROI possible. So, being efficient in its drive train and optimizing it for a specific speed is probably what they did.
Navy ships are also this way, direct drive. I think the smaller ships used to have four diesel engines that combined through a bull-gear to drive two shafts. Like two engines per shaft. They then retrofitted to gas turbine engines and became direct drive.
A cruise ship might have a diesel/electric arrangement as you describe. Makes sense as you’ll need power for all the passengers and various other services and the flexibility of turret style electric engines for maneuverability into a port for docking.
from wiki: Low-speed diesel engines do not require a transmission system because they are directly connected to the propellers by drive shafts.
You’re looking at 73 revolutions per minute which apparently is in the operating speed of the propeller.
The efficiency of the ship is in the deliberate operation at lower speeds so the whole thing is designed around specific parameters. They started with a premise and worked backwards and this combination is what fits the bill.
I’m picking the answer is no. I’ve had a tour of a cargo ship engine room, in this case a single propeller one, and the engine is directly connected. Starting the engine means starting thrust. For reverse thrust, the engine starts in reverse. All other uses of power is supplied by engine generator sets separate from the propulsion system.
The torque required to turn a prop is a function of RPM squared, so it is feasible to hard couple the prop. No need to disengage for starting. Airplanes do it almost universally.
In order to reverse, the engine is stopped, the valve drive is reconfigured, and the engine is started backward. Starting is normally done by connecting compressed air to appropriate cylinders.
Fixed drive, with no neutral or reverse used to be fairly common on small, cheap outboard fishing boat motors. though there was usually right angle gearing, flexible shafts or sampan arraignments can even avoid that.
These outboards usually had no steering stops, so you could spin them 180 degrees if reversing was needed.
I think Eska was the last to produce motors like this. I have a 1970s vintage one. (Eska with Sears branding)
Looks like you’re right about the lift profile, but that the camshaft is just rotated through a “lost motion angle” via hydraulic servo.
No way could they swap intake with exhaust. Big marine diesels are pretty much always supercharged two-stokes. You’d have to do some major reconfiguration to get the supercharger hooked up to the right manifold on a swap. Plus, you really don’t want to make both manifolds resistant to high temperatures.
This page http://www.boattest.com/Resources/view_news.aspx?NewsID=3055 gives some details of the engine on that ship. You may note that it is a two stroke and weighs 2000 tons. the 14 cylinder engine for example with a displacement of 25,480 Liters ( 1.56 million cubic inches ) burns up 1,660 gallons of crude (“bunker”) oil every hour.
100,000-hp was actually achieved on a test bed in the workshop with the 14 cylinder model, running the engine flat out at just under 102 rpm.
When my brother was serving on a much smaller oil tanker he took me for a tour while it was in Milford Haven. When we reached the engine room, I asked where the engine was - he pointed to one wall of the space we were in - “That’s it,” he said.
I said it that way because there are several possibilities. On a very low rpm engine, it might be feasible, and even economical to actuate each valve with a hydraulic cylinder, in which case they could be electronically controlled…no idea if that is actually done.
Another way is to reverse the drive to the camshaft. In this case the camshaft will also need to be rotated to the correct starting position for compressed air start, as the indexing to the crankshaft also has to change.
Another way is that the camshaft will have two cams for each valve, and some way to select which cam drives the valve. This was typical on steam engines, though it was usually eccentrics rather than cams.
The injection timing also has to be altered, as would the spark timing on a gasoline engine. (Not aware of reversible gas engines though). Lubrication pumps must also work in reverse. By driving the oil pump and injection pump off the camshaft, reversing the camshaft drive takes care of all of this, so it is by far the most common way.
This scheme has advantages in controllability, but is almost certainly less efficient: there are losses in the prime mover (e.g. diesel engine), the generator, power transmission, and the large drive motor.
The use of a huge direct-drive diesel engine is clearly due to efficiency advantages: these things are slightly over 50% efficient (IOW, they convert more than have of their fuel’s chemical energy to mechanical power). That’s a remarkable figure (30% is considered really good in most applications), and not one a diesel-electric system could hope to match.
The Messerschmitt KR200, a bubble-car built after WWII with surplus aircraft parts, had a reversible gas engine. From Wiki:
*
The KR200 ran on a 191 cc (11.7 cu in) Fichtel & Sachs air-cooled single cylinder two-stroke engine positioned in front of the rear wheel,[6] just behind the passenger’s seat.[3] The engine had two sets of contact breaker points and, to reverse, the engine was stopped and then restarted, going backwards. This was effected by pushing the key further in the ignition switch than normal, whether intentionally or not. One result of this was that the KR200’s sequential, positive-stop transmission provided the car with the same four gear ratios available in reverse as in forward movement.*
Wow, that IS impressive. I didn’t realize diesels had come so far (and running on the least-refined fuel, no less!).
So, does anybody give me an idea how much the maintenance costs would increase with a custom engine of that size (everything seems to be much more complicated at large scales)?
There are two sets of cam lobes on the cam shaft. one for forward one for reverse. The cam shaft slides back and forth. For forward move the cam shaft in the forward direction and start engine. For astern slide the shaft the opposite direction and start.
And they are direct drive.
A tanker or a freighter Loads up and leaves port to straight to the next major port. As soon as the harbor pilot gets off the ship full speed ahead is rung up and there be no speed change until the next pilot is picked up. A cruse ship will be going into port and out of port every day or two. With electric motors drives they can be hung under the hull and turn and used as stern thrusters. A little loss of efficiency, but easier to dock with out tugs.
Also with motors and electronics today the motors speed can change without the cycles changing.
The efficiency of diesels to day put an end to the erra of steam. Bunker “C” is really not a refined fuel. It is the crud left when refining oil. And it can be heavier than water.
There have been many mechanisms used over the years for reversing direct connected diesels on ships. The one in the link has been common in recent decades, but now more and more ships have electronically controlled ‘camshaft-less’ low speed diesels.
Switching exhaust and intake for reversing was actually used on some diesels way back when, but it’s true it’s unworkable for modern two strokes where the intake is through ports in the cylinder liner and the only actual valve is the exhaust valve.
Direct connected low speed diesels with fixed pitch props represent a large majority of merchant ship power plants counting by horsepower, since it’s the overwhelmingly dominant arrangement in large container ships, tankers and bulk carriers. Geared medium speed 4 stroke diesels though are numerous in smaller merchant ships. They are almost always mated with controllable pitch propellers nowadays (low speeds are sometimes mated to CPP’s too, but not usually). Direct reversing medium speeds are now rare: with the modern engines’ high degree of turbocharging they just don’t have the torque ‘umph’ to arrest the propeller and reverse it in good time at low rpm when the turbocharger isn’t functioning much. A low speed diesel is being scavenged by an electric blower at low RPM, before its own turbocharger kicks in seriously.
Cruise ships usually have diesel electric drive (a type of ship many people have had direct contact with) and various types of offshore service vessels, and occasionally other types like tankers in some cases, and now warships. But electric drive is very much the exception if you count all ships.
I would imagine that they actually have to do that every so often. Another issue is passing through canals like the Suez or Panama Canals. They are in the process of widening the Panama Canal to fit larger ships, but I believe the Triple E still won’t fit.